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Managing summer apple scab epidemics using leaf scab incidence threshold values for fungicide sprays
Crop Protection 35 (2012) 36e40
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Managing summer apple scab epidemics using leaf scab incidence threshold values for fungicide sprays Odile Carisse*, Tristan Jobin Horticultural Research and Development Center, Agriculture and Agri-Food Canada, 430 Gouin Blvd., Saint-Jean-sur-Richelieu, Qc J3B 3E6, Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 June 2011 Received in revised form 25 November 2011 Accepted 12 December 2011
In several apple production areas, most fungicide sprays applied to orchards target apple scab management. Recently, action thresholds were established to aid in decision making as to whether fungicide sprays are required to manage summer scab. To facilitate grower adoption of these thresholds, a sequential sampling for classification procedure (SSCP) was developed. The SSCP with an action threshold of 0.5% foliar scab was evaluated from 2005 to 2008 in seven commercial orchards as a tool to time fungicide sprays. At each site, at the end of the primary infection period, the orchard was divided into two sections: in one, summer scab was managed using the grower’s standard practices and in the other, summer scab was managed based on scan incidence. Starting in mid-June, each orchard section managed based on scab incidence was classified using the SSCP as below or above the action threshold or as in the “no decision” zone. When an orchard section was classified as above the action threshold, a fixed-interval spray program was initiated and generally continued until harvest. When an orchard section was classified as below the action threshold, initiation of the fungicide spray program was postponed until the threshold was reached. When an orchard was in the “no decision” zone, sampling was repeated one or two weeks later depending on the frequency of rain events. Similar incidences of foliar scab of 0.27% and 0.22% and of fruit scab of 1.11% and 1.27% were observed in orchard sections managed based on scab incidence and standard grower’s practices, respectively. However, fruit scab at harvest was above the economic threshold of 2.0% at three and six occasions in the sections managed using foliar scab incidence and using the growers’ standard practices, respectively. For the four years of the study, an average of 6.4 and 9.1 fungicide sprays were used to control summer scab in the orchard sections managed based on scab incidence and in those managed according to the growers’ standard practices, respectively. Considering that the same fungicide was used in all orchards, we concluded that the reduced number of fungicide sprays (30%) resulted from the elimination of sprays when scab incidence was below the action threshold of 0.5% leaves scabbed. Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.
Keywords: Apple diseases Action threshold Disease management Integrated disease management
1. Introduction In several apple-growing areas throughout the world, apple scab caused by the fungus Venturia inaequalis (Cke.) Wint. is the key disease for scheduling fungicide sprays (MacHardy, 1996). In eastern North America, the disease is mostly controlled by the application of fungicides (Koehler, 1998, 2005; Reardon et al., 2005). Most scab management schemes are divided into two programs, with the first one aiming at controlling the monocyclic phase of the disease, i.e. the primary scab infections caused by ascospores, and the second one aiming at controlling the polycyclic phase of the disease, i.e. the secondary infections caused by conidia. * Corresponding author. Tel.: þ1 450 515 2023; fax: þ1 450 346 7740. E-mail address: [email protected] (O. Carisse).
Considering that the secondary inoculum is produced on lesions resulting from the primary infections, it is crucial to achieve effective primary scab control to avoid epidemic build-up caused by secondary infections and to reduce the need for fungicide applications during the summer months. To achieve this goal, fungicide applications generally begin in the spring at the green-tip phenological stage and continue throughout spring into early summer, resulting in some 8 to 10 applications (Reardon et al., 2005). If this approach fails to control scab adequately, then fungicides must be applied during the summer, and the annual number of fungicide sprays could rise to 14 to 20, depending on the prevailing weather conditions during the summer months (MacHardy, 1996; Reardon et al., 2005). Several tools have been developed to help manage primary infections and include procedures to estimate potential ascospore
0261-2194/$ e see front matter Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.cropro.2011.12.014
O. Carisse, T. Jobin / Crop Protection 35 (2012) 36e40
dose (PAD) (Gadoury and MacHardy, 1986; MacHardy, 1996; Berkett et al., 1997), weather-based models to estimate ascospore maturity (Gadoury and MacHardy, 1982), and weather-based models to estimate periods of infection risks (Mills, 1944; MacHardy and Gadoury, 1989; Gadoury et al., 1995; Stensvand et al., 1997) and leaf growth (Carisse et al., 2008). Although infection models developed to estimate risk of primary infections have been adapted for secondary infections (Hartman et al., 1999), there are relatively few tools available to manage secondary infections (Holb, 2008). Most summer scab management schemes aimed at controlling fruit scab and maintaining leaf scab at an acceptable level to avoid excessive risk of fruit infections and of production of overwintering structures (pseudothecia) during the autumn and hence reduce the amount of primary inoculum the following spring. During the summer, the frequency of fungicide sprays and the expected level of control depend on the weather, cultivar susceptibility, and the marketing goal, i.e. mainly whether the fruits will be stored, processed, or sold for the fresh market. Nevertheless, it is generally admitted that the incidence of fruit scab at harvest should not exceed 2% (Schwabe et al., 1984; MacHardy, 1996; Holb et al., 2003; Reardon et al., 2005). To achieve this goal, various strategies have been proposed (Gadoury et al., 1989; MacHardy, 1996; Berkett et al., 1997; Koehler, 1998, 2005; Brun et al., 2010). These strategies are based on fixed-interval spraying programs, delayed fixed-interval spraying programs, or early-ended fixedinterval spraying programs. In some strategies, the intervals between sprays are determined by the frequency of rain (Berkett et al., 1997; Koehler, 1998, 2005). Nevertheless, most fungicide programs do not consider the level of leaf scab present in the orchard. Regardless of the strategy employed for summer scab management, the key element in making informed management decisions is the level of disease present in the orchard. If the level of scab is low and the forecasted weather is hot and dry, fungicide applications may not be needed. However, if the level of scab is high and the forecasted weather is warm and wet, fungicide sprays are required. Establishing a unique action threshold for applying fungicide sprays to manage summer scab is complex because the threshold is influenced by several factors, including tree size and density, orchard site characteristics, orchard operations, cultivar susceptibility, marketing goal and, not the least, the weather during the summer months. Therefore, a conservative threshold of 0.5% leaves scabbed was proposed as an action threshold (Koehler, 1998, 2005; Carisse et al., 2009). Because this threshold is low, it is expected that a large number of leaves will need to be assessed to determine if the incidence of leaf scab is above or below this threshold. Carisse et al. (2009) recently developed a sequential sampling for classification procedure (SSCP) to classify orchard scab incidence as above or below the action threshold of 0.5% foliar scab or as in the “no decision” zone. This SSCP had not yet been evaluated in the field. Therefore, the objective of this research was to compare summer scab management based on scab incidence monitored using this SSCP with grower’s standard practices in terms of foliar and fruits scab at harvest and of numbers of fungicide applications. 2. Materials and methods 2.1. Data collection The experiment was conducted from 2005 to 2008 in seven commercial orchards located in the Missisquoi-Estrie and Montérégie apple-growing regions, for a total of 28 evaluations (seven orchards four years). These orchards were planted with the cultivars ‘McIntosh’, ‘Cortland’ and ‘Lobo’ grafted on various rootstocks (Table 1). At the end of the primary infection period, at
37
Table 1 Description of the seven apple orchards used to compare programs for management of summer scab by Venturia inaequalis in 2005e2008. Site
Varieties
Orchard
County
1 2 3 4 5 6 7
Frelighsburg Frelighsburg Dunham Dunham Rougemont Stanbridge Station St-Paul
McIntosh McIntosh McIntosh McIntosh McIntosh, Lobo McIntosh McIntosh, Lobo
Rootstock
M.7 M.7 MM.106 MM.106 M.7 MM.106 M.7
Spacing (m) Between rows
Within rows
6.2 6.1 4.9 4.8 6.0 4.9 6.0
4.1 3.9 3.1 3.0 4.0 3.2 4.0
each site, an orchard section of 0.3e0.5 ha was selected for the experiment and split into two sections: in one, summer scab was managed following the grower’s standard practices which consisted in applying a fungicide prior to a forecast rain event or at fixed intervals, and in the other section, summer scab was managed based on a foliar scab action threshold of 0.005 scabbed leaves per shoot and classification of leaf scab incidence as above or below the threshold using the SSCP developed by Carisse et al. (2009). At all sites, primary scab infections were managed based on the growers standard practices in both sections, hence the differences in foliar and fruits scab at harvest could be attributed to difference in summer scab management. Weather data were obtained from the Environment Canada weather station located within seven Km from the commercial orchards.
2.2. Classification of the orchard sections as above or below the action threshold The first orchard classification was conducted in mid-June. At each sampling, a minimum of 50 shoots, both cluster and terminal shoots, which were representative of the orchard (at the bottom, in the centre and at the top of the trees) were examined. The number of scabbed leaves on each shoot was counted. Based on the total number of scabbed leaves, the orchard sections were classified, using the previously described SSCP proposed by Carisse et al. (2009), as above or below the action threshold of 0.005 scabbed leaves per shoot, or as in the “no decision” zone (Fig. 1). When an orchard section was classified in the “no decision” zone an additional 10 shoots were examined, until a decision could be made or a maximum of 240 shoots were examined. The following criteria were used to manage summer scab: (i) If the orchard section was classified as above the action threshold, a 7- to 14-day summer spray program was initiated and continued until harvest unless a prolonged dry period was forecasted, in which case the orchard section was reassessed. Unless that reassessment classified the orchard section as below the threshold, the fixed-interval summer spray program was maintained. (ii) If the orchard section was classified as below the action threshold, fungicide spray applications were postponed until the threshold was reached. In these situations, the orchards section was monitored at every 7e14 days depending on frequency of rainy days. If the action threshold of 0.5% foliar scab was not reached by mid-August, a less conservative threshold of 1.0% foliar scab was used from then until harvest (Carisse et al., 2009). (iii) If the orchard section was classified as in the “no decision” zone, sampling was repeated 7e14 days later depending on the frequency of rainy days. With the SSCP, a minimum of 50 and a maximum of 240 shoots per orchard section were sampled (Fig. 1), and care was taken to select shoots that were representative of the trees (i.e. shoots at the top, bottom, edges, and centre of the canopy). The time and the
O. Carisse, T. Jobin / Crop Protection 35 (2012) 36e40
A
25
Cumulative number of scabbed leaves (TN)
38
20
3. Results Pth = 0.005
5 Below threshold of 0.005 scabbed leaves per shoot
0 0
25
50
75
100
125
150
175
200
225
250
275
300
20
10
Continue sampling
Rain (mm)
Above threshold of 0.005 scabbed leaves per shoot
30
75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0
Rain (mm)
40
Pth = 0.01
75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0
Rain (mm)
50
75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0
Rain (mm)
B Cumulative number of scabbed leaves (TN)
Number of samples (shoot)
75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0
Below threshold of 0.005 scabbed leaves per shoot
0 0
25
50
75
100
125
150
175
200
225
250
Number of samples (shoot) Fig. 1. Sequential sampling for classification plans for classifying scab incidence as above or below action thresholds of 0.005 (A) and 0.01 (B) scabbed leaves per shoot (adapted from Carisse et al., 2009).
number of shoot samples required to reach a decision were noted. At all sites, summer scab was managed with the fungicide Captan (Maestro 80-DF, Aryst Life Sciences, North America, North Carolina, U.S.A) at a rate of 2.00e3.75 kg/ha. A few days before harvest, leaf scab incidence was estimated from a sample of 250 shoots randomly selected within each orchard section, and fruit scab incidence was estimated from 25 fruits on 10 trees selected at random within each section. In the autumns of 2004e2008, each orchard section was assessed to determine the inoculums level using the sequential sampling technique proposed by MacHardy et al. (1999). In each orchard section, 10 extension shoots on 10 trees for a total of 100 shoots were sampled and the cumulative number of scabbed leaves noted. The orchard sections were classified as low, moderate or high inoculums orchards, if the total number of scabbed leaves was 5, 6e17 or >17 per 100 shoots, respectively. 2.3. Data analysis Analysis of variance was used to test the effect of the spray programs on the number of fungicide sprays, on leaf and fruit scab incidence at harvest, and on inoculums level. Within each year, each orchard was considered as a repetition and significance between treatment means was detected using Tukey’s mean separation test at a 0.05 confidence. Statistical analysis was conducted using the SAS software program (version 9.1, SAS Institute, Inc., Cary, NC).
2005
20 18 16 14 12 10 8 6 4 2 0
Rain intensity Rain duration Threshold for summer scab infection
20 18 16 14 12 10 8 6 4 2 0
2006
2007
2008
18/06
02/07
16/07
30/07
13/08
27/08
Rain duration (hours)
Continue sampling
Rain duration (hours)
10
20 18 16 14 12 10 8 6 4 2 0
Rain duration (hours)
15
The four years of the study were characterized by 25, 30, 41, and 35 days with rain (>1 mm), respectively (Fig. 2). The number of fungicide sprays to manage summer scab varied from season to season, with, on average, 6.0, 5.1, 7.5 and 6.4 sprays in 2005, 2006, 2007 and 2008, respectively, in the orchard sections managed using the action threshold, and 8.1, 7.8, 10.3 and 9.1 sprays in 2005, 2006, 2007, and 2008, respectively, in the orchard sections managed using the growers’ standard practices (Table 2). Overall, significantly (P < 0.0001) fewer fungicide sprays were used to manage summer scab in the orchard sections where an action threshold of 0.005 scabbed leaves per shoot was used (average of 6.4 sprays) than in the sections where the growers’ standard practices were used (average of 9.1 sprays). At harvest, the incidence of leaf scab was 0.22, 0.24, 0.26 and 0.36% in 2005, 2006, 2007 and 2008, respectively, in the orchard sections managed based on scab incidence, and 0.15%, 0.16%, 0.31% and 0.28% in 2005, 2006, 2007 and 2008, respectively, in the orchard sections managed using the growers’ standard practices (Table 2). For the four years of the
20 18 16 14 12 10 8 6 4 2 0
Rain duration (hours)
Above threshold of 0.005 scabbed leaves per shoot
10/09
Days Fig. 2. Quantity (in mm) and duration (h) of rain event during the summers in 2005e2008 recorded at the Environment Canada weather station located at the Agriculture and AgriFood Canada experimental farm located in Frelighsburg, Quebec, Canada. The threshold for conidial infection was calculated from the relationship between temperature, leaf wetness duration and conidial infection (Hartman et al., 1999).
O. Carisse, T. Jobin / Crop Protection 35 (2012) 36e40 Table 2 Incidence of leaf scab at harvest, incidence of fruit scab at harvest, and number of summer fungicide sprays used to manage summer apple scab, in orchards managed using the growers’ standard practices (STD) and based on monitoring leaf scab incidence (INC). Orchards
Year 2005 Program
Year 2006 Program
Year 2007 Program
Year 2008 Program
INC
INC
INC
STD
INC
STD
0.18 0.21 0.17 0.41 0.30 0.28 0.29 0.26a
0.16 0.33 0.16 0.58 0.27 0.20 0.50 0.31a
0.21 0.58 0.22 0.59 0.21 0.26 0.45 0.36a
0.16 0.21 0.19 0.12 0.15 0.58 0.53 0.28a
0.27 1.12 0.13 2.97 0.50 1.11 1.89 1.14a
0.21 2.92 0.11 3.48 0.43 1.32 4.95 1.92a
0.73 4.87 0.36 1.92 0.79 0.97 2.98 1.80a
0.56 2.31 0.58 1.79 1.34 2.57 4.53 1.95a
8 9 7 8 7 7 7 7.57b
11 9 12 9 11 9 11 10.29a
6 7 9 7 6 8 6 7.00b
8 10 12 10 12 11 10 10.43a
STD
STD
Percent leaf scabbed at harvest 1 0.28 0.15 0.26 0.20 2 0.16 0.14 0.20 0.17 3 0.08 0.07 0.52 0.13 4 0.31 0.17 0.13 0.12 5 0.21 0.14 0.16 0.16 6 0.21 0.16 0.19 0.16 7 0.28 0.21 0.20 0.17 a 0.22a 0.15a 0.24a 0.16a Mean Percent fruit scabbed at harvest 1 1.74 1.19 0.75 0.45 2 0.25 0.21 0.40 0.36 3 0.13 0.09 1.93 1.78 4 1.76 1.56 0.13 0.19 5 0.92 0.10 0.05 0.52 6 0.58 0.18 0.43 0.09 7 0.98 1.32 0.40 0.32 a 0.91a 0.66a 0.58a 0.53a Mean Number of fungicide sprays per summer 1 5 7 4 8 2 4 8 4 7 3 5 7 6 9 4 6 8 6 8 5 7 8 4 7 6 7 9 5 8 7 8 10 7 8 6.00b 8.14a 5.14b 7.86a Meana
a Values are average of observations made in seven orchards and values with the same letter are not significantly different according to a pair-wise (INC vs STD) Tukey’s mean separation test at a 0.05 confidence level.
study, there were no significant (P ¼ 0.437) differences between the percent leaves scabbed in the two management schemes, with an average of 0.27 and 0.22% in the orchard sections managed based on scab incidence and those managed with the growers’ standard practices, respectively. At harvest, the mean percent fruit scabbed for all sites was 0.91%, 0.58%, 1.14% and 1.80% in 2005, 2006, 2007 and 2008, respectively, in the orchard sections managed using the action threshold, and 0.66%, 0.53%, 1.92% and 1.95% in 2005, 2006, 2007 and 2008, respectively, in the orchard sections managed using the growers’ standard practices. For the Four years of the study, there were no significant (P ¼ 0.636) differences between the percent fruit scabbed in the two management schemes, with 1.11% and 1.27% in the orchard sections managed using the action threshold and those managed with the growers’ standard practices, respectively (Table 2). At one site, however, the percent fruit scabbed at harvest was higher in the section managed using the action threshold than in the section managed using the grower’s standard practices. The reverse situation was observed at three orchards, where the percent fruit scabbed at harvest was higher in the sections managed using the growers’ standard practices. Regardless of the summer fungicide spray program, the inoculum levels remain constant throughout the four years of the study (Table 3). 4. Discussion During the last half century, several studies were conducted on various aspects of apple scab, including host resistance, hostepathogen relationships, epidemiology and pathogen ecology (MacHardy, 1996). Despite the large amount of knowledge about
39
Table 3 Number of scabbed leaves per 100 shoots in orchards managed using the growers’ standard practices (STD) and based on monitoring leaf scab incidence (INC). Orcharda
2004
2005
Leaf scab incidence (INC) 7 1.00 9b 2.00 7 4 3.00 5 2 4.00 11 8 5.00 9 5 6.00 7 5 7.00 11 7 c 8a 5a Mean Growers’ standard practices (STD) 1.00 10 4 2.00 6 4 3.00 4 2 4.00 9 4 5.00 8 7 6.00 6 4 7.00 8 5 7a 4a Meanc
2006
2007
2008
8 6 15 4 5 5 6 7a
4 4 4 10 8 7 7 6a
4 12 5 15 5 6 11 8a
6 5 4 9 5 4 4 5a
4 8 4 15 7 5 12 8a
4 5 5 3 4 15 13 7a
a
The orchards are described in Table 1. The inoculums level was estimated based on the number of scabbed leaves on 100 shoots (10 shoots on 10 trees). Orchard sections with 5, 6e17 or >17 scabbed leaves per 100 shoots, are classified as low, moderate or high inoculums orchards respectively (MacHardy et al., 1999; Reardon et al., 2005). c Within the same column, values with the same letter were not significantly different according to Tukey’s mean separation test at a 0.05 confidence level. b
apple scab, it is an unforgiving disease that can result in significant crop losses if left unmanaged. Epidemiological research studies on apple scab have focused predominantly on the analysis of primary infections (MacHardy, 1996). Much less attention has been devoted to secondary infections, mainly because they are often assumed to depend largely on primary infections (Holb et al., 2003, 2005). In the absence of management tools for summer scab, however, riskaverse growers may apply fungicides routinely, potentially resulting in an additional 8 to 10 sprays, whereas growers who are convinced that they have achieved adequate primary infection control may not spray enough during the summer, potentially resulting in yield losses (MacHardy, 1996; Holb et al., 2003, 2005). Achieving adequate control of secondary scab infections is crucial to keep yield losses below the economic threshold of 2% fruit scabbed at harvest (van der Scheer, 1992; MacHardy, 1996; Holb et al., 2003; Turechek and Wilcox, 2005). Furthermore, the potential ascospore dose which is an estimation of the amount of primary inoculum available in the spring is proportional to the foliar scab present in the fall (MacHardy, 1994). Environmental considerations are also becoming increasingly important; making it less acceptable to apply fungicide when it is not needed. Recently, Holb (2008) reported results of a trial on apple scab management schemes based various combination of sanitations practices and dates for initiation of the fixed-interval spray programs and dates for the last fungicide sprays (mid-July, mid-August and mid-September). The experiment was conducted in a commercial organic orchard and hence only fungicides recommended by Hungarian organic production guidelines were used (copper sulphate and elementary sulphur) (Holb, 2008). On the moderately scab-susceptible cultivar Jonathan, regardless of the sanitation practices and initiation of the fungicide spray program, final leaf and fruit scab incidence was significantly higher when sprays were ended in mid-July than when last sprays were applied mid-August or mid-September (Holb, 2008). From these results, it was concluded that one to three fungicide sprays may be omitted at the end of the growing season if sanitation practices aiming at reducing scab primary inoculum are used (Holb, 2008). Similar results were reported by van der Scheer (1992) and Holb et al. (2003) for experiments
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O. Carisse, T. Jobin / Crop Protection 35 (2012) 36e40
conducted with scab-susceptible cultivars in well managed non organic orchards. The results of these studies clearly showed that fungicide applications to control summer scab cannot be omitted without knowledge of amount of disease present in the orchard. Once the primary infection period is finished (i.e. the ascospore supply is depleted), growers must decide whether to delay or loosen up their summer fungicide spray program on the basis of their perception of whether they have achieved adequate control of primary scab infections and, sometimes, on the basis of an assessment of the incidence of foliar scab. To estimate the foliar scab incidence, Carisse et al. (2009) developed a sequential classification procedure which was evaluated in this study as an aid to manage summer scab during four seasons at seven commercial orchards. In general, applying fungicides only when the orchard foliar scab incidence was above the threshold of 0.005 scabbed leaves per shoot allowed growers to maintain fruit scab incidence below the economic threshold of 2% fruit scabbed with fewer fungicide sprays compared to the sections managed with the growers’ standard practices. Out of the 28 evaluations, however, this management strategy failed on three occasions to maintain fruit scab below the economic threshold of 2% fruit scabbed. A similar result was obtained in the orchard sections managed using the growers’ standard practices, where there were six cases of control failure. Nevertheless, fruit scab at harvest did not exceed 5% in any year or at any site, regardless of the management strategy used. Similarly, inoculum level estimated from foliar scab incidence in the autumn was generally low, meaning that the orchards were considered low or moderate inoculum orchards for the following year (MacHardy, 1996; Reardon et al., 2005). Using action thresholds to manage pests or diseases is not a new concept and is in fact the basis of many integrated pest management programs. The primary purpose of this study was to evaluate, under commercial apple production conditions, a management strategy for summer scab based on an action threshold for fungicide sprays of 0.5% leaves scabbed. A threshold of 7% scab incidence on developing fruit was recently proposed (Turechek and Wilcox, 2005). That strategy was based on mid-season sampling, but implies that there is a way to considerably reduce further fruit infections or stop already established infections. In practice, however, growers mostly apply protectant fungicides because of economical considerations and because leaves and fruits are becoming less susceptible as the season progresses (Tomerlin and Jones, 1983; Schwabe et al., 1984). Timing fungicide sprays based on disease incidence or potential ascospore dose (Reardon et al., 2005) imply that the growers or scouts monitor the orchard during the summer months. The SSCP used in this study was developed to facilitate monitoring by using scab incidence rather than severity, eliminating the need to count the number of lesions per leaves, and by minimizing the number of samples required to make a decision (Carisse et al., 2009). In this study, most orchards were sampled at least twice an others up to five times. Overall, an average of 80 shoots needed to be sampled to classify the orchards which took around 45 min. Managing apple scab is complex, and both primary and secondary infection components are integral parts of the disease management process. These components cannot be disconnected, even though they are caused by different types of spores. The results of this study show that using an action threshold of 0.5% scabbed leaves per shoot and a sequential sampling for classification procedure makes it possible to reduce the number of fungicide sprays by about 30% while keeping both fruit and foliar scab below the economic thresholds. Because the same fungicide was used in all orchard sections regardless of the summer scab management schedule, the reduction in the number of fungicide sprays could be
attributed to the elimination of unnecessarily sprays. This additional tool should help apple growers make informed decisions about fungicide spraying to manage summer scab. Acknowledgements The authors are grateful to Daniel Rolland, Annie Lefebvre, and Catherine Meloche for their assistance in orchard sampling. This work was financially supported by Agriculture and Agri-Food Canada. References Berkett, L.P., MacHardy, W.E., Sutton, D.K., Neff, G., Bradshaw, T., 1997. “Whole farm” apple scab IPM project. Proc. 103th Annu. Meet. Mass. Fruit Grow. Assoc. Inc. 103, 132e137. Brun, L., Guinaudeau, J., Gros, C.H., Parisi, L., Simon, S., 2010. Assessment of fungicide protection strategies in experimental apple orchards. IOBC/WPRS Bull. 54, 103e107. Carisse, O., Jobin, T., Bourgeois, G., 2008. Predicting apple leaf emergence from degree-day accumulation during the primary scab period. Can. J. Plant Sci. 88, 229e238. Carisse, O., Meloche, C., Boivin, G., Jobin, T., 2009. Action thresholds for summer fungicide sprays and sequential classification of apple scab incidence. Plant Dis. 93, 490e498. Gadoury, D.M., MacHardy, W.E., 1982. A model to estimate the maturity of ascospores of Venturia inaequalis. Phytopathology 72, 901e904. Gadoury, D.M., MacHardy, W.E., 1986. Forecasting ascospore dose of Venturia inaequalis in commercial apple orchards. Phytopathology 76, 112e118. Gadoury, D.M., MacHardy, W.E., Rosenberger, D.A., 1989. Integration of pesticide application schedules for disease and insect control in apple orchards of the northeastern United States. Plant Dis. 73, 98e105. Gadoury, D.M., Seem, R.C., Stensvand, A., 1995. New developments in forecasting the risk of apple scab. N. Y. Fruit Q. 2, 5e8. Hartman, J.R., Parisi, L., Bautrais, P., 1999. Effect of leaf wetness duration, temperature, and conidial inoculum dose on apple scab infections. Plant Dis. 83, 531e534. Holb, I.J., 2008. Timing of first and final sprays against apple scab combined. Crop Prot. 27, 814e822. Holb, I.J., Heijne, B., Jeger, M.J., 2003. Summer epidemics of apple scab: the relationship between measurements and their implications for the development of predictive models and threshold levels under different disease control regimes. J. Phytopathol 151, 335e343. Holb, I.J., Heijne, B., Withagen, J.C.M., Gáll, J.M., Jeger, M.J., 2005. Analysis of summer epidemic progress of apple scab at different apple production systems in the Netherlands and Hungary. Phytopathology 95, 1001e1020. Koehler, G.W., 1998. New England Apple Pest Management Guide. Cooperative Extension Service, Universities of Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island, and Vermont. Koehler, G.W., 2005. Apple Pest Reports. Apple Integrated Pest Management Program. Cooperative Extension, The University of Maine. January 2006. http:// pmo.umext.maine.edu/apple/AppPestReport.html. MacHardy, W.E., 1994. A “PAD” action threshold: the key to integrating practices for managing apple scab. In: Butt, D.J. (Ed.), Integrated Control of Pome Fruit Diseases. Norw. J. Agric. Sci. Suppl. 17, 75e82. MacHardy, W.E., 1996. Apple Scab: Biology, Epidemiology, and Management. The American Phytopathological Society, St. Paul, MN. MacHardy, W.E., Gadoury, D.M., 1989. A revision of Mills’s criteria for predicting apple scab infection periods. Phytopathology 79, 304e310. MacHardy, W.E., Berkett, L.P., Neefus, C.D., Gotlieb, A.R., Sutton, D.K., 1999. An autumn foliar scab sequential sampling technique to predict the level of “scabrisk” next spring. Phytopathology 89, S47. Mills, W.D., 1944. Efficient use of sulfur dusts and sprays during rain to control apple scab. Cornell Extension Bull. 630, 1e4. Reardon, J.E., Berkett, L.P., Garcia, M.E., Gotlieb, A., 2005. Field evaluation of a new sequential sampling technique for determining apple scab “risk”. Plant Dis. 89, 228e236. Schwabe, W.F.S., Jones, A.L., Jonker, J.P., 1984. Changes in the susceptibility of developing apple fruit to Venturia inaequalis. Phytopathology 74, 118e121. Stensvand, A., Gadoury, D.M., Amundsen, T., Semb, L., Seem, R.C., 1997. Ascospore release and infection of apple leaves by conidia and ascospores of Venturia inaequalis at low temperatures. Phytopathology 87, 1046e1053. Tomerlin, J.R., Jones, A.L., 1983. Development of apple scab on fruit in the orchard and during cold storage. Plant Dis. 67, 147e150. Turechek, W.W., Wilcox, W.F., 2005. Evaluating predictors of apple scab with receiver operating characteristic curve analysis. Phytopathology 95, 679e691. van der Scheer, H.A.Th., 1992. Management of scab and powdery mildew on apple with emphasis on threshold values for control of both diseases. Acta Phytopathol. Entomol. Hung 27, 621e630.
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Managing summer apple scab epidemics using leaf scab incidence threshold values for fungicide sprays Odile Carisse*, Tristan Jobin Horticultural Research and Development Center, Agriculture and Agri-Food Canada, 430 Gouin Blvd., Saint-Jean-sur-Richelieu, Qc J3B 3E6, Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 June 2011 Received in revised form 25 November 2011 Accepted 12 December 2011
In several apple production areas, most fungicide sprays applied to orchards target apple scab management. Recently, action thresholds were established to aid in decision making as to whether fungicide sprays are required to manage summer scab. To facilitate grower adoption of these thresholds, a sequential sampling for classification procedure (SSCP) was developed. The SSCP with an action threshold of 0.5% foliar scab was evaluated from 2005 to 2008 in seven commercial orchards as a tool to time fungicide sprays. At each site, at the end of the primary infection period, the orchard was divided into two sections: in one, summer scab was managed using the grower’s standard practices and in the other, summer scab was managed based on scan incidence. Starting in mid-June, each orchard section managed based on scab incidence was classified using the SSCP as below or above the action threshold or as in the “no decision” zone. When an orchard section was classified as above the action threshold, a fixed-interval spray program was initiated and generally continued until harvest. When an orchard section was classified as below the action threshold, initiation of the fungicide spray program was postponed until the threshold was reached. When an orchard was in the “no decision” zone, sampling was repeated one or two weeks later depending on the frequency of rain events. Similar incidences of foliar scab of 0.27% and 0.22% and of fruit scab of 1.11% and 1.27% were observed in orchard sections managed based on scab incidence and standard grower’s practices, respectively. However, fruit scab at harvest was above the economic threshold of 2.0% at three and six occasions in the sections managed using foliar scab incidence and using the growers’ standard practices, respectively. For the four years of the study, an average of 6.4 and 9.1 fungicide sprays were used to control summer scab in the orchard sections managed based on scab incidence and in those managed according to the growers’ standard practices, respectively. Considering that the same fungicide was used in all orchards, we concluded that the reduced number of fungicide sprays (30%) resulted from the elimination of sprays when scab incidence was below the action threshold of 0.5% leaves scabbed. Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.
Keywords: Apple diseases Action threshold Disease management Integrated disease management
1. Introduction In several apple-growing areas throughout the world, apple scab caused by the fungus Venturia inaequalis (Cke.) Wint. is the key disease for scheduling fungicide sprays (MacHardy, 1996). In eastern North America, the disease is mostly controlled by the application of fungicides (Koehler, 1998, 2005; Reardon et al., 2005). Most scab management schemes are divided into two programs, with the first one aiming at controlling the monocyclic phase of the disease, i.e. the primary scab infections caused by ascospores, and the second one aiming at controlling the polycyclic phase of the disease, i.e. the secondary infections caused by conidia. * Corresponding author. Tel.: þ1 450 515 2023; fax: þ1 450 346 7740. E-mail address: [email protected] (O. Carisse).
Considering that the secondary inoculum is produced on lesions resulting from the primary infections, it is crucial to achieve effective primary scab control to avoid epidemic build-up caused by secondary infections and to reduce the need for fungicide applications during the summer months. To achieve this goal, fungicide applications generally begin in the spring at the green-tip phenological stage and continue throughout spring into early summer, resulting in some 8 to 10 applications (Reardon et al., 2005). If this approach fails to control scab adequately, then fungicides must be applied during the summer, and the annual number of fungicide sprays could rise to 14 to 20, depending on the prevailing weather conditions during the summer months (MacHardy, 1996; Reardon et al., 2005). Several tools have been developed to help manage primary infections and include procedures to estimate potential ascospore
0261-2194/$ e see front matter Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.cropro.2011.12.014
O. Carisse, T. Jobin / Crop Protection 35 (2012) 36e40
dose (PAD) (Gadoury and MacHardy, 1986; MacHardy, 1996; Berkett et al., 1997), weather-based models to estimate ascospore maturity (Gadoury and MacHardy, 1982), and weather-based models to estimate periods of infection risks (Mills, 1944; MacHardy and Gadoury, 1989; Gadoury et al., 1995; Stensvand et al., 1997) and leaf growth (Carisse et al., 2008). Although infection models developed to estimate risk of primary infections have been adapted for secondary infections (Hartman et al., 1999), there are relatively few tools available to manage secondary infections (Holb, 2008). Most summer scab management schemes aimed at controlling fruit scab and maintaining leaf scab at an acceptable level to avoid excessive risk of fruit infections and of production of overwintering structures (pseudothecia) during the autumn and hence reduce the amount of primary inoculum the following spring. During the summer, the frequency of fungicide sprays and the expected level of control depend on the weather, cultivar susceptibility, and the marketing goal, i.e. mainly whether the fruits will be stored, processed, or sold for the fresh market. Nevertheless, it is generally admitted that the incidence of fruit scab at harvest should not exceed 2% (Schwabe et al., 1984; MacHardy, 1996; Holb et al., 2003; Reardon et al., 2005). To achieve this goal, various strategies have been proposed (Gadoury et al., 1989; MacHardy, 1996; Berkett et al., 1997; Koehler, 1998, 2005; Brun et al., 2010). These strategies are based on fixed-interval spraying programs, delayed fixed-interval spraying programs, or early-ended fixedinterval spraying programs. In some strategies, the intervals between sprays are determined by the frequency of rain (Berkett et al., 1997; Koehler, 1998, 2005). Nevertheless, most fungicide programs do not consider the level of leaf scab present in the orchard. Regardless of the strategy employed for summer scab management, the key element in making informed management decisions is the level of disease present in the orchard. If the level of scab is low and the forecasted weather is hot and dry, fungicide applications may not be needed. However, if the level of scab is high and the forecasted weather is warm and wet, fungicide sprays are required. Establishing a unique action threshold for applying fungicide sprays to manage summer scab is complex because the threshold is influenced by several factors, including tree size and density, orchard site characteristics, orchard operations, cultivar susceptibility, marketing goal and, not the least, the weather during the summer months. Therefore, a conservative threshold of 0.5% leaves scabbed was proposed as an action threshold (Koehler, 1998, 2005; Carisse et al., 2009). Because this threshold is low, it is expected that a large number of leaves will need to be assessed to determine if the incidence of leaf scab is above or below this threshold. Carisse et al. (2009) recently developed a sequential sampling for classification procedure (SSCP) to classify orchard scab incidence as above or below the action threshold of 0.5% foliar scab or as in the “no decision” zone. This SSCP had not yet been evaluated in the field. Therefore, the objective of this research was to compare summer scab management based on scab incidence monitored using this SSCP with grower’s standard practices in terms of foliar and fruits scab at harvest and of numbers of fungicide applications. 2. Materials and methods 2.1. Data collection The experiment was conducted from 2005 to 2008 in seven commercial orchards located in the Missisquoi-Estrie and Montérégie apple-growing regions, for a total of 28 evaluations (seven orchards four years). These orchards were planted with the cultivars ‘McIntosh’, ‘Cortland’ and ‘Lobo’ grafted on various rootstocks (Table 1). At the end of the primary infection period, at
37
Table 1 Description of the seven apple orchards used to compare programs for management of summer scab by Venturia inaequalis in 2005e2008. Site
Varieties
Orchard
County
1 2 3 4 5 6 7
Frelighsburg Frelighsburg Dunham Dunham Rougemont Stanbridge Station St-Paul
McIntosh McIntosh McIntosh McIntosh McIntosh, Lobo McIntosh McIntosh, Lobo
Rootstock
M.7 M.7 MM.106 MM.106 M.7 MM.106 M.7
Spacing (m) Between rows
Within rows
6.2 6.1 4.9 4.8 6.0 4.9 6.0
4.1 3.9 3.1 3.0 4.0 3.2 4.0
each site, an orchard section of 0.3e0.5 ha was selected for the experiment and split into two sections: in one, summer scab was managed following the grower’s standard practices which consisted in applying a fungicide prior to a forecast rain event or at fixed intervals, and in the other section, summer scab was managed based on a foliar scab action threshold of 0.005 scabbed leaves per shoot and classification of leaf scab incidence as above or below the threshold using the SSCP developed by Carisse et al. (2009). At all sites, primary scab infections were managed based on the growers standard practices in both sections, hence the differences in foliar and fruits scab at harvest could be attributed to difference in summer scab management. Weather data were obtained from the Environment Canada weather station located within seven Km from the commercial orchards.
2.2. Classification of the orchard sections as above or below the action threshold The first orchard classification was conducted in mid-June. At each sampling, a minimum of 50 shoots, both cluster and terminal shoots, which were representative of the orchard (at the bottom, in the centre and at the top of the trees) were examined. The number of scabbed leaves on each shoot was counted. Based on the total number of scabbed leaves, the orchard sections were classified, using the previously described SSCP proposed by Carisse et al. (2009), as above or below the action threshold of 0.005 scabbed leaves per shoot, or as in the “no decision” zone (Fig. 1). When an orchard section was classified in the “no decision” zone an additional 10 shoots were examined, until a decision could be made or a maximum of 240 shoots were examined. The following criteria were used to manage summer scab: (i) If the orchard section was classified as above the action threshold, a 7- to 14-day summer spray program was initiated and continued until harvest unless a prolonged dry period was forecasted, in which case the orchard section was reassessed. Unless that reassessment classified the orchard section as below the threshold, the fixed-interval summer spray program was maintained. (ii) If the orchard section was classified as below the action threshold, fungicide spray applications were postponed until the threshold was reached. In these situations, the orchards section was monitored at every 7e14 days depending on frequency of rainy days. If the action threshold of 0.5% foliar scab was not reached by mid-August, a less conservative threshold of 1.0% foliar scab was used from then until harvest (Carisse et al., 2009). (iii) If the orchard section was classified as in the “no decision” zone, sampling was repeated 7e14 days later depending on the frequency of rainy days. With the SSCP, a minimum of 50 and a maximum of 240 shoots per orchard section were sampled (Fig. 1), and care was taken to select shoots that were representative of the trees (i.e. shoots at the top, bottom, edges, and centre of the canopy). The time and the
O. Carisse, T. Jobin / Crop Protection 35 (2012) 36e40
A
25
Cumulative number of scabbed leaves (TN)
38
20
3. Results Pth = 0.005
5 Below threshold of 0.005 scabbed leaves per shoot
0 0
25
50
75
100
125
150
175
200
225
250
275
300
20
10
Continue sampling
Rain (mm)
Above threshold of 0.005 scabbed leaves per shoot
30
75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0
Rain (mm)
40
Pth = 0.01
75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0
Rain (mm)
50
75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0
Rain (mm)
B Cumulative number of scabbed leaves (TN)
Number of samples (shoot)
75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0
Below threshold of 0.005 scabbed leaves per shoot
0 0
25
50
75
100
125
150
175
200
225
250
Number of samples (shoot) Fig. 1. Sequential sampling for classification plans for classifying scab incidence as above or below action thresholds of 0.005 (A) and 0.01 (B) scabbed leaves per shoot (adapted from Carisse et al., 2009).
number of shoot samples required to reach a decision were noted. At all sites, summer scab was managed with the fungicide Captan (Maestro 80-DF, Aryst Life Sciences, North America, North Carolina, U.S.A) at a rate of 2.00e3.75 kg/ha. A few days before harvest, leaf scab incidence was estimated from a sample of 250 shoots randomly selected within each orchard section, and fruit scab incidence was estimated from 25 fruits on 10 trees selected at random within each section. In the autumns of 2004e2008, each orchard section was assessed to determine the inoculums level using the sequential sampling technique proposed by MacHardy et al. (1999). In each orchard section, 10 extension shoots on 10 trees for a total of 100 shoots were sampled and the cumulative number of scabbed leaves noted. The orchard sections were classified as low, moderate or high inoculums orchards, if the total number of scabbed leaves was 5, 6e17 or >17 per 100 shoots, respectively. 2.3. Data analysis Analysis of variance was used to test the effect of the spray programs on the number of fungicide sprays, on leaf and fruit scab incidence at harvest, and on inoculums level. Within each year, each orchard was considered as a repetition and significance between treatment means was detected using Tukey’s mean separation test at a 0.05 confidence. Statistical analysis was conducted using the SAS software program (version 9.1, SAS Institute, Inc., Cary, NC).
2005
20 18 16 14 12 10 8 6 4 2 0
Rain intensity Rain duration Threshold for summer scab infection
20 18 16 14 12 10 8 6 4 2 0
2006
2007
2008
18/06
02/07
16/07
30/07
13/08
27/08
Rain duration (hours)
Continue sampling
Rain duration (hours)
10
20 18 16 14 12 10 8 6 4 2 0
Rain duration (hours)
15
The four years of the study were characterized by 25, 30, 41, and 35 days with rain (>1 mm), respectively (Fig. 2). The number of fungicide sprays to manage summer scab varied from season to season, with, on average, 6.0, 5.1, 7.5 and 6.4 sprays in 2005, 2006, 2007 and 2008, respectively, in the orchard sections managed using the action threshold, and 8.1, 7.8, 10.3 and 9.1 sprays in 2005, 2006, 2007, and 2008, respectively, in the orchard sections managed using the growers’ standard practices (Table 2). Overall, significantly (P < 0.0001) fewer fungicide sprays were used to manage summer scab in the orchard sections where an action threshold of 0.005 scabbed leaves per shoot was used (average of 6.4 sprays) than in the sections where the growers’ standard practices were used (average of 9.1 sprays). At harvest, the incidence of leaf scab was 0.22, 0.24, 0.26 and 0.36% in 2005, 2006, 2007 and 2008, respectively, in the orchard sections managed based on scab incidence, and 0.15%, 0.16%, 0.31% and 0.28% in 2005, 2006, 2007 and 2008, respectively, in the orchard sections managed using the growers’ standard practices (Table 2). For the four years of the
20 18 16 14 12 10 8 6 4 2 0
Rain duration (hours)
Above threshold of 0.005 scabbed leaves per shoot
10/09
Days Fig. 2. Quantity (in mm) and duration (h) of rain event during the summers in 2005e2008 recorded at the Environment Canada weather station located at the Agriculture and AgriFood Canada experimental farm located in Frelighsburg, Quebec, Canada. The threshold for conidial infection was calculated from the relationship between temperature, leaf wetness duration and conidial infection (Hartman et al., 1999).
O. Carisse, T. Jobin / Crop Protection 35 (2012) 36e40 Table 2 Incidence of leaf scab at harvest, incidence of fruit scab at harvest, and number of summer fungicide sprays used to manage summer apple scab, in orchards managed using the growers’ standard practices (STD) and based on monitoring leaf scab incidence (INC). Orchards
Year 2005 Program
Year 2006 Program
Year 2007 Program
Year 2008 Program
INC
INC
INC
STD
INC
STD
0.18 0.21 0.17 0.41 0.30 0.28 0.29 0.26a
0.16 0.33 0.16 0.58 0.27 0.20 0.50 0.31a
0.21 0.58 0.22 0.59 0.21 0.26 0.45 0.36a
0.16 0.21 0.19 0.12 0.15 0.58 0.53 0.28a
0.27 1.12 0.13 2.97 0.50 1.11 1.89 1.14a
0.21 2.92 0.11 3.48 0.43 1.32 4.95 1.92a
0.73 4.87 0.36 1.92 0.79 0.97 2.98 1.80a
0.56 2.31 0.58 1.79 1.34 2.57 4.53 1.95a
8 9 7 8 7 7 7 7.57b
11 9 12 9 11 9 11 10.29a
6 7 9 7 6 8 6 7.00b
8 10 12 10 12 11 10 10.43a
STD
STD
Percent leaf scabbed at harvest 1 0.28 0.15 0.26 0.20 2 0.16 0.14 0.20 0.17 3 0.08 0.07 0.52 0.13 4 0.31 0.17 0.13 0.12 5 0.21 0.14 0.16 0.16 6 0.21 0.16 0.19 0.16 7 0.28 0.21 0.20 0.17 a 0.22a 0.15a 0.24a 0.16a Mean Percent fruit scabbed at harvest 1 1.74 1.19 0.75 0.45 2 0.25 0.21 0.40 0.36 3 0.13 0.09 1.93 1.78 4 1.76 1.56 0.13 0.19 5 0.92 0.10 0.05 0.52 6 0.58 0.18 0.43 0.09 7 0.98 1.32 0.40 0.32 a 0.91a 0.66a 0.58a 0.53a Mean Number of fungicide sprays per summer 1 5 7 4 8 2 4 8 4 7 3 5 7 6 9 4 6 8 6 8 5 7 8 4 7 6 7 9 5 8 7 8 10 7 8 6.00b 8.14a 5.14b 7.86a Meana
a Values are average of observations made in seven orchards and values with the same letter are not significantly different according to a pair-wise (INC vs STD) Tukey’s mean separation test at a 0.05 confidence level.
study, there were no significant (P ¼ 0.437) differences between the percent leaves scabbed in the two management schemes, with an average of 0.27 and 0.22% in the orchard sections managed based on scab incidence and those managed with the growers’ standard practices, respectively. At harvest, the mean percent fruit scabbed for all sites was 0.91%, 0.58%, 1.14% and 1.80% in 2005, 2006, 2007 and 2008, respectively, in the orchard sections managed using the action threshold, and 0.66%, 0.53%, 1.92% and 1.95% in 2005, 2006, 2007 and 2008, respectively, in the orchard sections managed using the growers’ standard practices. For the Four years of the study, there were no significant (P ¼ 0.636) differences between the percent fruit scabbed in the two management schemes, with 1.11% and 1.27% in the orchard sections managed using the action threshold and those managed with the growers’ standard practices, respectively (Table 2). At one site, however, the percent fruit scabbed at harvest was higher in the section managed using the action threshold than in the section managed using the grower’s standard practices. The reverse situation was observed at three orchards, where the percent fruit scabbed at harvest was higher in the sections managed using the growers’ standard practices. Regardless of the summer fungicide spray program, the inoculum levels remain constant throughout the four years of the study (Table 3). 4. Discussion During the last half century, several studies were conducted on various aspects of apple scab, including host resistance, hostepathogen relationships, epidemiology and pathogen ecology (MacHardy, 1996). Despite the large amount of knowledge about
39
Table 3 Number of scabbed leaves per 100 shoots in orchards managed using the growers’ standard practices (STD) and based on monitoring leaf scab incidence (INC). Orcharda
2004
2005
Leaf scab incidence (INC) 7 1.00 9b 2.00 7 4 3.00 5 2 4.00 11 8 5.00 9 5 6.00 7 5 7.00 11 7 c 8a 5a Mean Growers’ standard practices (STD) 1.00 10 4 2.00 6 4 3.00 4 2 4.00 9 4 5.00 8 7 6.00 6 4 7.00 8 5 7a 4a Meanc
2006
2007
2008
8 6 15 4 5 5 6 7a
4 4 4 10 8 7 7 6a
4 12 5 15 5 6 11 8a
6 5 4 9 5 4 4 5a
4 8 4 15 7 5 12 8a
4 5 5 3 4 15 13 7a
a
The orchards are described in Table 1. The inoculums level was estimated based on the number of scabbed leaves on 100 shoots (10 shoots on 10 trees). Orchard sections with 5, 6e17 or >17 scabbed leaves per 100 shoots, are classified as low, moderate or high inoculums orchards respectively (MacHardy et al., 1999; Reardon et al., 2005). c Within the same column, values with the same letter were not significantly different according to Tukey’s mean separation test at a 0.05 confidence level. b
apple scab, it is an unforgiving disease that can result in significant crop losses if left unmanaged. Epidemiological research studies on apple scab have focused predominantly on the analysis of primary infections (MacHardy, 1996). Much less attention has been devoted to secondary infections, mainly because they are often assumed to depend largely on primary infections (Holb et al., 2003, 2005). In the absence of management tools for summer scab, however, riskaverse growers may apply fungicides routinely, potentially resulting in an additional 8 to 10 sprays, whereas growers who are convinced that they have achieved adequate primary infection control may not spray enough during the summer, potentially resulting in yield losses (MacHardy, 1996; Holb et al., 2003, 2005). Achieving adequate control of secondary scab infections is crucial to keep yield losses below the economic threshold of 2% fruit scabbed at harvest (van der Scheer, 1992; MacHardy, 1996; Holb et al., 2003; Turechek and Wilcox, 2005). Furthermore, the potential ascospore dose which is an estimation of the amount of primary inoculum available in the spring is proportional to the foliar scab present in the fall (MacHardy, 1994). Environmental considerations are also becoming increasingly important; making it less acceptable to apply fungicide when it is not needed. Recently, Holb (2008) reported results of a trial on apple scab management schemes based various combination of sanitations practices and dates for initiation of the fixed-interval spray programs and dates for the last fungicide sprays (mid-July, mid-August and mid-September). The experiment was conducted in a commercial organic orchard and hence only fungicides recommended by Hungarian organic production guidelines were used (copper sulphate and elementary sulphur) (Holb, 2008). On the moderately scab-susceptible cultivar Jonathan, regardless of the sanitation practices and initiation of the fungicide spray program, final leaf and fruit scab incidence was significantly higher when sprays were ended in mid-July than when last sprays were applied mid-August or mid-September (Holb, 2008). From these results, it was concluded that one to three fungicide sprays may be omitted at the end of the growing season if sanitation practices aiming at reducing scab primary inoculum are used (Holb, 2008). Similar results were reported by van der Scheer (1992) and Holb et al. (2003) for experiments
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conducted with scab-susceptible cultivars in well managed non organic orchards. The results of these studies clearly showed that fungicide applications to control summer scab cannot be omitted without knowledge of amount of disease present in the orchard. Once the primary infection period is finished (i.e. the ascospore supply is depleted), growers must decide whether to delay or loosen up their summer fungicide spray program on the basis of their perception of whether they have achieved adequate control of primary scab infections and, sometimes, on the basis of an assessment of the incidence of foliar scab. To estimate the foliar scab incidence, Carisse et al. (2009) developed a sequential classification procedure which was evaluated in this study as an aid to manage summer scab during four seasons at seven commercial orchards. In general, applying fungicides only when the orchard foliar scab incidence was above the threshold of 0.005 scabbed leaves per shoot allowed growers to maintain fruit scab incidence below the economic threshold of 2% fruit scabbed with fewer fungicide sprays compared to the sections managed with the growers’ standard practices. Out of the 28 evaluations, however, this management strategy failed on three occasions to maintain fruit scab below the economic threshold of 2% fruit scabbed. A similar result was obtained in the orchard sections managed using the growers’ standard practices, where there were six cases of control failure. Nevertheless, fruit scab at harvest did not exceed 5% in any year or at any site, regardless of the management strategy used. Similarly, inoculum level estimated from foliar scab incidence in the autumn was generally low, meaning that the orchards were considered low or moderate inoculum orchards for the following year (MacHardy, 1996; Reardon et al., 2005). Using action thresholds to manage pests or diseases is not a new concept and is in fact the basis of many integrated pest management programs. The primary purpose of this study was to evaluate, under commercial apple production conditions, a management strategy for summer scab based on an action threshold for fungicide sprays of 0.5% leaves scabbed. A threshold of 7% scab incidence on developing fruit was recently proposed (Turechek and Wilcox, 2005). That strategy was based on mid-season sampling, but implies that there is a way to considerably reduce further fruit infections or stop already established infections. In practice, however, growers mostly apply protectant fungicides because of economical considerations and because leaves and fruits are becoming less susceptible as the season progresses (Tomerlin and Jones, 1983; Schwabe et al., 1984). Timing fungicide sprays based on disease incidence or potential ascospore dose (Reardon et al., 2005) imply that the growers or scouts monitor the orchard during the summer months. The SSCP used in this study was developed to facilitate monitoring by using scab incidence rather than severity, eliminating the need to count the number of lesions per leaves, and by minimizing the number of samples required to make a decision (Carisse et al., 2009). In this study, most orchards were sampled at least twice an others up to five times. Overall, an average of 80 shoots needed to be sampled to classify the orchards which took around 45 min. Managing apple scab is complex, and both primary and secondary infection components are integral parts of the disease management process. These components cannot be disconnected, even though they are caused by different types of spores. The results of this study show that using an action threshold of 0.5% scabbed leaves per shoot and a sequential sampling for classification procedure makes it possible to reduce the number of fungicide sprays by about 30% while keeping both fruit and foliar scab below the economic thresholds. Because the same fungicide was used in all orchard sections regardless of the summer scab management schedule, the reduction in the number of fungicide sprays could be
attributed to the elimination of unnecessarily sprays. This additional tool should help apple growers make informed decisions about fungicide spraying to manage summer scab. Acknowledgements The authors are grateful to Daniel Rolland, Annie Lefebvre, and Catherine Meloche for their assistance in orchard sampling. This work was financially supported by Agriculture and Agri-Food Canada. References Berkett, L.P., MacHardy, W.E., Sutton, D.K., Neff, G., Bradshaw, T., 1997. “Whole farm” apple scab IPM project. Proc. 103th Annu. Meet. Mass. Fruit Grow. Assoc. Inc. 103, 132e137. Brun, L., Guinaudeau, J., Gros, C.H., Parisi, L., Simon, S., 2010. Assessment of fungicide protection strategies in experimental apple orchards. IOBC/WPRS Bull. 54, 103e107. Carisse, O., Jobin, T., Bourgeois, G., 2008. Predicting apple leaf emergence from degree-day accumulation during the primary scab period. Can. J. Plant Sci. 88, 229e238. Carisse, O., Meloche, C., Boivin, G., Jobin, T., 2009. Action thresholds for summer fungicide sprays and sequential classification of apple scab incidence. Plant Dis. 93, 490e498. Gadoury, D.M., MacHardy, W.E., 1982. A model to estimate the maturity of ascospores of Venturia inaequalis. Phytopathology 72, 901e904. Gadoury, D.M., MacHardy, W.E., 1986. Forecasting ascospore dose of Venturia inaequalis in commercial apple orchards. Phytopathology 76, 112e118. Gadoury, D.M., MacHardy, W.E., Rosenberger, D.A., 1989. Integration of pesticide application schedules for disease and insect control in apple orchards of the northeastern United States. Plant Dis. 73, 98e105. Gadoury, D.M., Seem, R.C., Stensvand, A., 1995. New developments in forecasting the risk of apple scab. N. Y. Fruit Q. 2, 5e8. Hartman, J.R., Parisi, L., Bautrais, P., 1999. Effect of leaf wetness duration, temperature, and conidial inoculum dose on apple scab infections. Plant Dis. 83, 531e534. Holb, I.J., 2008. Timing of first and final sprays against apple scab combined. Crop Prot. 27, 814e822. Holb, I.J., Heijne, B., Jeger, M.J., 2003. Summer epidemics of apple scab: the relationship between measurements and their implications for the development of predictive models and threshold levels under different disease control regimes. J. Phytopathol 151, 335e343. Holb, I.J., Heijne, B., Withagen, J.C.M., Gáll, J.M., Jeger, M.J., 2005. Analysis of summer epidemic progress of apple scab at different apple production systems in the Netherlands and Hungary. Phytopathology 95, 1001e1020. Koehler, G.W., 1998. New England Apple Pest Management Guide. Cooperative Extension Service, Universities of Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island, and Vermont. Koehler, G.W., 2005. Apple Pest Reports. Apple Integrated Pest Management Program. Cooperative Extension, The University of Maine. January 2006. http:// pmo.umext.maine.edu/apple/AppPestReport.html. MacHardy, W.E., 1994. A “PAD” action threshold: the key to integrating practices for managing apple scab. In: Butt, D.J. (Ed.), Integrated Control of Pome Fruit Diseases. Norw. J. Agric. Sci. Suppl. 17, 75e82. MacHardy, W.E., 1996. Apple Scab: Biology, Epidemiology, and Management. The American Phytopathological Society, St. Paul, MN. MacHardy, W.E., Gadoury, D.M., 1989. A revision of Mills’s criteria for predicting apple scab infection periods. Phytopathology 79, 304e310. MacHardy, W.E., Berkett, L.P., Neefus, C.D., Gotlieb, A.R., Sutton, D.K., 1999. An autumn foliar scab sequential sampling technique to predict the level of “scabrisk” next spring. Phytopathology 89, S47. Mills, W.D., 1944. Efficient use of sulfur dusts and sprays during rain to control apple scab. Cornell Extension Bull. 630, 1e4. Reardon, J.E., Berkett, L.P., Garcia, M.E., Gotlieb, A., 2005. Field evaluation of a new sequential sampling technique for determining apple scab “risk”. Plant Dis. 89, 228e236. Schwabe, W.F.S., Jones, A.L., Jonker, J.P., 1984. Changes in the susceptibility of developing apple fruit to Venturia inaequalis. Phytopathology 74, 118e121. Stensvand, A., Gadoury, D.M., Amundsen, T., Semb, L., Seem, R.C., 1997. Ascospore release and infection of apple leaves by conidia and ascospores of Venturia inaequalis at low temperatures. Phytopathology 87, 1046e1053. Tomerlin, J.R., Jones, A.L., 1983. Development of apple scab on fruit in the orchard and during cold storage. Plant Dis. 67, 147e150. Turechek, W.W., Wilcox, W.F., 2005. Evaluating predictors of apple scab with receiver operating characteristic curve analysis. Phytopathology 95, 679e691. van der Scheer, H.A.Th., 1992. Management of scab and powdery mildew on apple with emphasis on threshold values for control of both diseases. Acta Phytopathol. Entomol. Hung 27, 621e630.