Evaluation of a forecaster for downy mildew of onion (Allium cepa L.)

C R O P P R O T E C T I O N (1987) 6 (2), 95-103

Evaluation of a forecaster for downy m i l d e w of onion (Allium cepa L.) G. D. JESPERSONAND J. C. SUTTON

Department of Environmental Biology, University of Guelph, Guelph, Ontario, Canada, N1 G 2WI ABSTRACT. A forecaster of onion downy mildew, referred to as DOWNCAST, was evaluated under field conditions to determine its validity and effectiveness for timing fungicide sprays. DOWNCAST utilized criteria for predicting sporulation and infection by the downy mildew pathogen, Peronospora destructor. The criteria were based on quantitative and temporal relationships of temperature, rain, high humidity, rate of dew deposition and dew duration with the infection cycle of the pathogen. DOWNCAST correctly predicted sporulation incidence on 111 of 119 nights during two growing seasons. Refinement of a temperature criterion was needed to predict sporulation correctly on the remaining eight nights. Indirect evidence showed that DOWNCAST correctly predicted infection incidence. Sequences of weather conditions were used to predict 'sporulation-infection periods', when the pathogen sporulated, survived and infected the onions, and to identify infection cycles. Fungicide programmes of Ridomil-MZ, Sandofan M8, Bravosan, Vinicur M-SC, Manzate 200 or Bravo 500, begun on the sixth or eighth day of the first infection cycle, did not usually reduce sporulation incidence early in the second infection cycle. However, all of the programmes except Bravosan and Bravo effectively restored disease management a few days later. Programmes of Ridomil-MZ begun in the second infection cycle usually were less effective than those begun in the first cycle. Relative effectiveness of the various fungicide programmes was reflected in increased yield and size of the onions. It was concluded that fungicide programmes should start at about the time of the first sporulation-infection period.

Introduction Epidemics of onion downy mildew, caused by Peronospora destructor (Berk.) Casp., often are explosive and difficult to manage. Practices for managing downy mildew include destruction of initial inoculum sources, such as cull and volunteer onions (Allium cepa L.), and fungicide sprays to reduce epidemic rates (Viranyi, 1981; Anonymous, 1983). Fungicides registered for this purpose in Canada include several protectants (maneb, mancozeb, zineb, anilazine and captafol) but none with systemic activity. Routine applications of the protectants have sometimes failed to manage the disease adequately and substantial losses have been incurred. Destructive epidemics developed sporadically in major onion-producing areas near Bradford, Cookstown and Thedford in Ontario during 1979-85. Critical timing of ihngicide sprays in relation to disease progress may facilitate dependable management of downy mildew. Several workers have concluded that spraying should begin before or shortly after disease first appears (Wooliams, 1957;

Kir'Ianova, 1979; Gladders and Pye, 1984), but both criteria assume an ability to predict or recognize initial disease. Prediction of downy mildew may be possible using weather variables monitored in the crop. Important relationships of weather variables and the infection cycle of P. destructor have been quantified in several studies (Yarwood, 1943; Hildebrand and Sutton, 1982, 1984a,b,c; Bashi and Aylor, 1983; Hildebrand, 1983; Sutton and Hildebrand, 1985). The infection cycle is characterized by long latent periods (about 9-16 days) and short periods (about 1-2 days) when the pathogen sporulates, is dispersed and infects the leaves (Hildebrand and Sutton, 1982). Stepwise increases in disease may be recognized when the pathogen sporulates on the green leaves, or when the leaves die a few days later. Dispersed spores survive on the host leaves for 1-3 days (Hildebrand and Sutton, 1984a). P. destructor may destroy the onion foliage almost completely during the course of four infection cycles (Hildebrand and Sutton, 1982). The relationships of weather and P. destructor were used to develop criteria for predicting sporulation and

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Forecasterfor onion downy mildew

infection by the pathogen (Hildebrand, 1983; Hildebrand and Sutton, 1984a,b,c; Sutton and Hildebrand, 1985). This quantitative information may facilitate prediction of periods when the pathogen successfully sporulates and infects the leaves, and may help to identify infection cycles during epidemics. In the study reported here, we tested the effectiveness of the predictive criteria and evaluated them for timing fungicide applications. The fungicides included the protectants mancozeb and chlorothalonil formulated with or without the systemic fungicides metalaxyl, cyprofuram or oxadixyl*. Effectiveness of metalaxyl against onion downy mildew has been demonstrated (Kir'Ianova, 1979; Schroeder and Ormrod, 1979; Schwinn, 1981). Cyprofuram and oxadixyl are known to be effective against various other downy mildews (Bruin and Edgington, 1983; Cohen and Samoucha, 1984). Materials and methods

Prediction of sporulation-infection periods Criteria used to predict sporulation and infection were modified from those proposed by Hildebrand (1983) and embodied observations of Hildebrand and Sutton (1982, 1984a, b,c) and Sutton and Hildebrand (1985). A sporulation-infection period was predicted when conditions were conducive to sporulation, spore dispersal, spore survival and infection. Conditions favouring dispersal were assumed to occur each day (Leach, Hildebrand and Sutton, 1982). The predictive scheme is referred to as DOWNCAST, an acronym for downy mildew forecaster. Sporulation of P. destructor was predicted at night when the following conditions were satisfied: (1) the mean hourly temperature between 0800 and 2000h EST of the preceding day was ~<24°C; (2) the mean hourly temperature at night was between 4°C and 24°C; (3) no rain occurred after 0100h, and (4) the relative humidity (r.h.) was >/95% at or before 0200 h and persisted without interruption until 0600h. To determine the effectiveness of the predictive criteria, the onions were examined each morning for sporulation of the pathogen. Infection by P. destructor was predicted to follow sporulation during the same wet period only when wetness persisted until 0900h or later at 6-22°C or until 1000h at 23-26°C (Hildebrand and Sutton, 1982, 1984a). Infection was predicted on the night succeeding that ofsporulation when dew deposition in the first 5h of leaf wetness was rapid and wetness lasted at least 3h at 6-22°C. No infection was predicted and the spores were assumed to have been * Chemical names of fungicides: chlorothalonil, tetrachloroisophthalonitrile; cyprofuram (-+)-a-[N-(3-chlorophenyl)cyclopropanecarboxamido]y-butyrolactone; mancozeb, manganese ethylenebis(dithiocarbamate) (polymeric) complex with zinc salt; metalaxyl, methyl N-(2-methoxyacetyl)-N-(2,6-xylyl)-DL-alaninate; oxadixyl, 2-methoxy-N-(2-oxo-1,3oxazolidin-3-yl)acet-2',6'-xylidide. C R O P P R O T E C T I O N Vol. 6 April 1987

killed when dew was deposited slowly (Hildebrand and Sutton, 1984a). When little or no dew formed, the spores were assumed to have survived (Hildebrand and Sutton, 1984a), and the infection criteria were applied to the second night after sporulation. When the spores were predicted to survive but not to infect on the second night, the criteria were applied to the third night. Spores produced on a given night were assumed to infect during only one infection period. Thus, when infection was predicted for the first night, it was not predicted for the second or third nights, regardless of weather conditions. For predicting sporulation and infection, weather variables were monitored continuously in plots of test onions using calibrated instruments. Air temperature and r.h. were measured with a hygrothermograph (Lambrecht, model 252, G&tingen, Federal Republic of Germany) located in a Stevenson screen at ground level. Rainfall was measured with a tipping bucket gauge (model 6010, Weathertronics Inc., Sacramento, CA 95841, USA). Leaf surface wetness was monitored at 0"2 m above ground level with a De Wit recorder (De Wit, Hengelo, The Netherlands). The area below the ink trace (ABT) of the De Wit recorder was used to estimate rate of dew deposition. The recorder was the same instrument equipped with the same string sensor used in a previous study in which the relationships of ABT values to infection and spore survival were determined (Hildebrand and Sutton, 1984a).

Field plots, 1983 Plots of onion cv. Canada Maple were established on sandy loam soil at the research station near Cambridge, Ontario. Untreated seeds were sown on 13 May in rows spaced 41cm apart at a density of about 50 seeds/m of row, using a precision seeder (Mini-Nibex, Verken AR, Markaryd, Sweden). An overhead sprinkler irrigation system was used to maintain soil moisture during dry periods. Weeds were removed by hand, and thrips were managed with Orthene 75 SP (acephate) applied to the foliage at a dose of 590 g in 7001 water/ha on 13 July. Fertilizer treatments followed local recommendations (Anonymous, 1983). To provide initial inoculum, two onion plants grown in a growth cabinet from sets inoculated with P. destructor (Hildebrand and Sutton, 1980) were placed in the centre of each plot. This inoculum source was maintained for the period of 28 June to 25 August by replacing the source plants after sporulation (every 3-12 days) with fresh source plants. Fungicide products timed according to DOWNCAST were (1) Ridomil-MZ 72W (8% metalaxyl plus 64% mancozeb) applied at 2- 5 kg/ha; (2) Sandofan M8 64WP (SAN 518, 8% oxadixyl plus 56% mancozeb) applied at 2.5 kg/ha; (3) Vinicur M-SC (4.64% cyprofuram plus 32.5% mancozeb) applied at 4.3 1/ha; (4) Manzate 200 80WP (80% mancozeb) applied at 3.0kg/ha, and (5) Bravo 500 (50% chlorothalonil) applied at 2.4 1/ha. The initial spray of each product

97

G . D . JESPERSON AND J. C. SUTTON

was applied during the latent periods of the first or second infection cycles of P. destructor on plants in the plots. Subsequent sprays were applied at about 7- or 14-day intervals. The fungicides were applied at 200kPa using an air-pressurized boom sprayer, equipped with four hollow-cone TeeJet T X Z drop nozzles (TeeJet Spraying Systems Co., Wheaton, ILL 60187, USA) which directed fungicide laterally to each side of two onion rows. The treatments were applied to the onion plots, each 6" 0 m × 3" 3 m, and arranged in a randomized complete block design with four replicates. Downy mildew was assessed by counting the number of plants with sporulation of P. destructor within four quadrats in each plot. Each quadrat comprised 1 m of row.

Field plots, 1984 Onion cv. Canada Maple was sown on 3 May in Burford-loam soil at the research station near Arkell, Ontario using the same procedure as in 1983. Dacthal 75WP (75% chlorthal dimethyl) was applied (27 kg in 600 1 water/ha) immediately after sowing, to control weeds. Fertilizer and insecticides were applied as in 1983. The onions were watered during dry periods using a trickle irrigation system (Twinwall, Chapin Watermatics Inc., Watertown, NY 13601). For initial inoculum, one onion near the centre of each plot was inoculated with P. destructor on 3 July. The inoculated plants were covered with a plastic bag overnight to maintain high r.h. and to encourage infection. Fungicides timed by the D O W N C A S T forecaster included Bravosan 80WP (9% oxadixyl plus 71%

chlorothalonil), applied at 2.25kg/ha, and all products used in 1983. Each fungicide was applied initially during the latent period of the first infection cycle on the non-inoculated plants in the plots. Ridomil-MZ programmes also were initiated during the latent period of the second cycle. Subsequent sprays were applied at about 7- or 14-day intervals. Fungicides were applied, using the same methods as in 1983, in plots ( 3 . 7 m x 3 . 8 m ) arranged in a randomized complete block design with four replications. Incidence of onion plants with sporulation of P. destructor was assessed on 27 and 29 July and 1, 7, 13 and 23 August, when 96, 96, 96, 25, 72 and 40 plants were assessed, respectively, in each plot. The percentage of green leaves bearing sporulation was estimated on 1, 13 and 23 August. The sampling pattern used in disease estimations was an X-shape across the eight rows nearest the centre of each plot. On 20, 21, and 29 August, each plot was assessed visually for dead and discoloured leaves using the Horsfall-Barratt scale (discussed in Horsfall and Cowling, 1978). For yield assessments, 200 onion bulbs were harvested from near the centre of each plot on 20 September, air dried for 7 days and graded into the following categories of bulb diameter: 4 . 4 - 7 . 6 c m (large); 3 . 2 - 4 - 4 c m (small); < 3 . 2 c m (culls). The number and weight of onions in each category were determined.

Statistical analyses Disease data of 1983 and 1984 were transformed to arcsin values for analysis of variance and means separation tests (Little and Hills, 1978). Assessments

TABLE 1. Frequency of prediction of no sporulation, sporulation, sporulation without infection, and sporulation with infection during various monitoring periods in 1983 and 1984 Monitoring periods Year

Number of days on which there was:

Dates

No sporulation because: > 24 ° C *

Raint

Sporulation§

Sporulation without infection

Sporulation with infection

Short humid period4:

1983

29 June to 11 July 12 July to 21 July 22 July to 31 July 1 August to 10 August 11 August to 20 August 21 August to 31 August 1 September to 11 September

3 10 6 5 3 5 5

0 0 2 2 1 2 0

6 6 2 1 2 1 3

6 0 3 4 5 5 5

6 0 2 0 4 1 3

0 0 1 4 1 4 2

Total

29 June to 11 September

37

7

21

28

16

12

1984

12 July to 21 22 July to 31 1 August to 11 August to 21 August to

4 3 8 6 2

0 0 0 0 1

2 2 1 2 1

5 7 2 2 8

2 4 0 2 4

3 3 2 0 4

Total

12 July to 31 August

23

1

8

24

12

12

July July 10 August 20 August 31 August

* Mean hourly temperature between 0800 and 2000h EST of the preceding day was >24°C. t Rain occurred between 0100 and 0600h. 4: Humid period began after 0200h or was interrupted between 0200 and 0600h. Observed directly except during 17-31 August 1984.

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Forecasterfor onion downy mildew

on the Horsfall-Barratt scale in 1984 were converted to percentages, then transformed to arcsin values. In 1984, data for percentages of onions of various sizes, but not of bulb weights, also were transformed. Individual treatment means were separated using the Waller-Duncan Bayesian k-ratio t test. In this test, a kratio of 100:1 was used, a value considered to be roughly equivalent to P - - 0 . 0 5 (Steel and Torrie, 1980). Means calculated from transformed data were back-transformed to the original units for presentation as results.

• P R E D I C T E D INFECTION

I SPORULATION ON SOURCE PLANTS SPORULATION ON PLOT PLANTS

tt m I

IttI t ll

It

t ~ • 1984 I 30

I 5

FIGURE 1.

Direct observations of sporulation confirmed that the weather criteria correctly predicted sporulation incidence except on two nights. High daytime temperatures resulted in negative predictions of sporulation on 37 nights and rain or short humid periods were inhibitory to sporulation on 7 and 21 nights, respectively (Table 1). Sporulation occurred on 3 and 19 August but was not predicted because the mean hourly temperatures of the respective preceding days were 2 4 . 6 ° C and 24"4°C, and thus exceeded the 24°C criterion. P. destructor sporulated on a total of 28 nights. Infection was predicted after only 12 nights when the pathogen sporulated (Table 1). Three infection periods were predicted during persistent wetness immediately after the pathogen sporulated and nine were predicted during the three nights succeeding that of sporulation. Rain fell on two nights of infection. According to D O W N C A S T , Slow dew deposition killed the spores and sporelings and prevented infection after 12 sporulation periods. Dew periods were too short for infection after four other sporulation periods. The first of the twelve sporulation-infection periods was predicted on 31 July, or 33 days after inoculum source plants were placed in the plots. Subsequent sporulation-infection periods ended on 2, 4, 5, 10 and 15 August and on later dates (Figure 1). The first infection cycle was completed on 16 August when P.

I I0

vv

TIIII|I I I vv

I i I 15 20 25 JULY

I 30

1

|II||H

•

vv

I 4

l!

~

I I I 9 14 19 AUGUST

•

I 24

I 29

I I 3 8 SEPT

I n c i d e n c e o f s p o r u l a t i o n (S) a n d p r e d i c t e d infection (I) by

destructor sporulated for the first time on the noninoculated plants. However, weather conditions were unfavourable for infection by these spores, and the second cycle evidently did not begin until the next sporulation-infection period was completed on 21 August. The pathogen sporulated on 31 August and on 1, 2 and 3 September but did not begin the third infection cycle until conditions favoured infection on the night of 3 September. The initial spray of each fungicide programme timed to begin early in the first predicted infection cycle was applied on 5 August, 6 days after the first sporulation-infection period and immediately after the fourth sporulation-infection period (Table 2). The first spray of the Ridomil-MZ programme, which was started late in the first infection cycle, was applied on 13 August after two more sporulation-infection periods had been predicted. Progress of downy mildew in the check plots was slow (Table 2). Sporulation incidence on the onion plants was only 4% on 23 August, but increased stepwise to 27% on 1 September. However, only 6% of the leaves showed sporulation on 1 September. Incidence of sporulation did not increase thereafter. The various fungicide treatments did not significantly reduce sporulation incidence estimated on 23 August, but all except Vinicur M-SC and Bravo 500 reduced sporulation of 8 September (Table 2). The

TABLE 2. E f f e c t s o f v a r i o u s f u n g i c i d e s p r a y p r o g r a m m e s i n i t i a t e d d u r i n g t h e first or s e c o n d i n f e c t i o n cycles o f P. i n c i d e n c e o f d o w n y m i l d e w in o n i o n s in 1983

destructor(5 a n d 13 A u g u s t respectively) o n

Incidence of plants w i t h s p o r u l a t i o n (%)*

Dates of application 5 Aug. . + + + + +

13 A u g . .

. + + +

23 A u g . .

30 A u g .

7 Sept.

+ + +

+ + + + +

23 A u g .

. + + + + +

* Values are weighted means obtained by backtransforming from arcsin values. t Means in a column followed by the same letter are not significantly different according to the Waller--Duncan Bayesian k-ratio t test (k-ratio= 100:1).

CROP PROTECTION Vol. 6 April 1987

1111 11

•w

Peronospora destructor on o n i o n s in field plots in 1983 a n d 1984.

FieM plots in 1983

Check Ridomil-MZ Ridomil-MZ Sandofan M8 M a n z a t e 200 Vinicur M-SC B r a v o 500

li •

1983

Results

Fungicide

•

v v w

4 3 1 1 2 5 3

at a a a a a a

8 Sept. 16 0 0 0 0 2 2

a b b b b ab ab

G.D.

JESPERSON

A~D J. C. SUTTOY

99

of the first infection cycle. However, incidence of plants with sporulation was low (< 1%). Downy mildew progressed explosively in check plots (Table 3). Estimated incidence of plants with sporulation increased from 0"8% on 27 July to 86% two days later and to 99°70 after three more days. Most leaves were killed by 23 August and no longer supported sporulation of P. destructor (Table 4). Foliage in the plots was almost totally discoloured by 29 August. The initial spray of each fungicide programme timed to begin during the first latent period was applied on 20 July. This spray was applied 8, 5 and 2 days after the first, second and third sporulationinfection periods, respectively, and 6 days before completion of the first infection cycle. None of the programmes started on 20 July significantly suppressed incidence ofsporulation (whole-plant basis) on 27 July or 1 August, and only Ridomil-MZ did so on 29 July (Table 3). Ridomil-MZ also was the only fungicide which suppressed sporulation incidence on the leaves on 1 August (Table 4). All of the programmes initiated on 20 July except Bravo 500 reduced incidence of sporulation on the plants on 7 August, which was 12 days after the second infection cycle began, and followed second or third fungicide sprays (Table 3). Most leaves with sporulation on I August were dead and without sporulation on 7 August. On 13 August, sporulation incidence on the plants was generally high; it was significantly lower than in the checks only in onions treated with Manzate 200, or with Ridomil-MZ (Table 3). In terms of leaves with sporulation, downy mildew intensity on 13 August was reduced significantly by Manzate 200, Ridomil-MZ and Sandofan M8 (Table 4). Sporulation incidence estimated on a plant basis on 23 August was low only in onions treated with Ridomil-MZ (Table 3) but Sandofan, Manzate 200 and Vinicur M-SC as well as Ridomil M Z reduced incidence on the leaves (Table 4). Most of the fungicides suppressed foliar discolouration estimated on 10 and 21 August (Table 4).

estimates of 23 August indicated that one spray of Ridomil-MZ (early or late in the first infection cycle), Sandofan M8 or Vinicur M-SC, or two sprays of Manzate 200 or Bravo 500 applied during the latent period of the first infection cycle, had not significantly reduced disease progress. However, Ridomil MZ, Sandofan M8 and Manzate 200 ultimately managed downy mildew in such a way that no sporulation of P. destructor was observed on 8 September. Field plots in 1984

The weather criteria of D O W N C A S T correctly predicted sporulation incidence on 38 of the 43 nights in the period of 4 July to 16 August, after which fresh sporulation was not readily confirmed because disease was severe. Sporulation was predicted on 10 nights but was observed on 15 of the 43 nights. On five of the 15 nights when P. destructor sporulated, D O W N C A S T predicted no sporulation because the mean hourly temperature of each preceding day exceeded the 24°C criterion, and ranged from 2 4 . 2 ° C to 26"3°C. Negative predictions for 28 of the 43 nights were confirmed by direct observation. In the period of 12 July to 31 August, most negative predictions of sporulation resulted from high daytime temperatures or short humid periods at night (Table 1). Infection was predicted after each of 12 nights of sporulation (Table 1). Two infection periods were predicted for the morning ofsporulation in prolonged dew and ten were predicted for wet periods during one of the three nights immediately after sporulation. Slow dew deposition resulted in negative predictions of infection after nine sporulation periods. Sporulation-infection periods were predicted during 12-18 July, 26-28 July, 2-7 August, 23-25 August and 1 September (Figure 1). Sporulation on inoculum source plants was light during the first sporulationinfection period (12 July), but heavy during the second and third sporulation-infection periods (16 and 18 July). P. destructor sporulated for the first time on the non-inoculated plot plants on 26 July, marking the end

TABLE 3. Effects~fvari~usfung~c~depr~grammes~initiatedduringthe~rstinfecti~ncyc~e(2~Ju~y)~r~ec~nd~nfecti~ncyc~e(3~Ju~y~r2August)~fP. destructor o n i n c i d e n c e o f o n i o n s w i t h s i g n s o f d o w n y m i l d e w i n 1984 Fungicide

I n c i d e n c e o f p l a n t s w i t h s p o r u l a t i o n (%)*

Product

A p p l i c a t i o n dates July 20

Check Ridomil-/~4Z Ridomil-MZ Ridomil-MZ Sandofan M8 M a n z a t e 200 Bravosan B r a v o 500 Vinicur M-SC

. + + + + + +

30 .

+ + -

29 J u l y

1 August

7 August

13 A u g u s t

23 A u g u s t

August

26 .

27 J u l y

2 .

+ -

. + + + + + + +

9

16

+ + + + -

+ + + + + + +

.

0.8 0.0 0.0 0.5 0.3 0" 1 0.1 0" 3 0" 0

at a a a a a a a a

86 26 77 85 46 58 52 62 46

a b a a ab ab ab ab ab

99 86 95 98 93 93 92 93 93

a a a a a a a a a

86 a 0 d 5 cd 1d 11 c d 11 c d 33 bc 62 ab 7 cd

100 27 62 96 88 71 99 100 77

a d cd abc abc bcd ab a abc

0 7 58 23 52 78 94 95 63

d:l: c b c b ab a a b

* , t As Table 2. 4: All plants dead. CROP

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Vol. 6 April 1987

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Forecaster for onion downy mildew

TABLE 4. Effects o f various fungicide p r o g r a m m e s , initiated d u r i n g the first infection cycle (20 July) or second infection cycle (30 J u l y or 2 August) o f P. destructor on incidence o f o n i o n leaves w i t h s p o r u l a t i o n o f the p a t h o g e n , a n d on foliar discolouration o f the onions in 1984 Fungicide

Incidence o f leaves w i t h s p o r u l a t i o n (%)*

Product

A p p l i c a t i o n dates July

Check Ridomil-MZ Ridomil-MZ Ridomil-MZ Sandofan M8 M a n z a t e 200 Bravosan Bravo 500 Vinicur M - S C

1 August

26

30

2

9

16

. + + + + + +

.

. + -

. + + + + + + +

+ + + + -

+ + + + + + +

+

23 A u g u s t

10 A u g u s t

21 A u g u s t

29 A u g u s t

August

20

+

13 A u g u s t

L e a f area discoloured (%)*

42 25 36 44 31 35 31 35 33

at b ab a ab ab ab ab ab

61 5 15 36 32 20 53 59 42

a e de bc bcd cd ab a ab

0 4 19 7 18 27 47 72 23

f4 e cd de cd bc b a c

44 9 11 11 9 16 20 20 11

a c bc bc c bc b b bc

91 25 34 42 33 40 75 55 45

a c c bc c bc ab bc bc

98 89 90 91 91 92 96 94 92

a d cd bcd bcd bcd ab abc bcd

*,t As Table 2. # Most leaves dead.

The first spray of the 14-day Ridomil-MZ programme initiated in the second infection cycle was applied on 30 July, which was four days after the second infection cycle began. This application did not suppress sporulation on 1 August, but markedly reduced incidence of sporulation on 7 August (Table 3). Sporulation incidence also was suppressed on 13 and 23 August after the second Ridomil application of 9 August (Tables 3 and 4). The two sprays of Ridomil-MZ in this programme managed downy mildew as effectively as three sprays applied weekly, beginning on 2 August, after six sporulation-infection periods. All fungicide programmes promoted onion yields and increased the proportion of onions categorized as large, except Bravo, which did not increase yield (Table 5). Ridomil-MZ scheduled at 14-day intervals promoted yield and frequency of large onions more effectively when initiated in the first infection cycle than when first applied in the second cycle. However, weekly applications of Ridomil-MZ begun in the

TABLE 5. Effects o f the various fungicide p r o g r a m m e s on the weight a n d size d i s t r i b u t i o n o f o n i o n bulbs h a r v e s t e d in 1984 P e r c e n t a g e o f onions in the following size categories*: Fungicide R i d o m i l - M Z (1)t Sandofan M8 Vinicur M-SC R i d o m i l - M Z (2)t M a n z a t e 200 R i d o m i l - M Z (3)t Bravosan B r a v o 500 Check

W e i g h t o f 200 onions (kg) 14.4 13.8 13.6 13.1 13.0 12.4 11 • 9 10.8 9.0

at ab ab ab ab bc bc cd d

Large

Small

Cull

70 68 65 63 59 56 53 44 27

25 27 27 28 34 32 39 43 53

5 5 8 8 7 11 8 13 19

a ab abc abc abc bcd cd d e

d d d cd bcd bcd bc ab a

c c bc bc bc b bc ab a

* The percentage values are weighted means obtained by backtransforrning arcsin values. t Ridomil-MZ programmes: 1, 14-dayintervals, begun on 20 July; 2, 7-day intervals, begun on 2 August; 3, 14-day intervals, begun on 30 July. :t: Values in a column followed by the same letter do not differ according to the Waller-Duncan Bayesian k-ratio t test (k-ratio = 100:1). CROP

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Vol. 6 A p r i l 1 9 8 7

second cycle were as effective as the 14-day programme begun in the first cycle. Sandofan M8, Vinicur M-SC, and Manzate 200 programmes, but not the Bravosan or Bravo 500 programmes, were as effective as Ridomil M Z in promoting yield and increasing the proportion of large onions when initially applied in the first infection cycle. Oxadixyl in combination with mancozeb (Sandofan M8) promoted yields better than when formulated with Bravo (Bravosan). Daily temperature values and sporulation

Because P. destructor sometimes sporulated when the mean temperature of the preceding day exceeded the D O W N C A S T criterion of 24°C, the weather data were used to re-examine relationships of temperature and sporulation. Temperature patterns for days (0800-2000h) averaging > 2 4 ° C were studied in relation to the success or failure ofsporulation on each succeeding night (12 nights with sporulation, 15 nights without sporulation). Data collected at the Arkell site in 1979 and 1980 (P. D. Hildebrand, personal communication) were included. Temperature peaks were lower, or were high for fewer hours, when sporulation occurred on the following night than when it did not occur. Temperature means and ranges for days followed or not followed by sporulation were, respectively, 25-1°C (24.4-26.4°C) and 26"0°C (24.5-26-5°C). P. destructor sporulated after daily temperatures exceeded 27, 28 and 29°C for the following number of hours, respectively, or less: 5, 3 and 2h; 6, 4 and 2h; and 7, 1 and Oh. The pathogen did not sporulate after the respective temperatures were exceeded for the following number of hours or more: 5, 0, and 0 h; 6, 1 and 0 h; 6, 4 and 0 h; and 7, 3 and 1 h. No sporulation was observed when temperatures exceeded 27°C for > 8 h , 28°C for > 4 h or 29°C for >2h. Discussion

The D O W N C A S T criteria correctly predicted sporulation incidence of P. destructor on 111 of 119

G. D. JESPERSONANDJ. C. SUTTON nights during the two field studies. On eight nights, sporulation occurred but was not predicted because the mean temperature of each preceding day exceeded the DOWNCAST criterion of K24°C. Mean daytime temperatures in the range of 24" 4 to 26.4°C failed to inhibit sporulation on these nights but were inhibitory on 15 other nights. The analyses of high daytime temperatures in relation to sporulation indicated that temperature patterns during the day were the basis of this paradox. Temperatures averaging 24.4-26.4°C at 0800-2000h were inhibitory only when high peaks occurred or when broad peaks of moderately high temperatures were recorded. The observations that P. destructor did not sporulate when temperatures of the preceding day were >27°C for >8h, >28°C for > 4 h or >29°C for > 2 h may be used to refine the temperature criterion of DOWNCAST. The temporal relationships of the predicted sporulation-infection periods and subsequent stepwise increases in sporulation incidence indicated that the DOWNCAST criteria for predicting infection probably functioned correctly. In 1983, for example, P. destructor first sporulated on the plot plants 16, 14, 12 and 11 days, respectively, after the first, second, third and fourth sporulation-infection periods were predicted. These observations were consistent with typical latent periods (9-16 days) of the pathogen (Hildebrand and Sutton, 1982, 1984b). Similarly, in 1984, sporulation was first observed in the plots 14 and 10 days after the first and second sporulation-infection periods were predicted, respectively. In both seasons, further stepwise increases in sporulation occurred about 9-16 days after sporulation-infection periods were predicted. The use of sporulation incidence for estimating downy mildew may be deceptive unless interpreted within the context of infection cycles and the continuum of disease progress. The estimates of sporulation incidence accounted only for those plants or leaves that exhibited a proportion of tissues which were infectious. They did not consider diseased plants or leaves that were preinfectious (in which the pathogen was entirely latent), or postinfectious (in which the pathogen was no longer able to sporulate). The brevity of the infection periods relative to the latent or postinfectious stages, and the abrupt changes from the preinfectious to the infectious stage and subsequently to the postinfectious stage, amplified apparent anomalies in the data. In check plots in 1984, for example, incidence of plants with sporulation abruptly increased from 0.8% on 27 July to 86% on 29 July and 99% on 1 August, but declined to 86% on 7 August (Table 3). Precipitous declines in sporulation incidence were observed between 1 and 7 August in the fungicide treatments, such as 86% to 0% in the Ridomil-MZ programme begun on 20 July. Downy mildew was more explosive in 1984 than in 1983. The epidemic of 1984 began in mid-July, about 2 weeks earlier than in 1983. In the check plots of 1984, downy mildew destroyed most of the onion foliage

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during three successive periods of overlapping infection cycles beginning on 12-18 July and ending on 23-25 August. In 1983, infection cycles of 31 July to 16 August and 21-31 August resulted in maximum disease incidence of only 27% on the plants and 6% on the leaves. Discontinuities in the chain of infection cycles were evident between 16 and 21 August and 31 August and 3 September. The greater explosiveness of downy mildew in 1984 than in 1983 probably was related to the time of occurrence and severity of sporulation-infection periods but not to the total number of these periods, which was the same in both seasons. Eight sporulationinfection periods predicted before 10 August in 1984, in contrast to five in 1983, undoubtedly favoured the early and rapid disease increase. Sporulation-infection periods in 1983 occurred during only two infection cycles, resulting in the asymptote in disease progress during the third cycle. In 1984, however, sporulationinfection periods were distributed over three cycles and contributed to the continued upthrust of disease until most onion foliage was destroyed. Weather conditions probably restricted severity of sporulationinfection periods to a greater extent in 1983 than in 1984 by limiting sporulation density, spore survival or infection ratios (sensu Butt and Royle, 1980). The frequent high temperatures and low humidities in 1983 may have restricted sporulation densities indirectly by limiting colonization of the leaves by P. destructor (Hildebrand and Sutton, 1984b; Sutton and Hildebrand, 1985). Death rates of detached spores were expected to be much higher, and germination rates much lower, in the high temperatures of 1983 than in the moderate temperatures of 1984 (Bashi and Aylor, 1983; Hildebrand and Sutton, 1984a,b). Quantification of the severity of sporulation-infection periods is a possible refinement for DOWNCAST. Fungicide programmes started on the sixth day of the first infection cycle (5 August) in 1983 did not significantly reduce sporulation incidence when estimated on day 2 of the second infection cycle (23 August). At the time of the disease assessment, the onions had received one application of products containing systemic fungicides or two applications of products containing only protectants. Subsequent applications ofRidomil-MZ, Sandofan M8 or Manzate 200 effectively restored disease control. The RidomilMZ programme initiated near the end of the first infection cycle (13 August) managed disease as effectively as the programme started on 5 August. Most of the fungicide programmes started during the first infection cycle (20 July) in 1984 also did not significantly reduce incidence of downy mildew estimated early in the second infection cycle (27 and 29 July, and 1 August). Only the Ridomil-MZ programme maintained good disease management in the second cycle and until late August. The Sandofan M8, Manzate 200 and Vinicur M-SC programmes achieved control of mildew by various times in the period 7-23 August; however, these plots also became severely CROP PROTECTION Vol. 6 April 1987

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Forecaster for onion downy mildew

diseased by late August. The Bravosan and Bravo 500 programmes reduced disease incidence only marginally. The Ridomil-MZ programme started in the second infection cycle (30 July) and the intensified (weekly) Ridomil-MZ programme initiated on 2 August managed disease adequately, but usually not as effectively as the Ridomil-MZ programme started on 20 July. Loss of disease control in late August was probably related to intense interplot interference by inoculum of P. destructor. Effectiveness of the various fungicides in managing downy mildew was reflected in increased yield and size of the onions. The frequent failure of the fungicides to suppress sporulation of P. destructor appeared to be related to spray timing in relation to the infection cycle of the pathogen. Failure of the systemic fungicides applied in the first cycle to reduce sporulation incidence in the second cycle indicated that these applications did not eradicate the pathogen effectively. However, Jesperson (1985) showed that Ridomil (metalaxyl) prevented sporulation when applied to onion foliage during 8 or more days before, or 4 days after, infection by P. destructor, but was only partially effective or ineffective when applied about 6 or more days after infection. Accordingly, in the present study, Ridomil-MZ applied 6 or 8 days after the first sporulation-infection period was too late to prevent sporulation and killing of some foliage but was well timed to manage subsequent infection and disease progress. Similarly, the Ridomil-MZ programme started on 30 July 1984 (second infection cycle) did not suppress sporulation of 1 August, which arose from colonies initiated during 12-18 July, more than 11 days before the fungicide was applied. However this spray application markedly suppressed sporulation on 7 August, which developed from infections during 26-28 July, only 2-4 days before spray application. The duration and intensity of eradicant activity of oxadixyl (ingredient of Sandofan M8 and Bravosan) or cyprofuram (ingredient of Vinicur M-SC) against P. destructor in onion leaves have not been quantified, but the present observations and those of Jesperson (1985) in controlled environments indicate shorter activity of these fungicides than was found for metalaxyl. The D O W N C A S T tests generally support conclusions of previous studies (Wooliams, 1957; Kir'I~nova, 1979; Gladders and Pye, 1984) that spraying should be started early in epidemics. Downy mildew was managed effectively, or moderately effectively, by several fungicides when spraying began in the first infection cycle. Further delay may be possible with certain fungicides, as was shown by the restoration of disease control by Ridomil-MZ programmes initiated early in the second infection cycle. Suppression of downy mildew by the fungicide programmes would probably have been greater in growers' fields than in the small plots where interference by inoculum from the checks or other sporulating plots was substantial. Timing of the first spray immediately before the first sporulation-infection period, or shortly CROP P R O T E C T I O N Vol. 6 April 1987

thereafter when effective eradicants are used, appears to represent an ideal approach to management, but this may not be readily attainable in practice. Although the D O W N C A S T criteria effectively identified sporulation-infection periods and timed fungicide sprays, limitations remain for implementation of the forecaster on the farm. A major difficulty is determining the occurrence of initial inoculum, which in most instances remains unknown until disease is discovered in the crop. D O W N C A S T may facilitate early recognition of mildew by identifying times when scouting should be intensified. Although sporulationinfection periods normally can be identified from microclimatic data, the rate of dew deposition remains an important variable which is not easily estimated, even coarsely, because of lack of adequate instrumentation (Sutton, Gillespie and James, 1987). Development of better instrumentation and prediction of sporulation-infection periods based on forecasted as opposed to current weather, present challenges for future improvements in DOWNCAST.

Acknowledgements This research was supported financially by the Natural Sciences and Engineering Research Council of Canada, the Ontario Ministry of Agriculture and Food, and the Pesticides Advisory Committee of the Ontario Ministry of the Environment.

References ANONYMOUS (1983). Vegetable Production Recommendations. Toronto, Ontario: Ontario Ministry of Agricultureand Food. 64 pp. BASHI, E. AND AYLOR, D. E. (1983). Survival of detached sporangia of Peronospora destructor and Peronospora tabacina. Phytopathology 73, 1135-1139. BRUIN, G. C. A. AND EDGINGTON,L. V. (1983). The chemical control of diseases caused by zoosporic fungi--a many sided problem. In: Zoosporic Plant Pathogens. A Modern Perspective, pp. 193-232 (ed. by S. T. Buczacki).London: AcademicPress. BUTT, D. J. AND ROYLE, D. J. (1980). The importanceof terms and definitions for a conceptuallyunified epidemiology.In: Comparative Epidemiology. A Toolfor Better Disease Management, pp. 29-45 (ed. by J. Palti and J. Kranz). Wageningen:

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G. D. JESPERSONAND J. C. SUTTON leaves by Peronospora destructor. Canadian Journal of Plant Pathology 6, 119-126. HILDEBRAND, P. D. AND SUTTON, J. C. (1984b). Relationships of temperature, moisture and inoculum density to the infection cycle of Peronospora destructor. Canadian Journal of Plant Pathology 6, 127-134. HILDEBRAND, P. D. AND SUTTON, J. C. (1984c). Interactive effects of the dark period, humid period, temperature and light on sporulation of Peronospora destructor. Phytopathology 74, 1444-1449. HORSFALL, J. G. AND COWLING, E. B. (1978). Phytopathometry: the measurement of plant disease. In: Plant Disease: An Advanced Treatise, Vol. 1, pp. 120-135 (ed. by J. G. Horsfall and E. B. Cowling). New York: Academic Press. JESPERSON, G. (1985). Management of Onion Downy Mildew with Fungicides Timed According to Weather Variables. MSc thesis, University of Guelph, Guelph, Ontario, Canada. 154 pp. KIR'IANOVA,E. V. (1979). Chemical method for protecting onions against downy mildew. Moskovskaia Sel'Skokhoziaistvennaia A kademia imeni Timiriazeva Doklady 256, 147-150. LEACH, C. M., HILDEBRAND,P. D. AND SUTTON, J. C. (1982). Sporangium discharge by Peronospora destructor: influence of humidity, red-infrared radiation, and vibration. Phytopathology 72, 1052-1056. LITTLE, T. M. ANDHILLS,F. J. (1978). Agricultural Experimentation. Design and Analysis. Toronto: Wiley. 350 pp. SCHROEDER, J. D. AND ORMROD, D. J. (1979). Fungicidal control of downy mildew and blast of storage onions. In: Pesticide

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Research Report, p. 410. Ottawa: Expert Committee for Pesticide Use in Agriculture. SCHWINN, F. J. (1981). Chemical control of downy mildews. In: The Downy Mildews, pp. 305-320 (ed. by D. M. Spencer). London: Academic Press. STEEL, R. G. D. AND TORRIE, J. H. (1980). Principles and Procedures of Statistics. Toronto: McGraw-Hill. 633 pp. SUTTON, J. C. AND HILDEBRAND, P. D. (1985). Environmental water in relation to Peronospora destructor and related fungi. Canadian Journal of Plant Pathology 7, 323-330. SUTTON, J. C., GILLESPIE, T. J. AND JAMES, T. D. W. (1987). Electronic monitoring and use of microprocessors in the field. In: Techniques in Plant Disease Epidemiology (ed. by J. Kranz and J. Rotem). Heidelberg: Springer-Verlag (in press). VIRANYI, F. (1981). Downy mildew of onion. In: The Downy Mildews, pp. 461-472 (ed. by D. M. Spencer). New York: Academic Press. WOOLIAMS, G. E. (1957). Downy mildew of onion and its control in the British Columbia interior. Canadian Journal of Plant Science 37, 237-244. YARWOOD, C. E. (1943). Onion downy mildew, Hilgardia 14, 595-691.

Received 10 March 1986 Revised 18 July 1986 Accepted 29 September 1986

CROP PROTECTION Vol. 6 April 1987