Modelling population densities of root-lesion nematode (Pratylenchus thornei) from soil profile temperatures to choose an optimum sowing date for wheat in a subtropical region

Field Crops Research 183 (2015) 50–55

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Modelling population densities of root-lesion nematode (Pratylenchus thornei) from soil profile temperatures to choose an optimum sowing date for wheat in a subtropical region J.P. Thompson Centre for Crop Health, Institute for Agriculture and the Environment, University of Southern Queensland, Toowoomba, Qld 4350, Australia

a r t i c l e

i n f o

Article history: Received 3 May 2015 Received in revised form 3 July 2015 Accepted 3 July 2015 Available online 5 August 2015 Keywords: Pratylenchus thornei Wheat sowing date Cultivar resistance Temperature model Thermal time Soil depth

a b s t r a c t The root-lesion nematode Pratylenchus thornei is widely distributed in many wheat growing countries and is particularly damaging to wheat in subtropical environments. This study aimed to investigate by simulation the effects of soil profile temperatures after different sowing dates on reproduction of P. thornei in susceptible and moderately resistant wheat cultivars in the subtropical grain region of eastern Australia. A quadratic regression model relating P. thornei population densities to thermal time was produced from experimental data for susceptible, intolerant wheat cv. Gatcher and moderately resistant, tolerant wheat cv. GS50a, and applied to soil profile temperatures after four sowing dates from 25 April to 24 July. Simulated final population densities of P. thornei throughout the soil profile to 60 cm depth were least at 18 weeks after 25 May sowing. For Gatcher and GS50a, respectively, there were 5.7 and 3.5 times as many nematodes after 24 July sowing as after 25 May sowing, 2.0 and 1.6 after 24 June, and 1.4 and 1.3 after 25 April sowings. GS50a had 78% (May) to 86% (July) fewer nematodes in the soil profile than Gatcher. These simulations indicated that an optimum sowing date can be chosen to limit P. thornei reproduction in both susceptible and moderately resistant wheat cultivars. Gatcher had a 61% increase in measured grain yield and GS50a had an 8% increase from sowing in late May rather than in the third week of June. Sowing wheat at an optimum time in subtropical grain regions to ensure roots grow in cool soil can be a useful component of integrated management by reducing the rate of P. thornei reproduction and increasing grain yield. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The root-lesion nematode species Pratylenchus thornei and P. neglectus are major pathogens of wheat (Triticum aestivum) in Australia (Thompson et al., 2008; Vanstone et al., 2008) and in many other countries (Smiley and Nicol, 2009). Root-lesion nematodes invade roots where they feed and reproduce in the root cortex causing loss of root function leading to poor uptake of nutrients and water from the soil. This results in wheat plant tops that are nutrient deficient (Thompson et al., 2012a) and water stressed (Whish et al., 2014) with consequent lower biomass production and grain yield. Wheat production in the subtropical grain region of northeastern Australia extends over 22–32◦ S latitude in a belt ∼200–400 km inland from the east coast (Webb et al., 1997). This region has a subtropical, semi-arid climate modified by

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elevation (∼200–500 m above sea level), with wheat grown on mainly deep, heavy-textured soils of high water-holding capacity. Average daily minimum temperatures range 0–9 ◦ C in winter and 12–20 ◦ C in summer, while average daily maximum temperatures range 12–20 ◦ C in winter and 27–30 ◦ C in summer (Webb et al., 1997). This temperature regime permits various crops to be grown throughout the year, but the rainfall (600–800 mm/year) is variable and unreliable, and dryland crop production is therefore dependent on both stored soil water and in-crop rainfall. P. thornei has higher incidence and population densities than P. neglectus in fields of the Australian subtropical grain region (Thompson et al., 2010). Hosts of P. thornei among crop species grown through winter in this region are the cereals wheat, barley (Hordeum vulgare) (Owen et al., 2008) and triticale (Triticum × Secale) (Owen et al., 2001), and the grain legumes chickpea (Cicer arietinum) (Thompson et al., 2000) and faba bean (Vicia faba) (Sheedy et al., 2009). Hosts of P. thornei among the crop species grown through summer are the grain legumes mungbean (Vigna radiata), black gram (Vigna mungo) and soybean (Glycine max) (Owen et al., 2014). P. thornei appears to be more damaging

J.P. Thompson / Field Crops Research 183 (2015) 50–55

Abbreviations a e k n P P Pi Pf R2 SE t T

proportion of Pi that initiates reproduction Euler’s number intrinsic rate of population increase number of individuals Pratylenchus probability initial population density final population density coefficient of determination standard error time thermal time

to wheat in this subtropical region where a practical threshold of damage is taken as 2000 nematodes/kg soil (Thompson et al., 2008) compared with the temperate region of southern Australia where the threshold is taken to be 20,000 nematodes/kg soil (A. McKay, pers. comm., 2015). Host crop cultivars can be characterised by their tolerance and resistance to nematodes. A tolerant cultivar yields well and conversely an intolerant cultivar yields poorly in soil with high populations of nematodes (Roberts, 2002). A resistant cultivar diminishes the nematode reproduction rate compared with a susceptible cultivar that favours a high nematode reproduction rate (Roberts, 2002). Inheritance of resistance to P. thornei in wheat, including synthetic hexaploid wheat, is quantitative with two to six effective genes involved depending on the cultivar (Thompson and Seymour, 2011; Thompson et al., 2012b). Root-lesion nematodes are polycyclic, that is, they pass through several life cycles during the growth of an annual crop, and their population increase can be described by a compound interest growth model (Van der Plank, 1968). In previous glasshouse experiments (Thompson et al., 2015a), the modified exponential equation that best described the change in P. thornei final populations (Pf ) in the soil and roots during growth of wheat cultivars was: Pf (t) = aPi ekt

(1)

where Pf is final population at time t, Pi is the initial population, a is the proportion of Pi that initiates reproduction, e is Euler’s number (a constant ≈2.71828) which is the base of natural logarithms, and k is the intrinsic rate of population increase for each cultivar. A moderately resistant wheat cultivar GS50a was shown to have lower k values than susceptible cultivars including Gatcher (Thompson et al., 2015a). These k values (with the time unit in weeks) were 0.004 and 0.08 for GS50a and Gatcher, respectively, where Pi was 5250 P. thornei/kg soil, and 0.05 and 0.35 where Pi was 1050 P. thornei/kg soil. GS50a is a tolerant and moderately resistant selection from the intolerant and susceptible commercial cultivar Gatcher (Thompson et al., 1999) and GS50a has similar phenology to Gatcher (Thompson et al., 2015a). Nematode reproduction is sensitive to both plant genetic effects and environmental conditions (Dropkin, 1980). Nematodes, like other invertebrates, are poikilothermic having body temperature the same as the environment. The reproduction rate of a particular nematode species can be related to the environmental temperature via three parameters. These are the base temperature (Tb ) below which no nematode reproduction occurs, the optimum temperature (To ) at which the rate of nematode reproduction is maximal, and the maximum temperature (Tm ) above which no nematode reproduction occurs (Trudgill et al., 2005). In a previous study with four wheat cultivars grown at various soil temperatures for 18 weeks in soil with Pi of 2500 P.

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thornei/kg soil, the optimum temperature was found to be in the range 20–25 ◦ C, the maximum was 30 ◦ C and the base temperature 10 ◦ C (Thompson et al., 2015b). These values were in general agreement with studies of the effects of temperature on P. thornei reproduction in carrot disc culture (Castillo et al., 1995; Thompson et al., 2015b), and chickpea roots (Castillo et al., 1996a,b). However, the effect of soil temperature on reproduction of P. thornei in the field is not well understood in the subtropical grain region. Integrated management of P. thornei in the subtropical grain region of Australia is mainly based on diagnosis of population densities in the soil before sowing, rotation with resistant crops and selecting wheat cultivars for growing on their tolerance and resistance ratings (Thompson et al., 2008). Tolerance and resistance ratings of wheat cultivars are published annually (Lush, 2014) and similar information on rotational crops is published periodically (Tips and Tactics, 2015) to enable growers’ sowing choices. Van Gundy et al. (1974) suggested that farmers in Sonora, Mexico (approximate position Lat. 29.65◦ N, Long.110.87◦ W) could give wheat a competitive advantage over P. thornei by sowing late (mid-November) so that roots grew in cool soil (defined as <15 ◦ C at 10.5 cm depth from beginning of December to late March), and suggested this as a useful component of an integrated management package. The effect of sowing date of a crop on the interaction between P. thornei and crop growth needs to be investigated for integrated management of P. thornei in the subtropical grain region of Australia. In this paper, I have extended the concept of Van Gundy et al. (1974) by developing a deterministic model of P. thornei reproduction based on thermal time (Trudgill et al., 2005). This model was then used to simulate P. thornei reproduction for susceptible and moderately resistant wheat genotypes in relation to soil temperature recorded at four depths to 60 cm in the soil profile at a site on the Darling Downs of Queensland in the subtropical grain region. Further, the effects of sowing date on soil temperature regimes and on consequent P. thornei population dynamics in the roots and soil during crop growth were explored using the model. 2. Materials and methods 2.1. Model development Data on the effects of temperature on the final populations of P. thornei determined at fortnightly intervals from 8 to 18 weeks were obtained from experiments in which wheat cultivars were grown in soil with an initial population (Pi ) density of 2500 P. thornei/kg soil at six soil temperatures ranging from 15 to 30 ◦ C (Thompson et al., 2015b). For the present study, thermal time (Trudgill et al., 2005) was calculated in degree days for all ambient soil temperatures up to and including 25 ◦ C using the formula: T = (Ta − Tb )t

(2) ◦C

where Ta and Tb are ambient and base temperatures in respectively, and t is time in days. Tb was estimated to be 10 ◦ C (Thompson et al., 2015b). Final population densities were graphed against thermal time for temperatures up to 25 ◦ C and regression equations were fitted for the susceptible wheat cultivar Gatcher, its moderately resistant selection GS50a, and an unplanted treatment, using Genstat (VSN International, 2014). 2.2. Temperature in the soil profile Soil temperature records (JK Leslie, unpublished data) were obtained for a 5-year period at depths of 8, 15, 30 and 60 cm in the soil profile of a self-mulching black Vertosol (Isbell, 1996) of the Waco Series (Beckman and Thompson, 1960) on a farm at Mt.

J.P. Thompson / Field Crops Research 183 (2015) 50–55

Air max

Air min

Soil 8 cm max

Soil 8 cm min

Soil 15 cm

Soil 30 cm

Soil 60 cm 35 Temperature ( ⁰C)

30 25 20 15 10 5 0

Pratylenchus thornei populaon density ln(nematodes/kg soil+1)

52

16

Gatcher

Maria (Lat 27.44◦ S, Long. 151.46◦ E; elevation ∼370 m). This site on the Darling Downs of Queensland, is approximately mid-latitude of the subtropical grain region. The daily maximum and minimum air temperature and soil temperature at 8 cm depth, and the mean daily temperature at 15, 30 and 60 cm depths (where there was little diurnal fluctuation) are presented in Fig. 1. The values plotted are temperature means across the 5 years in pentads, i.e. periods of 5 consecutive days.

12 10 8 6 4 2 0

At each soil depth (8, 15, 30, 60 cm), a running daily cumulative thermal time in degree days was calculated for a period of 18 weeks after four sowing dates from mid-autumn to mid-winter, namely, 25 April, 25 May, 24 June and 24 July. The mean daily temperature across 5 years at each of the depths was used for this purpose. Nematode population densities were simulated by solving each of the three equations for Gatcher, GS50a and the unplanted control using the daily cumulative thermal time at each depth until 18 weeks after sowing at the four dates (48 simulations in all composed of 3 cultivar/control treatments × 4 sowing dates × 4 soil profile depths). Nematode population densities were simulated in ln(P. thornei/kg soil + 1) units and back-transformed to number of P. thornei/kg soil for presentation of results. 2.4. Measured grain yields Grain yields for Gatcher and GS50a were obtained for 6 years from field experiments at a P. thornei-infested site used for tolerance testing as described by Thompson et al. (1999). This site at Formartin (Lat 27.46◦ S, Long. 151.43◦ E; 364 m elevation) is 3.7 km from the Mt. Maria site where soil temperature data were collected. Two experiments comparing wheat cultivars were sown at Formartin each year with the first at a planned sowing date in the last week of May, and the second in the third week of June. However, because of the variable rainfall only 4 years of field experiments were sown close to the two target sowing dates, and in one of these years the wheat heads in the early sown experiment suffered some frost damage. Therefore comparisons of the grain yield of Gatcher and GS50a were made for three experimental years (1997, 1999, 2000) for which earlier sown experiments had a mean sowing date

500 1000 1500 Thermal me (⁰C days above 10⁰C)

2000

Fig. 2. Regression relationships for wheat cultivars Gatcher and GS50a between Pratylenchus thornei population density as ln(nematodes/kg soil + 1) and thermal time (T) in degree days above 10 ◦ C based on harvest times between 8 and 18 weeks and temperatures up to 25 ◦ C where: ln (P. thornei + 1) = −0.000003(0.0000009)T 2 Gatcher : +0.009(0.0019)T + 5.4671(0.904), R2 = 0.80, P < 0.001, n = 24 ln (P. thornei + 1) = −0.000002(0.0000007)T 2 GS50a : +0.0063(0.0014)T + 5.1569(0.678), R2 = 0.82, P < 0.001, n = 24

Unplanted control :

2.3. Simulation of population dynamics of P. thornei in relation to temperature in the soil profile

Unplanted

14

0

Fig. 1. Air and soil temperatures at four depths in the soil profile averaged over a 5year period at Mt. Maria, on the Darling Downs, Queensland, Australia. Graphs show daily minimum and maximum temperatures for air and soil at 8 cm depth, and daily mean temperature for 15, 30 and 60 cm soil depths (drawn from unpublished data provided by Dr. J.K. Leslie).

GS50a

ln (P.thornei + 1) = 0.0013(0.00018)T + 5.4151(0.193), R2 = 0.70, P < 0.001, n = 24

where values in parentheses are standard errors of the coefficients and constants.

of 29 May ± 2 days and later sown experiments had a mean sowing date of 21 June ± 1day. 3. Results 3.1. Thermal time model of P. thornei population density Final population densities graphed against thermal time for temperatures up to 25 ◦ C for the susceptible wheat cultivar Gatcher, the moderately resistant selection GS50a, and an unplanted treatment are given in Fig. 2. The relationship between ln(number of P. thornei/kg soil + 1) and thermal time was best described by quadratic regression equations for Gatcher and GS50a, and by a linear regression equation for the unplanted control. The derived regression equations were: ln (P. thornei + 1) = −0.000003(0.0000009)T 2 Gatcher : +0.009(0.0019)T + 5.4671(0.904),

(3)

R2 = 0.80, P < 0.001, n = 24 ln (P. thornei + 1) = −0.000002(0.0000007)T 2 GS50a : +0.0063(0.0014)T + 5.1569(0.678),

(4)

R2 = 0.82, P < 0.001, n = 24 ln (P. thornei + 1) = 0.0013(0.00018)T Unplanted control : +5.4151(0.193), R2 = 0.70, P < 0.001, n = 24

(5)

(a) Gatcher

(a) Gatcher

120

0

25 April

100

53

Pratylenchus thornei populaon density (nematodes/kg soil x 1000) 125 25 50 75 100

0

25 May

80

24 June 60 24 July 40

15

Soil depth (cm)

Pratylenchus thornei populaon density (nematodes/kg soil x 1000)

J.P. Thompson / Field Crops Research 183 (2015) 50–55

Unplanted

25 April 25 May

30

24 June 24 July

45

Unplanted

20 60

0 0

5

10 Weeks aer sowing

15

20

(b) GS50a

15

0

14

25 April

12

25 May

10

24 June

8

24 July

Soil depth (cm)

(b) GS50a

16 (nematodes/kg soil x 1000)

Pratylenchus thornei populaon density

0

Pratylenchus thornei populaon density (nematodes/kg soil x 1000) 2.5 5 7.5 10 12.5

15 25 April 25 May 30 24 June 24July 45

6

Unplanted

Unplanted

4 60

2 0 0

5

10 Weeks aer sowing

15

20

Fig. 3. Simulated population densities of Pratylenchus thornei after growth of two wheat cultivars (a) Gatcher and (b) GS50a, for 18 weeks based on mean thermal time at 8 cm depth in the soil profile after sowing at four dates from April to July compared with unplanted soil on the Darling Downs, Queensland, Australia. Note the different scales on the vertical axes of (a) and (b).

where T is the cumulative thermal time in degree days at time t for soil temperatures above a base temperature of 10 ◦ C up to the upper limit of the optimal range of 25 ◦ C and where values in parentheses are standard errors of the coefficients and constants. 3.2. Simulated population densities of P. thornei over time The simulated population densities of P. thornei in the soil increased exponentially with time after sowing during 18 weeks growth of Gatcher and GS50a. This is illustrated in Fig. 3 for Gatcher and GS50a, based on mean thermal time at 8 cm depth in the soil after sowing at the four dates of 25 April, 25 May, 24 June and 24 July. Simulated population densities of P. thornei in the first 18 weeks of the wheat crop’s growth were considerably greater with later sowing dates. The simulated reproduction curves at the various sowing dates were a similar shape for Gatcher and GS50a with always lower population densities for GS50a than Gatcher. Thus the Pf values for GS50a after 18 weeks ranged from only 13% to 24% of those for Gatcher (at 24 July and 25 May sowing dates, respectively). The simulated P. thornei populations in the unplanted controls were low after all sowing dates, compared to those in the presence of both wheat cultivars. Therefore results for the unplanted control

Fig. 4. Simulated final population densities of Pratylenchus thornei after growth of two wheat cultivars (a) Gatcher and (b) GS50a, based on mean thermal time at four depths in the soil profile at 18 weeks after sowing at four dates from April to July compared with unplanted soil on the Darling Downs, Queensland. Note the different scales on the horizontal axes of (a) and (b).

are presented for only the 24 July sowing date, which gave the highest values (Fig. 3).

3.3. Simulated population densities of P. thornei in the soil profile at 18 weeks after sowing The final P. thornei populations simulated for Gatcher and GS50a at each depth in the soil profile at the end of 18 weeks after each of the four sowing dates are given in Fig. 4. There was some nematode reproduction at all depths to 60 cm after all four sowing dates, however, final populations varied considerably with sowing date. The greatest final nematode populations were with the 24 July sowing date. At this late sowing date, nematode reproduction was relatively greater in the topsoil at 8 cm depth than it was in the subsoil at 60 cm depth, because in spring soil temperature increased more quickly in the topsoil than in the subsoil. In contrast, at the earliest sowing date (25 April) final nematode populations were less at 8 cm than at 60 cm, because of residual warmth from summer remaining in the subsoil longer than in the topsoil. These patterns of nematode densities in relation to sowing date and soil depth were evident for both Gatcher (Fig. 4a) and GS50a (Fig. 4b) with the latter cultivar resulting in considerably lower population densities than the former.

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J.P. Thompson / Field Crops Research 183 (2015) 50–55

Gatcher 29 May Gatcher 21 June GS50a 29 May GS50a 21 June 0

1000

2000

3000

4000

Grain yield (kg/ha) Fig. 5. Grain yield (measured) of the susceptible, intolerant wheat cultivar Gatcher and the moderately resistant, tolerant cultivar GS50a with early sowing (black bars, mean date 29 May ± 2 days) or later sowing (grey bars, mean date 21 June ± 1 day) at the P. thornei-infested site of Formartin on the Darling Downs, Queensland. Solid bars are means of three field experiments in different years with SE indicated by capped line.

3.4. Measured grain yields Measured mean grain yields of Gatcher and GS50a from the high P. thornei site at Formartin for three years of replicated field experiments at mean sowing times of 29 May and 21 June are given in Fig. 5. The mean grain yield of Gatcher was increased by 61% by early sowing compared with late sowing, whereas for GS50a this value was 8%. The yield of GS50a was 1.9 and 2.8 times the yield of Gatcher for 29 May and 21 June sowing dates, respectively. 4. Discussion The sowing window for wheat on the Darling Downs within the subtropical grain region of Australia extends from the last week of April to the last week of July (Lush, 2014). The recommended sowing window is dictated by the risk of grain yield loss from either post-head-emergence radiant frost (Frederiks et al., 2015) or heat shock around anthesis causing sterility and abortion of formed grains, as well as reduced duration of grain filling (Barlow et al., 2015). The aim is to sow crops early enough to complete grain development before conditions become too hot in early summer and late enough to avoid anthesis occurring in winter and early spring when the risk of frost is high. The risks vary depending on the genetically controlled rate of phenological development of each wheat cultivar, and the position of the field in the topography, whether on the uplands where frost risk is lower or on the plains where frost risk is greater. The simulations in this study were for two wheat cultivars of similar phenology, but of different resistance to P. thornei, with Gatcher being susceptible and GS50a moderately resistant. The simulated results indicated a greater risk of higher nematode reproduction from sowing wheat late (late July in this environment), because the roots develop in warmer soil, than with earlier sowing, particularly in late May. This greater increase of nematode population density with late sowing can put greater pressure on a susceptible, intolerant wheat cultivar, which may suffer more yield loss than if sown earlier so that roots grow in cooler soil. As reported here, sowing the susceptible, intolerant cultivar Gatcher at the end of May resulted in 61% more grain yield on average than sowing it later in the third week of June, when soil temperatures over the ensuing 18 weeks and consequent nematode multiplication were greater. In comparison, the average yield advantage from sowing the moderately resistant cultivar GS50a at the earlier date was 8%. Furthermore, in assessments of wheat cultivars for field tolerance to P. thornei based on vegetative symptoms of stunting and chlorosis (Thompson et al., 1999), sowing in July on the Darling Downs has been found to give stronger symptom expression and better discrimination between cultivars than early sowing, no doubt due to the more favourable soil temperatures for nematode reproduction in wheat roots from July sowing.

The simulations indicated that the last week of May would be an optimum time to sow wheat in this environment to ensure soil temperatures during early root growth limit the rate of P. thornei reproduction. The principle of choosing sowing date to manage P. thornei populations is similar to that advocated by Van Gundy et al. (1974) for the Sonoran region of Mexico, but in that subtropical environment in the Northern Hemisphere it was later sowing (mid-November) that resulted in wheat roots developing in cooler soil (below 15 ◦ C) from the start of December. The current simulations demonstrate the importance of early sowing of wheat in the Australian subtropical grain region to limit nematode reproduction in the roots and lessen yield loss of susceptible, intolerant cultivars (Thompson et al., 2015b). However, at the earliest sowing date of late April, temperatures in the subsoil at 60 cm were more favourable for P. thornei reproduction than at the later date of late May. Thus it is best to indicate an optimal sowing date to limit P. thornei reproduction rather than a blanket statement of sowing early (Darling Downs, Queensland) or late (Sonora, Mexico). In the deep soils of the subtropical grain region, P. thornei can occur in the soil profile to 120 cm depth. When soil profile distributions are examined, peak population densities of P. thornei are seen to occur in some fields in the topsoil (0–15 cm), but in other fields at various depths, for example, in the 15–30, 30–45 or 45–60 cm layers (Thompson et al., 1999; Owen et al., 2014). The simulations conducted in this paper have been valuable to explore the effects of wheat genotype and sowing date on population development of P. thornei as affected by temperatures in the soil profile during crop growth in the field. Other factors that might influence P. thornei population densities such as different initial populations, soil moisture, and root mass at various depths in the soil profile, and changes in phenology with sowing date, have been considered constants in these simulations. Integration of temperature with such other factors in a crop growth model accounting for root-lesion nematodes will be undertaken in further studies as indicated by Whish et al. (2014). Similar overall patterns of response in P. thornei reproduction to thermal time occurred for the two cultivars despite the difference between them in the intrinsic rate of increase in P. thornei populations (Thompson et al., 2015a). However, these simulations also showed the strong effect of wheat genotype on P. thornei population increase during crop growth with the moderately resistant GS50a limiting the final population density to only 14–22% of that with the susceptible Gatcher depending on the sowing date. Thus the principle of choosing an optimum sowing date so that wheat roots develop in cool soil to limit P. thornei population increase should apply generally to all wheat cultivars, from moderately resistant to susceptible. Since there is no evidence that P. thornei populations from different geographic locations have different temperature requirements (Thompson, 2015b), the model developed here could be applied to other locations where wheat is grown globally.

5. Conclusions The model of P. thornei reproduction in wheat roots against thermal time developed here, was valuable in exploring the effects that soil profile temperatures after various sowing dates have on nematode population densities. Simulations showed that sowing wheat in late May on the Darling Downs in the Australian subtropical grain region can limit population density of P. thornei in the soil profile for both susceptible and moderately resistant wheat cultivars, by allowing roots to grow in soil cooler than after sowing at other dates in a 3-month sowing window. The moderately resistant and tolerant cultivar GS50a produced lower simulated population densities of P. thornei than the susceptible cultivar Gatcher at all sowing dates, and produced higher measured grain yield at both early (late

J.P. Thompson / Field Crops Research 183 (2015) 50–55

May) and conventional (third week of June) sowing dates on a P. thornei-infested site. Gatcher had a 61% increase in measured grain yield and GS50a had an 8% increase from sowing in late May rather than in the third week of June. Therefore sowing at an optimum time to limit P. thornei reproduction within the recommended sowing window (dictated by the subsequent risk of frost or heat damage at anthesis) could be a useful component of integrated management of P. thornei. Acknowledgements The author thanks the Grains Research and Development Corporation (GRDC) for funding, and Dr. J.K. Leslie for soil temperature records. References Barlow, K.M., Christy, B.P., O’Leary, G.J., Riffkin, P.A., Nuttall, J.G., 2015. Simulating the impact of extreme heat and frost events on wheat crop production: a review. Field Crops Res. 171, 109–119. Beckman, G.G., Thompson, C.H., 1960. Soils and land use in the Kurrawa area, Darling Downs, Queensland. CSIRO Div. Soils, Soils Land Use Ser., No. 37, Melbourne, Australia. Castillo, P., Trapero-Casas, J.L., Jiménez-Díaz, R.M., 1995. Effect of time, temperature, and inoculum density on reproduction of Pratylenchus thornei in carrot disk culture. J. Nematol. 27, 120–124. Castillo, P., Gomez-Barcina, A., Jiménez-Díaz, R.M., 1996a. Plant parasitic nematodes associated with chickpea in southern Spain and effect of soil temperature on reproduction of Pratylenchus thornei. Nematologica 42, 211–219. Castillo, P., Gomez-Barcina, A., Jiménez-Díaz, R.M., 1996b. The effect of temperature on hatching and penetration of chickpea roots by Pratylenchus thornei. Plant Pathol. 45, 310–315. Dropkin, V.H., 1980. Introduction to Plant Nematology. John Wiley & Sons, New York, 293 pp. Frederiks, T.M., Christopher, J.T., Sutherland, M.W., Borrell, A.K., 2015. Post-head-emergence frost in wheat and barley: defining the problem, assessing the damage, and identifying resistance. J. Exp. Bot. 66, 3487–3489. Isbell, R.F., 1996. The Australian Soil Classification, Rev. ed. CSIRO Publishing, Melbourne. Lush, D., 2014. Queensland 2014 Wheat Varieties Guide. Grains Res. Dev. Corp. and Qld. Dep. Agric. Fish. For, ISSN1838-9279. Available at www.nvtonline.com.au Owen, K.J., Clewett, T.G., Thompson, J.P., 2001. Hosting ability for summer and winter grain crops for root-lesion nematodes (Pratylenchus thornei and P. neglectus). In: Proc. 13th Bienn. Plant Pathol. Conf., Cairns. Australas. Plant Pathol. Society, Cairns, p. 201. Owen, K.J., Sheedy, J.G., Thompson, J.P., Clewett, T.G., O’Reilly, M.M., 2008. Resistance of Australian spring barley and wheat cultivars to root lesion nematode (P. thornei) 2007 Plant Dis. Man. Rep. (Online). Report 2:N034. American Phytopathological Society, St. Paul, MN, USA. Owen, K.J., Clewett, T.G., Thompson, J.P., 2014. Wheat biomass and yield increased when populations of the root-lesion nematode (Pratylenchus thornei) were reduced through sequential rotation of partially-resistant winter and summer crops. Crop Pasture Sci. 65, 227–241. Roberts, P.A., 2002. Concepts and consequences of resistance. In: Starr, J.L., Cook, R., Bridge, J. (Eds.), Plant Resistance to Parasitic Nematodes. CABI Publishing, Wallingford, UK, pp. 23–41 (Chapter 2).

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Sheedy, J.G., Clewett, T.G., Thompson, J.P., Paull, J., Rose, I.A., 2009. Resistance to root lesion nematode (P. thornei) of Australian faba bean cultivars, 2008 Plant Disease Management Reports (Online). Report 3:N035. American Phytopathological Society, St. Paul, MN, USA. Smiley, R.W., Nicol, J.M., 2009. Nematodes which challenge global wheat production. In: Carver, B.F. (Ed.), Wheat Science and Trade. Wiley-Blackwell, Ames, Iowa, USA, pp. 171–187 (Chapter 8). Thompson, J.P., Seymour, N.P., 2011. Inheritance of resistance to root-lesion nematode (Pratylenchus thornei) in wheat landraces and cultivars from the West Asia and North Africa (WANA) region. Crop Pasture Sci. 62, 82–93. Thompson, J.P., Brennan, P.S., Clewett, T.G., Sheedy, J.G., Seymour, N.P., 1999. Progress in breeding wheat for tolerance and resistance to root-lesion nematode (Pratylenchus thornei). Australas. Plant Pathol. 28, 45–52. Thompson, J.P., Greco, N., Eastwood, R., Sharma, S.B., Scurrah, M., 2000. Integrated control of nematodes of cool-season food legumes. In: Knight, R. (Ed.), Linking Research and Marketing Opportunities for Pulses in the 21st Century. Kluwer Acad. Pub., Dordrecht, The Netherlands, pp. 491–506. Thompson, J.P., Owen, K.J., Stirling, G.R., Bell, M.J., 2008. Root-lesion nematodes (Pratylenchus thornei and P. neglectus): a review of recent progress in managing a significant pest of grain crops in northern Australia. Australas. Plant Pathol. 37, 235–242. Thompson, J.P., Clewett, T.G., Sheedy, J.G., Reen, R.A., O’Reilly, M.M., 2010. Occurrence of root-lesion nematodes (Pratylenchus thornei and P. neglectus) and stunt nematode (Merlinius brevidens) in the northern grain production region of Australia. Australas. Plant Pathol. 39, 254–264. Thompson, J.P., Mackenzie, J., Sheedy, G.H., 2012a. Root-lesion nematode (Pratylenchus thornei) reduces nutrient response, biomass and yield of wheat in sorghum–fallow–wheat cropping systems in a subtropical environment. Field Crops Res. 137, 126–140. Thompson, J.P., Zwart, R.S., Butler, D., 2012b. Inheritance of resistance to root-lesion nematodes (Pratylenchus thornei and P. neglectus) in five doubled-haploid populations of wheat. Euphytica 188, 209–219. Thompson, J.P., Clewett, T.G., O’Reilly, M.M., 2015a. Optimising initial population, growth time and nitrogen nutrition for assessing resistance of wheat cultivars to root-lesion nematode (Pratylenchus thornei). Australas. Plant Pathol. 44, 133–147. Thompson, J.P., Clewett, T.G., O’Reilly, M.M., 2015b. Temperature response of root-lesion nematode (Pratylenchus thornei) reproduction on wheat cultivars has implications for resistance screening and wheat production. Ann. Appl. Biol. 167, 1–10. Tips and Tactics, 2015. Root-Lesion Nematodes: Northern Region. GRDC Grownotes. GRDC, Canberra, 8 pp. Available at www.grdc.com.au Trudgill, D.L., Honek, A., Li, D., Van Straalen, N.M., 2005. Thermal time – concepts and utility. Ann. Appl. Biol. 146, 1–14. Van der Plank, J.E., 1968. Disease Resistance in Plants. Academic Press, New York, pp. 206. Van Gundy, S.D., Perez, J.G.B., Stolzy, L.H., Thomason, I.J., 1974. A pest management approach to the control of Pratylenchus thornei on wheat in Mexico. J. Nematol. 6, 107–1165. Vanstone, V.A., Hollaway, G.J., Stirling, G.R., 2008. Managing nematode pests in the southern and western regions of the Australian cereal industry: continuing progress in a challenging environment. Australas. Plant Pathol. 37, 220–224. VSN International, 2014. Genstat for Windows, 17th ed. VSN International, Hemel Hempstead, UK. Webb, A.A., Grundy, M.J., Powell, B., Littleboy, M., 1997. The Australian subtropical cereal belt: soils, climate and agriculture. In: Clarke, A.L., Wylie, P.B. (Eds.), Sustainable Crop Production in the Sub-tropics – An Australian Perspective. Qld. Dep. Prim. Ind., Toowoomba, Australia, pp. 8–23. Whish, J.P.M., Thompson, J.P., Clewett, T.G., Lawrence, J.L., Wood, J., 2014. Pratylenchus thornei populations reduce water uptake in intolerant wheat cultivars. Field Crops Res. 161, 1–10.