Relationship between ascochyta blight on field pea (Pisum sativum) and spore release patterns of Didymella pinodes and other causal agents of ascochyta blight

Abstract

<jats:p>Ascochyta blight of field pea, caused by <jats:italic><jats:styled-content style="fixed-case">D</jats:styled-content>idymella pinodes</jats:italic>,<jats:italic> <jats:styled-content style="fixed-case">P</jats:styled-content>homa medicaginis</jats:italic> var<jats:italic>. pinodella</jats:italic>,<jats:italic> <jats:styled-content style="fixed-case">P</jats:styled-content>homa koolunga</jats:italic> and <jats:italic><jats:styled-content style="fixed-case">D</jats:styled-content>idymella pisi</jats:italic>, is controlled through manipulating sowing dates to avoid ascospores of <jats:italic><jats:styled-content style="fixed-case">D</jats:styled-content>. pinodes</jats:italic>, and by field selection and foliar fungicides. This study investigated the relationship between number of ascospores of <jats:italic><jats:styled-content style="fixed-case">D</jats:styled-content>. pinodes</jats:italic> at sowing and disease intensity at crop maturity. Field pea stubble infested with ascochyta blight from one site was exposed to ambient conditions at two sites, repeated in 2 years. Three batches of stubble with varying degrees of infection were exposed at one site, repeated in 3 years. Every 2 weeks, stubble samples were retrieved, wetted and placed in a wind tunnel and up to 2500 ascospores g<jats:sup>−1</jats:sup> h<jats:sup>−1</jats:sup> were released. Secondary inoculum, monitored using seedling field peas as trap plants in canopies arising from three sowing dates and external to field pea canopies, was greatest in early sown crops. A model was developed to calculate the effective number of ascospores using predictions from G1 <jats:sc>blackspot manager</jats:sc> (Salam <jats:italic>et al</jats:italic>., 2011b; <jats:italic>Australasian Plant Pathology</jats:italic>,<jats:bold> 40</jats:bold>, 621–31), distance from infested stubble (Salam <jats:italic>et al</jats:italic>., 2011a; <jats:italic>Australasian Plant Pathology</jats:italic>,<jats:bold> 40</jats:bold>, 640–7) and winter rainfall. Maximum disease intensity was predicted based on the calculated number of effective ascospores, soilborne inoculum and spring rainfall over two seasons. Predictions were validated in the third season with data from field trials and commercial crops. A threshold amount of ascospores of <jats:italic><jats:styled-content style="fixed-case">D</jats:styled-content>. pinodes</jats:italic>, 294 g<jats:sup>−1</jats:sup> stubble h<jats:sup>−1</jats:sup>, was identified, above which disease did not increase. Below this threshold there was a linear relationship between ascospore number and maximum disease intensity.</jats:p>