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

Abstract

Ascochyta blight of field pea, caused by Didymella pinodes, Phoma medicaginis var. pinodella, Phoma koolunga and Didymella pisi, is controlled through manipulating sowing dates to avoid ascospores of D. pinodes, and by field selection and foliar fungicides. This study investigated the relationship between number of ascospores of D. pinodes 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 À1 h À1 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 BLACKSPOT MANAGER (Salam et al., 2011b; Australasian Plant Pathology, 40, 621-31), distance from infested stubble (Salam et al., 2011a; Australasian Plant Pathology, 40, 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 D. pinodes, 294 g À1 stubble h À1 , was identified, above which disease did not increase. Below this threshold there was a linear relationship between ascospore number and maximum disease intensity.

Intra-diurnal and daily changes in Didymella ascospore concentrations in the air of an urban site

“…Other studies, which examined dispersal of this pathogen (Salam et al, 2011;Davidson et al, 2013) did not apply volumetric air sampling and thus are not comparable with our study. Kaiser and Küsmenoglu (1997) reported a dispersal distance of D. rabiei for several kilometres from the inoculum source, which at typical wind speeds equal to 2 m s -1 would delay arrival in the city centre by an hour.…”

Survival ofPhoma koolunga, a causal agent of ascochyta blight, on field pea stubble or as pseudosclerotia in soil

“…() found that P. koolunga was coincidental with D. pinodes in field pea cropping soils. As a consequence, the general recommendation of delayed sowing or distance from field pea stubble infested with D. pinodes (Davidson et al ., ) as well as a 4 year interval between crops of field pea should be applied to minimize the risk of an ascochyta blight epidemic. In addition, other legume crops may be alternative hosts of P. koolunga (Bretag et al ., ; Davidson et al ., ).…”

“…Although this research showed that infested field pea stubble left on the soil surface did not initiate severe disease after 5 months, this window may not be sufficient in all conditions and also could allow early sown or volunteer field pea or alternative hosts to be infected by P. koolunga. Burying stubble would shorten survival of the pathogen and limit release and dissemination of conidia (Davidson et al, 2013). On the other hand, burying plant debris is not compatible with zero tillage or residue retention farming systems, which have been adopted in recent decades.…”

“…Incidence and severity of ascochyta blight increase when field pea crops are sown in the vicinity of field pea stubble infested with D. pinodes (Davidson & Ramsey, 2000;Bretag et al, 2006;Davidson et al, 2013). This is attributed to production of survival structures of the causal agents on the stubble (Dickinson & Sheridan, 1968;Zhang et al, 2005;Bretag et al, 2006;McDonald & Peck, 2009;Davidson et al, 2013). The major pathogen of the ascochyta blight complex, D. pinodes, can survive from 4 to 18 months as pseudothecia or sclerotia on field pea stubble, or as chlamydospores in infested soil for at least 12 months (Zhang et al, 2005;Bretag et al, 2006).…”

Novel Technologies for the detection of Fusarium head blight disease and airborne inoculum

“…Air sampling has been used to study the epidemiology of crop diseases in order to design a control strategy -how to alter time of sowing or harvest to escape disease (Davidson et al 2013), when precisely to apply fungicides or other crop protection agents (Brachaczek et al 2016;Carisse et al 2009;Thiessen et al 2016;West et al 2002), and whether it is possible to separate susceptible crops from inoculum sources (Maldonado-Ramirez et al 2005;Marcroft et al 2003). Spore sampling is useful to monitor changes in pathogen populations for both fundamental research and applied purposes such as providing a direct forecast of imminent disease risk by detecting the inoculum before infection events start.…”