Epidemiology and management of ascochyta blight of field pea (Pisum sativum) in South Australia.

Abstract

Ascochyta blight disease (synonym: blackspot) of field pea has worldwide distribution and regularly causes AUD$25 million loss per annum in Australian field pea (Pisum sativum) crops. This study provides new information on the causal pathogens and management strategies to reduce loss from this disease. Research involving sowing dates, genotypes and fungicide treatments was conducted to identify optimal management strategies. Earlier sowing generally resulted in higher yield except when ascochyta blight was severe. Yield response to fungicide application varied with disease severity, sowing date and genotype. The optimum sowing period was within a week of the first autumn rains in low rainfall regions and 3 weeks after the first autumn rains in medium and medium - high rainfall regions. Earlier flowering genotypes were the highest yielding particularly when sown early and subjected to strategic fungicide applications. The pathogen, Phoma koolunga, was recognised for the first time as a component of the ascochyta blight disease complex in southern Australia. The species was described morphologically. Sequences of the internal transcribed spacer region were distinct from those of the accepted causal pathogens of ascochyta blight of field pea viz. Didymella pinodes, Phoma medicaginis var. pinodella and Ascochyta pisi. Symptoms on field pea seedlings caused by P. koolunga were indistinguishable from those caused by D. pinodes, other than a 24 h delay in manifestation of symptoms. P. koolunga was detected across field pea cropping soils in South Australia but rarely from other Australian states while D. pinodes plus P. medicaginis var. pinodella were widespread. The quantity of DNA of these pathogens detected in soils was positively correlated with ascochyta blight lesions in a pot bioassay. Soil-borne inoculum gradually decreased in the 3 years following a field pea crop. DNA tests and pathogen isolation from naturally infected field pea plants showed P. koolunga to be an important component of the disease complex in South Australia. P. koolunga and D. pinodes were equally responsible for disease symptoms, while P. medicaginis var. pinodella had a minor role in the disease complex. Interaction between D. pinodes, P. medicaginis var. pinodella and P. koolunga was investigated in controlled conditions. Colony diameter of the former was reduced on potato dextrose agar (PDA) amended with filtrate from broth cultures of P. koolunga, as was colony diameter of D. pinodes on PDA amended with filtrate from P. medicaginis var. pinodella or D. pinodes. This effect was shown to be fungistatic rather than fungicidal. When coinoculated onto leaves on field pea plants, or onto excised leaf discs, either the quantity of DNA of D. pinodes and of P. medicaginis var. pinodella, or the mean lesion diameter of these pathogens, was significantly reduced when co-inoculated with P. koolunga. P. koolunga was not influenced by co-inoculation. D. pinodes demonstrated self-antagonism. D. pinodes is considered the principal pathogen of concern in this complex. This study further investigated the relationship between ascospore numbers of D. pinodes at sowing and disease at the end of the season. Ascospores released from stubble infested with ascochyta blight were counted periodically in a wind tunnel. A model was developed to predict disease severity in relation to ascospore numbers, distance from infested field pea stubble, and rainfall. The model was validated with an independent dataset. A threshold level of ascospores of D. pinodes was identified above which disease did not increase. The findings from this study have been incorporated into management recommendations for field pea in southern Australia. Growers are encouraged to manipulate sowing dates according to the temporal release of ascospores, and select a cultivar that has the best agronomic yield potential for the sowing date, and to use fungicide strategically. The recommendation also emphasises field selection based on commercial testing for the presence of soil-borne inoculum of D. pinodes, P. medicaginis var. pinodella and P. koolunga.