Differential responses of field pea (Pisum sativum L.) to ascochyta blight (Mycosphaerella pinodes): rating disease in the field

Ascochyta blight (Mycosphaerella pinodes) is a serious foliar disease of field pea and infects most pea (Pisum sativum L.) crops in southern Australia. Despite efforts to develop resistance to ascochyta blight, pea crops throughout the world are still severely affected by the disease and this study re-evaluates the way disease responses are assessed in order to have greater precision in rating disease in the field. Ten genotypes of pea with differential morphology, maturity date, and responses to M. pinodes were selected and these were field tested at 2 sites, on 3 sowing dates. Detailed measures of leaf and stem disease were recorded each fortnight throughout the season. When disease pressure was low in the early part of the growing season, disease was most accurately measured in leaves as percentage of leaf area affected and in stems as percentage of stem length affected. As disease pressure increased, the simplest and more reliable measures taken for diseased tissues of the main stem were the percentage of senescent leaves and the percentage of stem girdling. Two wild type pea genotypes had thin stems which senesced after becoming severely girdled and these were able to avoid disease by sprouting new shoot growth after the height of the epidemic. Early season resistance (4-8 weeks after seeding) was detected in the lines SA 1160, JI 252, and Austrian Winter. Towards the height of the epidemic there was little difference in the rankings for genotypes that maintained their main stems. It was concluded that single measures of disease at the height of the epidemic are unlikely to identify correctly all genotypes with differential responses to M. pinodes infection.