Effect of sowing date on the severity of ascochyta blight in field peas (Pisum sativum L.) grown in the Wimmera region of Victoria

Abstract

Field experiments were established at Horsham, in the Wimmera region ofVictoria, in 1987, 1988 and 1989 to compare the severity of ascochyta blightand grain yield of field peas sown in May, June and July. In each year, theseverity of ascochyta blight on all the pea cultivars studied was greatest onthe May-sown plots and least severe on the July-sown plots. The level of seedinfestation by ascochyta blight fungi was also highest in grain harvested fromthe plots sown earliest. In 1987, the average length of lesions girdling themain stem was 28.7 cm in the May-sown plots and 1.0 cm in the July-sown plots.In 1988, the average percentage of stem area affected by ascochyta ranged from 60.2% in the May-sown plots to 13.1% in the July-sown plots,while in 1989 the range was from 38.3% in the May-sown plots to5.8% in the July-sown plots. In 1988, delaying sowing until Julyresulted in a significant reduction in disease with only a small reduction inyield. However, in 1989 while July sowing reduced the severity of disease by 17%, compared to a June sowing, the later sowing also reduced grainyields by 40%.In a separate trial at Horsham in 1988, using cv. Buckley, disease progresswas most rapid on the April-sown plots and slowest on the August-sown plots.The final disease levels ranged from 100% of stem area affected (Aprilsowing) to 2% of stem area affected (August sowing). The yield lossescaused by the disease were greater the earlier the plots were sown.These studies suggest that the severity of disease in commercial crops may bereduced by delaying sowing until after mid-June, thus avoiding exposure ofyoung plants to high levels of primary inoculum.