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

Field experiments were established at Horsham, in the Wimmera region of Victoria, in 1987, 1988 and 1989 to compare the severity of ascochyta blight and grain yield of field peas sown in May, June and July. In each year, the severity of ascochyta blight on all the pea cultivars studied was greatest on the May-sown plots and least severe on the July-sown plots. The level of seed infestation by ascochyta blight fungi was also highest in grain harvested from the plots sown earliest. In 1987, the average length of lesions girdling the main 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 to 5.8% in the July-sown plots. In 1988, delaying sowing until July resulted in a significant reduction in disease with only a small reduction in yield. However, in 1989 while July sowing reduced the severity of disease by 17%, compared to a June sowing, the later sowing also reduced grain yields by 40%. In a separate trial at Horsham in 1988, using cv. Buckley, disease progress was 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 (April sowing) to 2% of stem area affected (August sowing). The yield losses caused by the disease were greater the earlier the plots were sown. These studies suggest that the severity of disease in commercial crops may be reduced by delaying sowing until after mid-June, thus avoiding exposure of young plants to high levels of primary inoculum.