Effects of crop rotation, residue retention and sowing time on the incidence and survival of ascochyta blight and its effect on grain yield of field peas (Pisum sativum L.)

Ascochyta blight is endemic in many regions where field pea (Pisum sativum) is grown, causing significant losses in grain yield. Crop management is the main method used to reduce the severity of disease and minimise yield losses, but with varying degrees of success. This is likely to be due to variation in the production and survival of inoculum. A 6-year experiment was conducted in which disease pressure was manipulated by crop rotation, residue retention, time of sowing and fungicide treatment. The experiment measured the amount and persistence of inoculum on crop residues and in soil as well as its dispersal in the air during the growing season, and assessed disease development and yield. Aerial inoculum from infected pea residues was the main source of infection, followed by soil-borne inoculum. Soil-borne inoculum declined slowly. The average rate of decline in soil inoculum was 15% per year, suggesting that a break of 6 years is required to reduce soil inoculum by 90%. This slow decline meant that increasing the disease break from 2 to 4 years did not consistently reduce the severity of disease or improve grain yield. By comparison, inoculum on pea crop residues declined rapidly after one season. Aerial inoculum was released from residues in two main periods: in autumn, before the pea crop was sown, and also in spring. Very little aerial inoculum was measured in winter. Rainfall in summer and autumn influenced inoculum release prior to sowing and reduced the disease pressure during establishment of the crop. Aerial inoculum was exhausted once 193mm of rainfall had been received after 1 December. Delayed sowing was the best strategy for reducing the level of disease by avoiding the autumn peak in aerial inoculum. Disease severity was more than halved and grain yield increased by 6% by delaying sowing by 3-4 weeks. However, in the absence of disease, grain yield of early-sown crops was 20% greater than that of late-sown crops. A strategy of early sowing and the strategic use of fungicide may enable the dual goals of reduced disease and improved yields to be achieved. Early sowing without the disease penalty may be possible in seasons that have high summer and autumn rainfall, which results in the release of ascospores prior to sowing.