Symptoms range from yellow blotches to small black speckles. Downy mildew is limiting production on some farms and entire crops continue to be lost. Almost all crops are at risk because the fungus requires only short periods of surface wetness to achieve infection.
HDC project FV 316 set out to compare varieties for susceptibility, investigate the role of seed treatments and find out whether crop covers influence the incidence of the disease.
For wild rocket, clear differences in resistance were found between the 26 varieties tested. The most resistant varieties decreased severity by more than 80 per cent compared with a susceptible standard. Nine salad rocket varieties were tested but no significant differences were found. Downy mildew was less severe than in wild rocket - symptoms were restricted to small spots.
Seed treatments were effective at protecting the crop for about three weeks. Downy mildew is able to overcome host resistance and fungicides so growers must be aware that there is a risk that there may be a breakdown of resistance if new races of downy mildew are selected.
Some types of crop covers caused increases in disease incidence and in crop vigour, compared with uncovered crops in 2008. In 2009, crop covers were used for 13 days from emergence. The Enviromesh, Ultrafine and Fleece covers provided an acceptable balance of features by reducing pest damage and increasing yield while slightly increasing downy mildew risk.
Horticultural Development Company
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