Diseases of cole crops (
Downy mildew
Downy mildew is caused by the fungal-like organism Peronospora parasitica. Initial signs and symptoms are grayish white growth on the underside of leaves during cool, moist weather. Infected areas become purplish then yellow to brown spots visible on both leaf surfaces (photo 1). P. parasitica is systemic and may cause black streaks in the floral tissue of broccoli and cauliflower (photo 2). Leaf surfaces must be wet from rain or dew for about 10-12 hours during cool nights of 45° - 60° F and days of less than about 75° F for disease to develop.
The fluffy growth on leaves produces sporangia that are easily disseminated by wind and water to other plants, resulting in rapid disease development under suitable environmental conditions. The sporangia are short lived, but P. parasitica also produces resting spores, called oospores, that can survive from one season to another in crop residue or soil.
Disease is controlled by rotation to non-cruciferous crops, control of cruciferous weeds (such as alyssums, mustards, winter cress, smallseed falseflax, shepherdspurse, london rocket) tolerant varieties and the timely application of fungicides. Fungicide application are very effective if applied early. Fosetyl-Al (Aliette), chlorothalonil (Bravo), maneb, and formulations of metalaxyl/mefenoxan (Ridomil Gold) are registered for use. Fungicides should be alternated to reduce development of resistance.
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February 10, 2003