How Cauliflower Downy Mildew Occurs and How to Control It
Cauliflower is a type of vegetable belonging to a variety of cabbage. Cauliflower is a cruciferous plant, a biennial or annual herb with broad leaves, a blunt base, wavy edges, smooth and hairless, bearing yellow-white flowers. Leaves grow from the root, the main stem and branch stems grow from the leaves, and the stems are covered with small granules that form a flower-like shape; the flower is a fleshy mass, overall resembling a large blossom, white and attractive. The cauliflower head is tender and fine in texture, sweet and delicious, and very easy to digest and absorb after eating.
The principle of cauliflower downy mildew:
Cauliflower downy mildew occurs commonly in both northern and southern regions. In the south it mainly damages seed-production plants, greatly affecting seed yield; in northern production fields, when the disease is severe, there is a certain loss of yield. Cauliflower leaves, curds, flower stalks and seed pods can all be affected. Leaves form yellow lesions with indistinct edges, bearing dark brown spots; on the underside of the leaves the lesions have dark brown spots that are fairly obvious and slightly raised, with a white mold layer on top, and in severe cases the leaves turn yellow and fall off. When flower stalks are affected they often break and die back, so that seed pods cannot set seed. This disease produces angular lesions limited by the leaf veins on the underside of the leaves, and in high humidity scattered frost-like white mold appears. The pathogen of cauliflower downy mildew is a fungus of the genus Peronospora in the subphylum Mastigomycotina; the morphology of the pathogen is the same as that of cabbage downy mildew, but the one infecting cauliflower is the cabbage type of the Brassica variety of cruciferous downy mildew. In northern regions, the primary infection sources for spring and autumn cauliflower come respectively from overwintering and oversummering mycelium and oospores; in southern regions the infection source comes from the pathogen on overwintering cruciferous winter vegetables. When the average temperature is 16-20°C, air humidity is relatively high, and water condenses on the plant surface, the disease readily occurs and spreads; in northern regions spring is more severe than autumn; in southern regions it occurs commonly in winter and spring, and when the curd-formation stage and the emergence of flower stalks coincide with continuous rainy weather and relatively low temperatures, the damage is more severe. In unheated protected cultivation, the disease is severe in winter.
The control methods for cauliflower downy mildew are:
1. Strengthen cultivation management, avoid continuous cropping and adjacent cropping with cruciferous vegetables, and rotation with field crops is best; in areas where the disease occurs frequently or in years forecast to have little rain, sow at the appropriate time but later.
2. Select disease-resistant varieties.
3. Seed disinfection: dress the seed with 35% metalaxyl powder at 0.3% of the seed weight, or 50% thiram, which can achieve fairly good results.
4. Chemical control: spray in the field at the early stage of disease onset; commonly used chemicals include 40% phosethyl-Al wettable powder diluted 200-300 times, or 25% metalaxyl wettable powder diluted 700 times, or 75% chlorothalonil wettable powder diluted 500 times, or 70% mancozeb wettable powder diluted 500 times, or 58% metalaxyl-mancozeb wettable powder diluted 500 times.