Hazards and Diagnosis: Light yellow irregular markings appear on the lower leaves, and grayish moldy layers appear on the back of leaves. The lesion is limited by veins. Upper leaves also have lesions. After the lesions have browned, the lower leaves die. The nature of the pathogenic virus:
The pathogenic bacteria attached to the roots of diseased plants or diseased leaves overwintering in the form of mycelium constitute the source of infection in the coming year. Conidia are formed on the lesions and spread by air currents. In the event of dewdrops, germ tubes sprout, sprouting hyphae, invading from cell junctions or stomata, spreading within the leaf tissue and forming lesions. The germ can also be transmitted through the seed. The optimum temperature for the development of germs is 17-13°C, the highest 25°C, and the lowest 3～4°C. The optimum temperature spread below 8°C. Downy mildew only parasitic in cruciferous plants.
Occurrence conditions and countermeasures: Often occurs in the late autumn and spring when the low temperature and humidity. The sick leaves should not be scattered and piled up or buried deeply. Attention should be given to rotation of vegetables or crops other than Cruciferae.
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