Symptoms of downy mildew in Chinese cabbage seedlings When the seedlings are sick, chlorosis spots and hoarfrost appear on the stems and leaves. In the later stage, the disease worsened, and the stems and leaves of seedlings turned yellow and died. The symptoms of downy mildew of adult cabbage mainly harm leaves. During the growing period, leaves will appear green spots soaked by water. With the development of the disease, the color of the spots gradually deepened and gradually changed from black to dark black. Most spots are almost round in the initial stage, and are irregular or polygonal due to the restriction of veins when expanding. These spots are slightly concave. When the weather is wet, the spots on the back of the leaves produce a white and sparse mold layer, that is, sporangia and sporangia of pathogenic bacteria.

When the temperature is high, the diseased spots often develop into yellow-brown or yellow-white withered spots. When the condition is serious, the diseased spots merge with each other to form dead spots. After the onset of old leaves, pathogenic bacteria sometimes infect stems, and then develop and infect leaf bulbs, resulting in irregular yellow to brown necrotic spots on midvein and mesophyll tissues, and the leaves dry up and fall off in the later stage. Have you ever seen the symptoms of cabbage downy mildew? The pathogen of downy mildew of Chinese cabbage is parasitic downy mildew, which belongs to downy mildew of Nostoc flagelliforme. Mycelium has no diaphragm and is colorless, and nutrients and water in cells are absorbed by haustorium. Asexual reproduction produces sporangia. Sporangium usually passes through stomata, and grows singly or in clusters of 2-5, with a total length of137.0-428.0 microns. The base of the trunk is slightly enlarged, and the top branches are bifurcated at an acute angle for 2-6 times.

The petiole at the top is sharply curved, and there is a sporangium at each end. Sporangium is oval, single cell, colorless, (17~29) × (15~28) micron, and oospores are formed sexually and are spherical. If the conditions are suitable, it can germinate directly, causing germ tube infection. Parasitic downy mildew is an obligate parasitic fungus, which can only survive in the living body. There is obvious physiological differentiation, which can be divided into three varieties: Brassica, radish and shepherd's purse. The main difference is that each variety is different in infectivity. Brassica species are highly contagious to the genus, but weakly contagious to radish, and generally do not infect shepherd's purse; Radish varieties are highly contagious to radish, but weakly contagious to Brassica, and cannot infect shepherd's purse. Shepherd's purse is a variety, which only infects shepherd's purse and does not infect other hosts.

Parasitic downy mildew sporangium can produce zoospores when it germinates, or it can directly germinate to produce germ tubes. Recent studies show that the sporangium germination of this fungus can be carried out by forming sporangium and producing secondary sporangium outside the bud tube. ) prevention and control methods. (1) Reasonable close planting, timely topdressing, and regular spraying of biological bacterial fertilizer or sodium nitrophenolate (love more and harvest more) can prevent plants from aging and enhance disease resistance. (2) Before sowing, seed is disinfected by dressing with 25% metalaxyl wettable powder which accounts for 0.3% of the seed weight; Spraying 58% metalaxyl manganese zinc wettable powder 500 times, 72% cymoxanil manganese zinc wettable powder 600 times, or 50% dimethomorph wettable powder 2 000 times at the initial stage of the disease. Pay attention to spraying middle and lower leaves and leaf backs to improve the control effect.