1. Pathogens and pathogenesis.
1. The bacterium produces oospores and conidia. Oospores are formed on the surface of the leaf in the dry diseased leaf tissue, while conidia are formed on the leaf surface. The pathogen overwinters on the leaves in the form of oospores and hyphae, and becomes the primary source of infection after forming conidia the following year. Conidia are formed at night and scattered during the day. The bubble invades from the leaf stomata and develops the disease after 5 to 10 days of incubation period, and many conidia are formed on the leaf surface, which becomes the source of reinfection. It is a fungus of the subphylum Flagellates. The sporangial peduncle is solitary or clumped, with 3 6 bifurcated branches at the apex, colorless, without septum. Sporangia unicellate, oval, light brown. The oospores are spherical, thick membrane, and yellowish-brown.
2. Weather and seasons Cool and humid weather is conducive to the development of pathogenic bacteria and the occurrence of diseases. The temperature is around l5, and it is most conducive to the onset of the disease when there is more rainfall. Downy mildew may be prevalent when it rains continuously from mid-April to early May (May to June in the Tohoku region) and heavy fog or fog is frequent. The disease is particularly severe in low-lying plots, flooded plots, densely planted and poorly grown. The onset of the disease is suppressed due to high temperatures in summer. The incidence of disease in autumn was also positively correlated with rainfall, and improper irrigation also induced aggravation of the disease. The field is seriously ill in autumn, and the disease is also severe in the spring of the following year.
2. Disease symptoms.
1. Locally infected plants only show symptoms at the infected site. After the green onion was infected by the pathogenic bacterial system, the diseased plant was dwarfed, the leaves were twisted and deformed, and a layer of loose white to light purple villous mold layer was formed on the surface of the leaves and flower stems when wet. When drying, the leaves turn white or appear yellowish and die, sometimes only the tip of the leaf is diseased, and then turn white and die. Locally infected leaves and flower stems produce oval, round lesions of varying sizes, white or yellowish, with inconspicuous edges. If the middle and lower parts of the leaves are diseased, the upper part of the leaves will droop and dry up.
2. The diagnostic characteristics of downy mildew of green onion are yellowish-white leaves, with white to light yellow villous mold layer on the surface, and the diseased leaves are soft and easy to break. In the later stage, saprophytic fungi sometimes breed in the diseased area, and black mold appears.
3. The overwintering green onions infected with the whole plant stop growing and the plant height decreases; The leaves are thickened, yellowed and curved. When it rains continuously in spring and autumn, a white mold layer can be formed, and the yellow turns wilted. In reinfected green onions, large yellowish-white oblong lesions appear on the leaves and peduncles, with a white mold layer on the surface, which later turns green or dark purple. In severe cases, the diseased plants are pale yellow and wilted. After the growth of miscellaneous bacteria on the lesion, sometimes a black velvety mold layer is formed.
3. Mode of transmission.
There are two types of downy mildew of green onions: systemic infection and local infection: the systematically infected strains are formed by the infection of pathogens transmitted by soil and planting materials.
1. The pathogen of downy mildew of green onion mainly relies on mycelium to latent on the bulb for overwintering, or oospores for overwintering on the diseased residues in the soil or attached to the surface of the seed, which becomes the primary source of infection in the second year. In the second year, the pathogen germinates, invades from the stomata of the plant or spreads through mycelium in the host.
2. Green onion downy mildew produces a large number of sporangia on diseased plants in the field, which is spread with the help of air currents, rainwater, insects, and agricultural operations. Once the sporangia are transmitted and come into contact with the host susceptible site, they can germinate and invade within a few hours when the conditions are suitable, and the incubation period is short, only 3 to 5 days when the conditions are suitable. Multiple infestations can be repeated during the growing season.
3. The optimal temperature for the formation of ascomycetes of green onion downy mildew is 15, which is not resistant to drying, has a short lifespan, and the appropriate temperature for germination is 10.
Four: prevention and control agents.
Downy mildew carb hydrochloride, mycloconazole, cyanosalazole, enomorpholine, pyroxystrobin, fluopyramide, fluorothiazolepyracetophenone, cyanide frosturea.
5. Atlas. 