Symptoms of downy mildew is also called yellowing and atrophy. The general incidence in China is between 10% and 20%, and it is as high as 50%. It usually occurs sporadically at the fields or at the ditch. The disease has different symptoms at different growth stages. The diseased seedlings are dwarfed at the seedling stage, and the leaves are light green or have slightly striped leaves. After returning to jointing, the diseased leaves became lighter, and yellowish-white strips appeared. The leaves became thicker, crumpled and twisted, and the diseased plants were dwarfed. They did not normally head or spike out of the flag leaf sheaths and bent into the deformed leading spikes. The 1000-grain weight of diseased strains at various levels fell by an average of 75.2%. Transmission routes and pathogenic conditions The pathogens use oospore to overwinter or run over the summer in the soil. The oospore has germination ability in water for 5 years. Generally after 5-6 months of dormancy, it germinates and produces zoospore. When there is water or a large temperature, it invades from young buds after germination and becomes a systemic infection. The oospore germination temperature was 19°C-20°C, the optimum germination temperature of sporangia was 16°C-23°C, and the suitable water temperature for zoospore germination was 18°C-23°C. After the sowing of wheat, wheat fields before flooding were flooded for more than 24 hours. In March of the following year, there was a cold spring. The low temperature favored the occurrence of the disease. Low-lying fields and Rice and wheat fields are also vulnerable to disease. Prevention and control methods (1) In regions or fields where heavy crop rotations occur, one or more crop rotations should be carried out with non-cereal crops. (2) Perfect irrigation and irrigation system area is strictly prohibited flood irrigation, timely drainage after rain to prevent moisture retention, found that diseased plants in a timely manner. (3) Before seed sowing, 50 kg of wheat seed should be soaked with 25% metalaxyl WP (25-37.5 g active Ingredient) and 3 kg of water. If necessary, spray 0.1% copper sulphate solution or 58% metalaxyl WP wettable powder 800-1000 times, 72% WP (wet urea manganese zinc) wettable powder 600-700 times, 69% safety. Manganese zinc wettable powder 900-1000 times, 72.2% propamocarb (Pric) 800 times liquid.
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