The symptoms, also known as blue mold, are the subject of external phytosanitary tests. Mainly harmful inspection objects. The main damage leaves. The young leaves develop, the diseased leaves stand upright; yellow round spots form in the early stages of the larger leaves; the middle spots of the later spots subside, and gray or blue moldy layers appear on the back; when severe, the lesions fuse into brown necrotic large spots, and the diseased leaves shrink and twist. . Systemic infections are often manifested, and the tobacco plants dwarf and wither. Spots also appear on flowers and pods.
The pathogen Peronospora tabacina Adam said that the tobacco downy mildew is a flagellin subphylum fungi. Hyphae without isolation, multi-nuclear tubular. The anamorphous state produces cysts and sporangia. The cysts are dendritic, colorless, 450-750 μm long, with sporangia on them. Sporangia oval or lemon-like, colorless and transparent, size 17-2813-17 (μm). The sporangia produce a temperature of 15-21 °C, a maximum of 30 °C, and a minimum of 2 °C. Sporulation germination temperature 15-20 °C, requires a relative humidity higher than 95%, the most suitable 98.5%.
Transmission Pathways and Pathogenic Conditions The oocysts were borne by the oospore in the soil with the diseased body and became the source of infestation early in the year. The onset of smoke produces sporocysts that are re-infected by airflow. It can also be transmitted through agricultural operations, agricultural implements, tobacco leaves, tobacco seeds, etc. Unfatuated manure with diseased bodies can also spread. Tobacco downy mildew at 16-23 °C, more than 95% relative humidity at night, for 3 hours to form sporangia. Downy mildew occurs at low temperatures and high humidity. Rainy, condensation at night facilitates the occurrence and prevalence of downy mildew. The density of tobacco plants is high, and low-lying and poor-draining tobacco fields are seriously affected.
Prevention methods (1) Strengthen plant quarantine, and strictly quarantine the imported tobacco seeds and tobacco leaf products. (2) After the tobacco is harvested, it is necessary to deepen the land and deep-seated debris. It is forbidden to collect seeds on diseased plants. Bed soil is to be sterilized or a piece of land not planted with tobacco is used as a seedbed. Apply well-divided organic fertilizer. Reasonable and reasonable close planting, to avoid the accumulation of water in the field, improve the resistance of tobacco plants. (3) Chemical control 25% metalaxyl WP 800-1000 times or 58% metalaxyl WP wettable powder 1000 times, 64% antivirus WP 600-800 times, 72% Dew wettable powder 900 times liquid, 69% ank manganese zinc wettable powder 1000 times liquid, 667m2 with good liquid medicine 50-70L, spraying attention to the front and back of the blade should be sprayed.
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