Tobacco damping-off
Symptoms Most of the diseases occur in the seedling stage before the Big Cross. The basal part of the stem on the first day of the onset showed a wet rot, followed by a brownish watery rot, and the diseased seedlings were wilting and dark green. The diseased seedlings often spread into lodgings around the surface. When the humidity is high, white filamentous mycelium can be seen on the seedbed surface and the diseased seedlings.
Pathogen Pythium aphanidermatum (Eds.) Fitzp. Fruit Pythium, a flagella flagellum subphylum fungi. Mycelium is colorless without separation. The sporangia are filiform or branched, or irregular in size, ranging from 63-7254.9-14.8 (μm), and the sporangia germinate to produce vesicles. The vesicles are spherical and contain 6-26 zoospores. The oviculum is spherical, with a size of 14.9-34.8 μm. Males are bag-shaped to wide-stemlike, with one or more filaments, mostly one, size 5.6-15.47.4-10 (μm). Oval spores spherical, smooth, partial, size 14.0-22.0 μm. In addition, P. debaryanum Hesse described Pythium debari, P.ultimum Trow as the ultimate Pythium, and can also cause tobacco damping-off.
Transmission routes and pathogenic conditions The bacterium is wintered in the soil with oospore or chlamydospore. Under suitable conditions, sporangia germinate and invade the host with zoospores or buds. The hyphae that live in the soil in the soil can also produce sporangia, which can infest the spores to invade the rhizomes of tobacco near soil surface. The re-infestation source in the field is mainly the sporangia and zoospore produced on the diseased seedlings, which are spread by spraying water or rain. After the pathogen invades the host, it expands in cortical parenchyma cells, and the mycelium spreads among cells or cells, and then forms oospore in the diseased tissue. The suitable temperature for pathogenic bacteria growth is 15-16°C, the suitable onset temperature is 10°C, and the temperature above 30°C is inhibited. Nursery period of low temperature and high humidity conditions to its pathogenesis.
Control methods (1) bed soil disinfection bed soil should use disease-free new soil. If old garden soil is used, seed bed disinfection should be carried out. Apply 50% seed dressing double powder or 40% pentachloronitrobenzene powder 8-10g or 25% metalaxyl wettable powder 9g and 70% mancozeb wettable powder 1g per m2, mix well with alumina, 1 /3 The medicine soil is sprinkled on the surface of the bowl, and the remaining 2-3 pieces of medicine soil cover the top of the seed, that is, overlying the lower cushion. (2) Strengthen the management of seedbeds, select a plot with high topography, low groundwater level, and well-drained water. Pouring water before sowing, watering as little as possible after emergence. Watering time is selected sunny morning, avoid flood irrigation. Nursery crickets (beds) release air in a timely manner, and dampness should be released on a cloudy day in a timely manner to prevent exhaustion. (3) In the initial stage of onset, spray 72.2% Preclosure 400 times or 3% Malignant Tonic 300 times, 95% Greenhenge 1st Premium 4000x, 25% Metalaxyl WP 600-800 times For each m2 seedbed, use 3L of good liquid.