Abstract:

The aims are to study the ecological relationship between Rhizopus oryzae and tobacco leaf mildew (TLM) at the curing stage, and to provide a basis for making the disease-control strategy. The natural incidence rate of different parts of the petioles and stems were defined by tissue moisturizing method. The relationship between the incidence rate and water content was studied by inoculating the pathogen on petioles. The content of R. oryzae in the rhizosphere soil of tobacco fields and subsequent crops was determined by the dilution plate method. The results showed that the main fungus in tobacco rhizosphere soil was R. oryzae which caused TLM. Higher disease incidence rate appeared when the stem was close to petiole. Stem of continuously cropping tobacco plants displayed more incidence compared to that of new tobacco plants. The pathogen could easily invade the stem through infecting 1-14 nodes in the middle and lower parts of the plant. Tobacco petiole did not show any disease when the water content was lower than 40%, and the disease was most severe when the water content was 70%-80%. The number of R. oryzae in the root soil of continuously cropping tobacco plant was greater compared to newly planted soil. Root soil with black shank disease plants displayed more pathogen number compared to the one with healthy plants. R. oryzae was more abundant in the soil surrounding the roots of summer squash and cauliflower, while it was less in the soil surrounding the roots of peas and broad beans. This study highlights the significance of formulating control strategy for TLM at curing stage of tobacco.

Key words: tobacco, curing stage, tobacco leaf mildew, Rhizopus oryzae, rhizosphere soil