| Bee chalk brood disease is caused by Ascosphaera apis Olive & Spiltoir。The present researches on chalk brood are mainly on biological character of the Ascosphaera apis such as the optimal cultivating conditions, prevalence situation, pathology and diagnosis etc. There are extracellular proteases in the body wall and midgut of insects.Peritrophic membrane is a thin layer of membrane in midgut. It is consisted of protein, chitin and other carbonhydrate compounds. And it is also a natural barrier in resistance for insects to fungal pathogens. More and more people pay attention to the role of extracellular protease that it could damage the peritrophic membrane in the midgut of insects and disrupt the growth and development of insects. The extracellular protease activities of 9 Ascosphaera apis isolated from different cities were evaluated and the reliabity for the indices of their virulence. The extracellular protease exhibited optimum catalytic activity at 60～65℃. Its optimal pH was 7.5～8. The activity could not be detected when pH was below 4.5, the extracellular protease between different isolates have differences. In the assays of enzymes, there are many differences in each isolates. the estimates of LT50's were 6.92～9.04 d. Regression analysis revealed that PU was significantly correlated to the daily cumulative mortality of brood. The author suggested that PU could be used as virulence index only for the early-stage selection of candidate isolates in large quantity but not for the correlation.
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