Germination, penetration, and invasion of Beauveria bassiana on silkworm, Bombyx mori, causing white muscardine

Abstract

White muscardine is a common disease of the silkworm Bombyx mori caused by an insect mycopathogen Beauveria bassiana. The present study revealed the germination, penetration, and invasion of B. bassiana on the larval integument of B. mori. The NB18 strain of B. mori was surface inoculated with concentrations of 1×106 conidia/ml. Each conidium, germinated approximately 8 h after inoculation, forming an appressorium within 24 h and the hypha entered the cuticle by forming a clear circular hole. The hyphae reached the haemocoel, where they grew and multiplied extensively, forming a mycelial complex, causing death of the host larva in about five‐six days. This occurred with minimal breakdown of the internal tissues. Death of the host was followed by ramification of the fungus through the mesodermal and epidermal tissues, leading to larval mummification about six days post‐inoculation. Extensive fungal outgrowths followed, mostly through the intersegmental regions of the larva, and formed aerial hyphae, which gave rise to branched conidiophores forming a confluent white fungal mat over the entire larval body in six‐seven days. The life‐cycle of the fungus on B. mori was completed in seven‐eight days.