The scanning electron microscopic study of the infection and conidial development of Aspergillus tamarii Kita on its host, the silkworm, Bombyx mori Linn.

Abstract

AbstractDevelopment of Aspergillosis on the integument of the silkworm, Bombyx mori Linn., was examined by scanning electron microscopy. Aspergillosis is a fungal disease caused by an insect mycopathogen Aspergillus tamarii Kita, which infects the silkworms in countries where sericulture (the rearing of silkworms)is prevalent. The present study showed the course of infection and the conidial development of A. tamarii on the integument of B. mori. Five different strains (KA, NB18, NB4 D2, NB7 and PM) of B. mori were inoculated on their body surface with ca. 1 × 106 conidia/ml. Among the five breeds tested, the conidial germination was greatest on the larval surface of KA breed, and least on PM. Most of the conidia germinated on the cuticle approximately 8–12 hours after inoculation, forming a suctorial appressorium within 24 hours. The hyphae reached the hemocoel, where they grew and multiplied extensively, forming a mycelial complex and causing death of the host larva in about 5–6 days. The death of the host was followed by growth of the fungus through mesodermal and epidermal tissues, leading to larval mummification about 6–7 days post-inoculation. Extensive aerial outgrowths of the fungus followed, mostly through the intersegmental regions of larvae. Abundant branched conidiophores developed, forming a confluent yellow brown mat over the entire host body 7 days after inoculation. Each conidiophore had an apical vesicle bearing numerous phialides from which conidia were developed in long chains.