Role of conidial fusion in infection by Botrytis cinerea on cucumber leaves.

Glucose or sucrose enhanced germination of conidia, formation of primary appressoria and elongation of superficial hyphae, but did not induce infection by Botrytis cinerea on cucumber leaves. By enhancing conidial concentration in glucose or sucrose solution, however, B. cinerea produced significant lesions. When the conidial concentration exceeded 1×107 conidia/ml in 5% glucose or 5% sucrose solution, inoculated conidia fused with each other and formed netted structures on cucumber leaves. Similar fusions were also reported in the process of infection by Fusarium nivale. Hyphae growing out from the netted structures were more vigorous than hyphae or germ tube from single conidia. The hyphae from the structures formed appressoria, which belong to the second type of simple appressoria group. From the appressoria, infection pegs developed through epidermal cell walls. In 24hr after inoculation, infection hyphae were observed in the epidermal cells, and also in mesophyll cells 48hr after inoculation. It seems that glucose induces the conidial fusion and formation of the netted structures under the condition of more than a given concentration of conidia. The netted structures are thought to play a role of supplying nutrients for the infection by B. cinerea.