Conidial concentration as an important factor determining the type of prepenetration structures formed by Botrytis cinerea on leaves of French bean (Phaseolus vulgaris)

inoculations of primary leaves of French bean ( Phaseolus vulgaris ) with conidia of Botrytis cinerea , suspended in 0.11 or 0.28 M glucose supplemented with 0.067 M KH 2 PO 4 or 1 mM Na‐ATP as infection stimulants, yielded mostly spreading lesions. Light microscopy revealed that the fungus could develop three types of prepenetration structures from which penetration started: germ tube apices (GA) either non‐swollen or appearing as an appressorium‐like swelling; appressoria (HA) of different forms produced on branched germ tubes or superficial liyphae; and infection cushions (IC). These structures were also formed on leaves of cucumber and tomato. Penetration from the prepenetration structures was not markedly influenced by the glucose concentration in the inoculum or by host plant species, and varied only slightly with the isolate of B. cinerea . Penetration occurring from GA was greatly enhanced at high conidial concentrations. Low conidial concentrations, however, enhanced penetration starting from HA or IC, penetration from these structures also being dependent on the nature of the stimulant added to the glucose in the inoculum. The high proportion of GA in inocula with a high conidial concentration was associated with a rapid penetration and a rapid formation of spreading lesions.