SUMMARY Ascospores of Sclerotinia sclerotiorum infected fully expanded sunflower leaves in the absence of added nutrients, wounds or senescent tissue. The site of infection was confined to a specific region around the junction of the leaf blade and the petiole and was associated with sites of sucrose secretion by the host. In these areas ascospores germinated and formed extensive colonies on the leaf surface. Simple appressoria were visible at 24 h, and complex appressoria at 48 h, after inoculation. Ascospores that germinated in areas where sucrose was not secreted produced germtubes that were shorter (less than 50 μm at 48 h after inoculation) than those produced at sites where sucrose was secreted (greater than 5 mm in length). Infections also developed on severely wounded main leaf veins. Leaf wetness was required for infection, with maximal infection occurring after a 72 h dew period.

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