Development and morphology of Uncinula necator following treatment with the fungicides kresoxim-methyl and penconazole

Epifluorescence microscopy and low temperature scanning electron microscopy were used to document the development of Uncinula necator on vine leaves and the antifungal effects of kresoxim-methyl and penconazole. Post-germinational growth and development followed a regular time course which was classified into 10 stages. Kresoxim-methyl was applied at a range of concentrations and at different times before and after inoculation. In glasshouse trials at moderate relative humidity (60%) all pre-infectional applications completely inhibited conidial germination. Lower efficacies were observed with detached leaves at high humidity in Petri dishes. Post-infectional applications of at least 8 mg a.i. l(-1) inhibited sporulation and mycelial growth and 67 mg al. l(-1) caused a partial collapse of surface structures. Penconazole applied at 17 mg a.i. l(-1) did not inhibit germination, but prevented hyphal development and caused growth distortion with hyphal tip swelling. Pre-and post-infectional treatments had similar effects. Applications made 3 d after inoculation increased multiple appressoria and conidiophore formation.