The Effect of Tricyclazole and Culture Medium on Production of the Melanin Precursor 1,8-Dihydroxynaphthalene by Sclerotinia sclerotiorum Isolate SS7

The fungal plant pathogen Sclerotinia sclerotiorum (Lib.) de Bary, isolate SS7, produced pentaketide melanin precursor 1,8-dihydroxynaphthalene (DHN) and the polyketide sclerin when grown on potato dextrose agar (PDA) purchased from Difco. Adding tricyclazole, a polyketide melanin pathway inhibitor, to Difco PDA reduced the production of DHN and sclerin by SS7 compared to control treatments in the late but not in the early stages of growth. Increasing concentrations of tricyclazole were marginally more effective at reducing DHN biosynthesis but were incrementally more inhibitory to sclerin production. Tricyclazole, even at 50 ppm, only slightly inhibited fungal growth and did not prevent the production of melanin. In contrast, SS7 cultures grown on PDA from BDH Inc. produced less than 10% of the amount of sclerin found with Difco medium and DHN was present only in trace or undetectable amounts. The medium used for culturing SS7 thus had a more significant impact on production of excreted polyketides than tricyclazole. In all treatments and media used, the sclerotia remained darkly pigmented. The results indicate that tricyclazole does inhibit the biosynthesis of polyketides in isolate SS7 of S. sclerotiorum but melanin pathways other than that which produces DHN may also be operational in this fungus.