Sensitivity of Crinipellis perniciosa to two triazole fungicides in vitro and their effect on development of the fungus in cocoa

Two triazole fungicides, hexaconazole and triadimenol, were evaluated for their effects on the growth of Crinipellis perniciosa in vitro, and for their ability to prevent broom formation on cocoa seedlings. Amounts of both fungicides required to reduce fungal growth, germ tube extension and basidiospore germination by 50% were found respectively to be less than 1, 10-150 and more than 200 mg/l. Reductions in germ tube length were associated with slower rates of cell growth, whereas growth of dikaryotic mycelium was reduced due to shorter cell lengths. Isolates of C. perniciosa showed different sensitivities to the fungicides, variation between collections from different localities being greater than between collections from the same locality. Hexaconazole gave good control of the disease applied as a spray to cocoa seedlings both before and after inoculation. Triadimenol showed good activity when used as a preinoculation soil drench and in some treatments where plants became infected no necrosis of host tissue was observed. Activity was greatly reduced by drench treatments applied 10 days after inoculation, and most infected plants formed necrotic cankers.