Biological Control of White Rot Disease of Onion (Sclerotium cepivorum) by Trichoderma harzianum

Abstract

Prolonged exposure of mycelia and conidia of T. harzianum to benomyl did not produce isolates tolerant of the fungicide. Exposure of 4 wild strs. of T. harzianum to chlorothalonil, procymidone, iprodione and vinclozolin resulted in selection of several tolerant isolates. Some tolerant isolates grew better radially on media containing the fungicides than their respective wild strs. did. Other isolates lost their tolerance after being cultured on fungicide-free media. Conidia of certain isolates of the wild strs. WT-6 and T, tolerant of chlorothalonil and iprodione, respectively, germinated better on media containing high concs. of the fungicides than did conidia of their respective wild strs. Exposure of conidia of the wild str. T-14 to 0.1% a.i. chlorothalonil for 4 weeks reduced germination by 80% when the conidia were placed on a fungicide-free medium. Similar exposure of conidia of T-14(3M), a chlorothalonil-tolerant isolate, reduced germination by only 20%. An iprodione-tolerant isolate from the Egyptian str. T produced more toxin, as measured by inhibition of mycelial growth of Sclerotium cepivorum, than did the wild str. One tolerant isolate of str. Th-1(Th-1[procy-2M]) reduced white rot of onion, caused by S. cepivorum, more effectively than did Th-1 or other tolerant isolates. The iprodione-tolerant isolate T(ipro-25M) and iprodione combined with T(ipro-25M) gave the best control of the disease in the field in Egypt.