Induction of New Isolates ofTrichoderma harzianumTolerant to Fungicides and Their Experimental Use for Control of White Rot of Onion

Abstract

Prolonged exposure of mycelia and conidia of Trichoderma harzianum to the fungicide benomyl did not produce isolates tolerant to the fungicide. Exposure of four wild strains of T. harzianum to the fungicides chlorothalonil, procymidone, iprodione, and vinclozolin resulted in selection of several isolates tolerant to these fungicides. Some of the fungicide-tolerant isolates grew better radially on media containing the fungicides than their respective wild strains did. Other isolates lost their tolerance after being cultured on fungicide-free media. Conidia of certain isolates of the wild strains WT-6 and T, tolerant to chlorothalonil and iprodione, respectively, germinated better on media containing high concentrations of the fungicides than did conidia of their respective wild strains. Exposure of conidia of the wild strain T-14 to 0.1% (active ingredient) chlorothalonil for 4 wk 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 derived from the Egyptian strain T produced more toxin, as measured by inhibition of mycelial growth of Sclerotium cepivorum, than did the wild strain. One fungicide-tolerant isolate of strain Th-1 (Th-1 [procy-2M]) reduced white rot of onion caused by S. cepivorum more effectively than did Th-1 or other fungicide-tolerant isolates. The iprodione-tolerant isolate T(ipro-25M) and iprodione combined with T(ipro-25M) gave the best control of white rot of onion in the field in Egypt.