Effect of the fumigant metham on Trichoderma spp.
Radial growth of 34 isolates of Trichoderma hamatum, T. harzianum, and T. viride was inhibited (mean effective dose (ED50)) by as little as 1.3 μg of vapors (mostly methylisothiocyanate) per cubic centimeter of atmosphere from metham decomposing in soil suspensions. Isolates of T. hamatum were generally more sensitive than those of T. harzianum to the vapors. Spore germination was more sensitive than hyphal growth to vapors, and whereas the toxic volatiles were only fungistatic to growth, they were fungicidal to germination. As little as 0.5 μg/cm3 of vapors from decomposing metham in soil reduced germination of conidia of three isolates (TMP, Tm-1, and T-1-R4) more than 50% and in another three (T-1, Th-6, and Th-20) 90%. Exposure of spores to metham in solution (350 μg active ingredient/mL) for 1 h reduced survival more than 50% in eight isolates. When conidia of T. viride (T-1-R4) and T. harzianum (WT-6-24) were added (ca. 20 × 103 spores/g) to soil treated with a sublabel rate of metham (equivalent to 234 L/ha), the number of colony-forming units (CFU) of isolates introduced 4 days before, or at time of fumigation were significantly less than those in nonfumigated soil after 3 weeks of incubation. CFU of isolates introduced 4 days after fumigation were similar to those in nonfumigated soils. Over a 9-week period, introduced populations of T-1-R4 and WT-6-24 remained static in nonfumigated soils. Indigenous Trichoderma spp. appeared unaffected by the levels of metham used. Addition of bran or use of soils high in natural organic matter did not affect results.