Occurrence of Trichoderma species in apple orchard and woodland soils

Abstract

Native populations of Trichoderma spp in soils associated with apple trees in Wisconsin were quantified and the diversity of species present was characterized. In all, 20 soils were sampled from four geographical regions of Wisconsin: 17 from around apple trees, two from woodlands immediately downhill from apple orchards and one from a virgin white pine forest. Trichoderma spp were enumerated on and recovered from dilution plates, containing a selective medium, at 8, 16 and 24°C. Because Trichoderma spp were considered as potential biological control agents of Phytophthora crown rot of apple trees, soils also were assayed for Phytophthora cactorum (Lebert & Cohn) J. Schröt., the predominant causal agent of crown rot. with apple cotyledon baits. For most soils, significantly fewer colony forming units (cfu) of Trichoderma spp developed at 8°C compared to 16 or 24°C. which had similar numbers of cfu. Populations of Trichoderma spp varied significantly among soils at each incubation temperature and were not correlated with soil physical characteristics. For 12 of the soils sampled, five isolates from each incubation temperature (180 isolates total) were retained and represented five species. T. hamatum (Bonord.) Bainier, T. harzianum Rifai. and T. koningii Oudem. occurred in more soils than did T. virens (Miller et al.) Arx (= Cliocladium virons Miller et al) and T.viride Pers.: Fr. Each species was recovered from all geographical regions sampled except T. virens, which was recovered only from soils collected in southern Wisconsin. T. viride was recovered only at 8 and 16°C, but other species were recovered at all three temperatures. P. cactorum was found in 17 of the 20 soils. Trichoderma spp appear well adapted to soils associated with apple trees in Wisconsin.