Biol Fertil Soils (1994) 17:85-90 9 Springer-Verlag 1994 Margaret A. Ousley 9 James M. Lynch 9 John M. Whipps Potential of Trichoderma spp. as consistent plant growth stimulators Received: 14 December 1992 Abstract In a series of repeated trials, six Trichoderma spp. strains, applied as a dried powder from a liquid fer- mentation in molasses/yeast medium, proved to be con- sistent at promoting the growth of Iettuce (Latuca sativa L.) seedlings grown in a peat-sand potting compost in the glasshouse. Strains WT, 92, 20, and 75 at 0.75~ or 1% w: w concentrations increased shoot dry weight by up to 26%, although WT did inhibit germination. For example, after 4 days only 13 % of seeds sown in WT 1% w:w treated compost had germinated, whereas in other treat- ments germination was consistently greater than 32~ WT increased shoot fresh and dry weights by 14.3 g and 0.6 g per pot, respectively, without affecting the root dry weights, to give concomitant increases in shoot:root ra- tios of fresh and dry weight. The potential use of these Trichoderma spp. strains for plant growth promotion is discussed. Key words Growth stimulator 9 Inocula 9Lettuce Trichoderma spp. Introduction Trichoderma spp. are known to increase plant growth in a range of bedding and crop plants, including alyssum (Berteroa incana), carnation (Dianthus caryophyllus), chrysanthemum (Dendranthema grandiflora), marigold (Tagetes sp.), periwinkle (Vinca sp.), moss rose (Portulaca sp.), petunia (Petunia x hybrida), snapdragon (Antir- rhinum majus), cucumber (Cucumis sativus), eggplant (Solarium melongena), Pea (Pisum sativum), pepper (Piper nigrum), radish (Raphanus sativus), tobacco M.A. Ousley - J.M. Lynchl (~) . j.M. Whipps Microbiology and Crop Protection Department, Horticulture Research International, Littlehampton, West Sussex BN17 6LP, UK Present address." ~School of Biological Sciences, University of Surrey, Guildford, Surrey GU2 5XH, UK (Nicotiana sp.) and tomato (Lycopersicon esculentum) plants (Baker et al. 1984; Chang et al. 1986; Paulitz et al. 1986; Baker 1988). The size of the increase in plant growth depends on the crop. For example, when Trichoderma spp. was added to soil as a conidial suspen- sion an increase in dry weight was obtained with tomato, pepper, and cucumber plants but not with bean and radish plants (Chang et al. 1986). In addition, the form of inoculum can also influence the stimulation of growth obtained. For instance, Baker et al. (1984) obtained a greater dry weight of radish shoots when Trichoderma sp. was applied to compost as a peat-bran inoculum than when a conidial suspension was used. Similarly, growth promotion by Trichoderma spp. has also been shown to be dependent on the strain used and the concentrations of inocula applied (Lynch et al. 1991). Considerable efforts have been made to obtain isolates of Trichoderma spp. which exhibit a consistent ability to control pathogens, such as Pythium spp. and Rhizocotonia spp. (Mihuta-Grimm and Rowe 1986; Ahmad and Baker 1988; Coley-Smith et al. 1991; Maplestone et at. 1991). However, there have been rela- tively few attempts to obtain isolates of Trichoderma spp. which reproducibly provide increases in plant growth in the absence of pathogens. This paper presents results on the germination and growth of lettuce from a series of tri- als on the effects of various Trichoderma spp. isolates ap- plied at a range of inoculum concentrations in which six isolates providing reproducible growth promotion were identified. Material and methods Sources of Trichoderma spp. strains Eleven strains of Trichoderma spp. were used for detailed study. T. harzianum (TH1, IMI 275950) was isolated from wheat straw (Lynch 1987), T. harzianum (T35, IMI 298374) was isolated as an antagonist of Rhizoctonia solani (Ridout et al. 1986), and T. har- zianum (WT; obtained from Prof. R. Baker at Colorado State Uni-