EVALUATION OF TRICHODERMA-SPP FOR BIOLOGICAL-CONTROL OF PHYTOPHTHORA CROWN AND ROOT-ROT OF APPLE SEEDLINGS

In a greenhouse bioassay, 223 isolates of five species of Trichoderma were evaluated as biological control agents of Phytophthora crown and root rot of apple seedlings. Effects of inoculum rate and ambient temperature on seedling mortality caused by Phytophthora cactorum were determined. Isolates of Trichoderma spp. were evaluated with two delivery methods. A viscous suspension of conidia in an aqueous gel was applied to seedling roots, or a colonized mixture of peat and wheat bran (peat-bran) was added to soil. After all isolates were evaluated twice with both delivery methods, six isolates delivered in peat-bran and five isolates delivered as conidium suspensions had increased survival time of seedlings compared with treatments without Trichoderma spp. These 11 treatments then were compared in a separate experiment. Isolate TW.055 of T. virens (= Gliocladium virens) in peat-bran consistently was superior to all other treatments; seedlings survived an average of 30 (out of 44) days whereas those in the control survived only 19 days. Isolates TW.105 and TW.189 of T. koningii in peat-bran (both with an average seedling survival time of 24 days) and a conidium suspension of isolate TW.138 of T. harzianum (with an average seedling survival time of 22 days) were effective in most trials. Trichoderma spp. were promising as biological control agents of Phytophthora crown and root of apple seedlings under experimental conditions conducive to disease development and, therefore, should be pursued as a potential means of biological control of the disease on apple trees in the orchard. The bioassay developed to evaluate isolates of Trichoderma spp. was effective and probably could be adapted to evaluate other microorganisms for potential to control Phytophthora crown and root rot of apple trees.