STIMULATION OF SEX ORGAN FORMATION IN PHYTOPHTHORA BY ANTAGONISTIC SPECIES OF TRICHODERMA

SUMMARY Two methods are described for the induction of sex organ formation in heterothallic Phytophthora species in response to Trichoderma, a volatile method and a direct inoculation (‘contact’) method. Tested for their ability to induce sexual reproduction in Phytophthora, isolates of Trichoderma koningii, T. piluliferum, T. polysporum and T. viride were found to be active, and isolates of T. hamatum, T. harzianum, T. longibrachiatum and T. pseudokoningii inactive. Response in Phytophthora was, without exception, confined to the A2 compatibility type. Response of different Phytophthora species, however, was more variable. All forty-one isolates of P. cam-bivora and P. cinnamomi tested responded to Trichoderma viride. Isolates of Phytophthora cryptogea, P. drechsleri, and P. nicotianae were apparently polymorphic for response. The black pepper form of P. palmivora responded, whereas P. palmivora forms 1 and 2 did not. A minimum induction period of c. 24 h exposure to volatiles of active Trichoderma was apparently necessary for consistent oogonial formation to occur. Oogonial formation occurred on a range of sterol-containing media but not in their absence and over a wide range of conditions of temperature and light. Antagonism of Phytophthora cultures by active Trichoderma isolates was readily apparent both in the volatile and contact methods. The ability of Trichoderma isolates to induce a sexual response in Phytophthora was correlated with their ability to produce volatile antibiotics. Possible implications of the ‘Trichoderma effect’ for the taxonomy, reproductive physiology and genetics of Phytophthora spp. are considered.