Almost all the PGPF reported so far have shown a pronounced suppressive effect against soil-borne diseases. One example of this is the suppression by Trichoderma harzianum of damping-off disease on barley, cucumber, radish, and tomato caused by Pythium ultimum (Ahmad and Baker 1988). Nonpathogenic R. solani AG4 has been reported to suppress damping-off disease caused by virulent R. solani and R. zeae by 76-94% on cotton, radish, and wheat. The sterile fungi, SBF, SDF, and SRF, have been shown to decrease the occurrence of take-all disease of wheat caused by Gaeumannomyces graminis var. tritici (Dewan and Siva-sithamparam 1989; Narita and Suzuki 1991; Speakman and Kruger 1984). The PGPF isolated from zoysiagrass rhizo-sphere have been shown high suppressive ability against soil-borne diseases caused by Pythium aphanidermatum, P. irregulare, R. solani, Sclerotium rolfsii, Fusarium oxysporum f. sp. melonis, F. o. f. sp. cucumerinum, G. graminis var. tritici and Cochliobolus sativus (Hyakumachi 1994). When cucumber plants inoculated with PGPF isolates from zoysiagrass rhizozphere, disease suppression was observed against the air-borne pathogen, Colletotrichum orbiculare (Meera et al. 1994). In this work the PGPF were applied to plant roots and leaves were used for pathogen inoculation thereby the PGPF and pathogen were physically separated. The result therefore suggests that induced systemic resistance is involved as one of the mechanisms for disease suppression by PGPF. The PGPF isolates from zoysiagrass rhizosphere, Trichoderma, Fusarium, Penicillium, Phoma, and sterile fungi, all provided significantly protection to air-borne anthracnose caused by C. orbiculare, bacterial angular leaf spot caused by Pseudomonas syringae pv. lacrymaus, and soil-borne Fusarium wilt by F. oxysporum f. sp. cucumeris (Koike et al. 2001). In the case of Fusarium wilt, a split-root system was used to ensure physical separation of PGPF and pathogen, and to assess induced resistance.
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