Effects of Fumigation and Fungal Antagonists on the Relationships of Inoculum Density to Infection Incidence and Disease Severity in Fusarium Crown Rot of Tomato

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Abstract
Fusarium crown rot of tomato was used as a model to quantify these effects. The model was chosen because the epidemic is severe when soils are treated with biocides that allow recolonization by F. oxysporum f. sp. radicis-lycopersici; thus, the introduction into soils of microorganisms that are antagonistic to the pathogen should provide protection against the disease. Chlamydospores of the pathogen were formed under axenic conditions so that defined concentrations of specific inocula could be added to freshly fumigated soil. The antagonists were selected for their abilities to increase populations rapidly in freshly fumigated soil, to establish high populations in the root zone of the host, and to interact with the pathogen to reduce the incidence of infection or disease. The inoculum levels of the pathogen at which 50% of the tomato plants were infected were 300, 900 and 6500 chlamydospores/g of soil that had been fumigated previously, not fumigated, or fumigated and infested with antagonists, respectively. In greenhouse experiments the mean lesion length on stems increased as the inoculum density was increased in fumigated soil; lesion length, however, did not increase as the inoculum density was increased in fumigated soil with antagonists added. [The antagonists used were Penicillium funiculosum, Trichoderma harzianum and Aspergillus ochraceus.].