Suppression ofFusarium roseum'Avenaceum' by Soil Microorganisms

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Abstract
F. roseum ''Avenaceum'' is a highly virulent root pathogen of lentils, but severity of disease is suppressed by natural soil. The suppressive effect of natural soil was eliminated by moist heat (60.degree. C for 30 min) treatment, but not by soil treatment with Bay 22555 [sodium p-(dimethylamino)-benzine diazosulfonate], pentachloronitrobenzine or benomyl. Suppression was less apparent when the dosage of inoculum of Avenaceum was increased in soil. Within 8 wk the suppressivity gradually returned to heat-treated soil amended with 1% (wt/wt) nontreated soil. The suppressive effect could not be attributed to induced host resistance. Instead, antagonists act against Avenaceum in the inoculum substrate in soil. Inoculum buried 48-72 h in nontreated soil, then transferred to heat-treated soil, no longer produced disease or supported growth of Avenaceum from the food base into the soil as did similar inocula from treated soil. Isolations from the inoculum substrate recovered from nontreated soil yielded species of common, fast-growing fungi: these included Mucor plumbeus, M. hiemalis, Trichoderma viride and Penicillium spp. which singly and in combinations controlled lentil root rot caused by Avenaceum when added to treated soil. In contrast, several other isolates of soilborne fungi and bacteria did not control the disease. The inhibition of Avenaceum in the inoculum food base apparently was due to competition between Avenaceum and certain common, fast-growing fungi for the food base needed by Avenaceum to produce disease.

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