Effects of Crop Residues and Colonization of Plant Tissues on Propagule Survival and Soil Populations ofFusarium oxysporumf. sp.apiiRace 2

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Abstract
Laboratory-grown conidia and chlamydospores of Fusarium oxysporum f. sp. apii race 2 persisted in soil-filled nylon bags for 2 yr in fallow organic soil, but populations had declined to 10% of the initial density of 1 .times. 106 colony-forming units of F. o. f. sp. apii per gram of soil. Colonization of highly susceptible and moderately resistant celery plant tissues by F. o. f. sp. apii in naturally infested soils afer 3 mo. was about equal in feeder roots, primary roots, and crown tissue but was greater in the aboveground tissues of the highly susceptible plants. Symptomless root colonization by a pathogenic orange-colored mutant was greater in species of monocots and in carrot than in susceptible celery. In a field soil naturally infested with F. o. f. sp. apii, colonization of roots of sweet corn, cabbage, onion, lamb''s-quarters, smartweed, barnyardgrass, and purslane was demonstrated. Soil populations of the orange mutant were suppressed in soil supplemented with residues of onion or mint, but celery-supplemented soils contained densities of the orange mutant isolate that were equal to the non-residue-supplemented soil for the 3-wk period. Fusarium yellows severity in celery increased in plants grown in soils supplemented with celery residues but was lower in celery grown in soils supplemented with onion residues. Root exudates of 11 crop plants stimulated chlamydospore germination equally in a chlamydospore-agar-soil overlay system. Aqueous extracts from onion residue caused greater germination than extracts from celery, mint, rye, or sudax residues.