Inhibition ofFusarium oxysporumf. sp.dianthiby Iron Competition with anAlcaligenessp.

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Abstract
Alcaligenes sp. strain MFA1, isolated from the roots of carnations [Dianthus caryophyllus] grown in a Fusarium wilt-suppressive soil and cultured on low-iron media, produced a siderophore that inhibited microconidium germination and germ tube growth of Fusarium oxysporum f. sp. dianthi. Germination of chlamydospores of F. o. dianthi in soil amended with glucose and asparagine at 0.2 mg/g each to simulate plant exudate was less than 12% in the presence of MFA1 population densities of 103 colony-forming units (cfu) per gram of soil in comparison to 28% without MFA1. Inordinately high population densities of MFA1 at 109 cfu per gram of soil did not significantly further suppress chlamydospore germination and, in the presence of higher nutrient levels, no suppression was observed. The inhibitory effect of MFA1 on chlamydospore germination in soil was related to siderophore production by the bacterium. The addition of 10-3 M FeCl3 to soil reversed the inhibitory effects of the bacterium, and four siderophore-negative mutant strains of MFA1 did not suppress chlamydospore germination, although they survived in soil as well as the parental strain. When carnation seeds were inoculated with a suspension of MFA1 at 108 cfu/ml, the frequency of root colonization by F. o. dianthi on 10-day-old roots was 24% lower than in untreated controls. The number of colonies of F. o. dianthi isolated from roots of rooted cuttings inoculated with MFA1 1 mo after treatment was 55% lower than that of uninoculated controls. The subsequent disease severity of MFA1-treated plants 3 and 4 mo after planting were 45 and 35% less, respectively, than that of control plants, but no significant difference was detected at 5 mo.