The Role of Competition for Iron and Carbon in Suppression of Chlamydospore Germination ofFusariumspp. byPseudomonasspp.

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Abstract
Inhibition of germination of chlamydospores of various formae speciales of Fusarium oxysporum in soil was correlated with production of siderophores by fluorescent pseudomonads and inhibition was totally or partially reversed by adding Fe to the system. Germination of some isolates of F. oxysporum, F. solani, and F. graminearum was also inhibited at low levels by a mutant of Pseudomonas spp. not producing siderophores and such inhibition was not reversed by added Fe, suggesting that competition for carbon by the bacterial biomass was instrumental in inhibition. Siderophore production was reduced proportionally more than cell division when fluorescent pseudomonads were grown in dilution series of synthetic medium containing low Fe. This suggested that in plants producing lower amounts of exudates in the rhizosphere, pseudomonads would produce lower amounts of siderophores. Germination of chlamydospores of Fusarium spp. in rhizospheres of nontreated plants growing in raw soil was inversely correlated with the amount of germination suppression in rhizospheres containing siderophore-producing pseudomonads. Thus, when there were more nutrients exuded in the rhizosphere, there was greater inhibition of germination of chlamydospores when pseudomonads were added to conducive soil due (by extrapolation) to production of greater amounts of siderophores by the pseudomonads. Inhibition of chlamydospore germination in soil by added pseudomonads was greater with one initial application of 1:4 glucose-asparagine (60-250 .mu.g of glucose per gram of soil) than by an equivalent total application divided into six pulses each delivered at 2-hr intervals. The amount of inhibition induced by fluorescent pseudomonads in pulsed treatments was similar to that observed in rhizospheres of plants treated with these antagonists. Addition of available Fe reduced siderophore activity and reduced inhibition of chlamydospore germination. Disease suppressiveness in soils was not correlated with siderophore production in liquid culture by pseudomonads in systems employing F. solani; however, significant direct correlations among these parameters was observed for F. oxysporum formae speciales. Chlamydospores of F. solani f. sp. phaseoli had greater weights, volume and Fe content per propagule than those of F. oxysporum f. sp. cucumerinum. Thus, the larger chlamydospores of F. solani may not require exogenous Fe and energy for complete germination and successful infection.