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Critical Iron Level Associated with Biological Control of Fusarium Wilt. L. A. Simeoni, Graduate research assistant, Department of Agronomy, Colorado State University, Fort Collins 80523; W. L. Lindsay(2), and R. Baker(3). (2)Professor, Department of Agronomy, Colorado State University, Fort Collins 80523; (3)Professor, Department of Plant Pathology and Weed Science, Colorado State University, Fort Collins 80523. Phytopathology 77:1057-1061. Accepted for publication 14 January 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-1057.

Chlamydospore germination in vitro of Fusarium oxysporum f. sp. cucumerinum was significantly decreased over the control in the presence of siderophore-producing Pseudomonas putida A12 at Fe3+ activities between 10-22 M and 10-28 M. A nonsiderophore-producing mutant, A1/UV/AB-6, had no effect on fungal spore germination with decreasing Fe activity. Microbial turnover during experimental procedures released 6 g of Fe per kilogram of solution. When this Fe was available to the fungal chlamydospores, spore germination was 21% higher than when the released Fe was trapped by chelation with ethylenediamine di-o-hydroxyphenylacetic acid (EDDHA). Toxic trace elements in the nutrient solution decreased bacterial survival unless enough EDDHA was present to bind the elements and reduce their effects. These results suggest that the critical level of Fe3+ activity in this system, below which chlamydospore germination was suppressed, was between 10-19 M and 10-22 M and that optimal suppression took place between Fe3+ activities of 10-22 M and 10-27 M.

Additional keywords: antagonism, biological control, suppressive soils.