Extent and pattern of early soybean seedling colonization by Fusarium oxysporum and F. solani in naturally infested soil

Abstract

Fusarium oxysporum colonized soybean [Glycine max (L.) Merrill cv. Essex] cotyledons 1 day after planting in a naturally infested soil and was present in 25% of tissue segments after 4 days. Both F. solani and F. oxysporum colonized the lower hypocotyl and emerging roots 2 and 3 days after planting, respectively. After 4 days, both fungi were found colonizing the elongating, upper portion of the hypocotyl, but the root tip remained free of the two fungi. The hypocotyl-root transition zone had the highest frequency of colonization (20–28% of 2 mm tissue segments) by each species at 4 days. F. solani had higher inoculum efficiencies for colonization than F. oxysporum. Tissue pasteurization tests indicated most early colonization is confined to the outer portions of tissues. The relative frequency and pattern of isolation of F. oxysporum and F. solani from lesions paralleled that of early colonization results, but only a small portion of hypocotyl and root colonization sites developed into lesions. These results support the hypothesis that non-pathogenic and pathogenic F. oxysporum and F. solani strains are primary and frequent colonizers of soybean tissues. Non-pathogenic isolates may have potential for biocontrol.