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Healthy-looking roots (symptomless) of cotton seedlings grown in soil collected from a cotton field were extensively colonized by Fusarium species. The species recovered included F. oxysporum, F. solani, F. equiseti, F. nygamai, F. pallidoroseum and F. moniliforme. F. oxysporum and F. solani were the dominant species. F. nygamai, which was frequently found, proved to be a new species record for the United States. Pathogenicity tests of 97 isolates of Fusarium showed that eleven isolates of F. solani and three of F. oxysporum were pathogenic to cotton and that isolates of F. equiseti, F. nygamai and F. semitectum were not pathogenic. Pathogenic isolates reduced seedling growth and caused necrotic lesions on taproots and secondary roots. Pathogenic isolates of F. solani were more virulent on cotton seedlings than those of F. oxysporum. It seems that pathogenic Fusarium isolates are present on apparently healthy seedling roots, but do not incite disease when environmental conditions are not conducive to disease. F. nygamai produced phytotoxins in culture media and symptoms produced by these phytotoxins could be mistaken for pathogenesis. Therefore, care must be taken in preparing inoculum and in drawing conclusions about the pathogenicity of Fusarium species on cotton plants. |
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