FREQUENCY OF ISOLATION AND THE PATHOGENICITY OF FUSARIUM SPECIES ASSOCIATED WITH ROOTS OF HEALTHY COTTON SEEDLINGS

Healthy-looking roots (symptomless) of cotton seedlings grown in soil collected from a cotton field were extensively colonized by Fusarium s pecies. The species recovered included F. oxysporum, F. solani, F. equ iseti, F. nygamai, F. pallidoroseum and F. moniliforme. F. oxysporum a nd F. solani were the dominant species. F. nygamai, which was frequent ly found, proved to be a new species record for the United States. Pat hogenicity tests of 97 isolates of Fusarium showed that eleven isolate s 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 p athogenic. Pathogenic isolates reduced seedling growth and caused necr otic lesions on taproots and secondary roots. Pathogenic isolates of F . solani were more virulent on cotton seedlings than those of F. oxysp orum. It seems that pathogenic Fusarium isolates are present on appare ntly healthy seedling roots, but do not incite disease when environmen tal conditions are not conducive to disease. F. nygamai produced phyto toxins in culture media and symptoms produced by these phytotoxins cou ld be mistaken for pathogenesis. Therefore, care must be taken in prep aring inoculum and in drawing conclusions about the pathogenicity of F usarium species on cotton plants.