Identification ofFusarium oxysporumf. sp.apiion the Basis of Colony Size, Virulence, and Vegetative Compatibility

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Abstract
Fusarium oxysporum was isolated from celery [Apium graveolens L. var. dulce (Miller) Pers.], roots showing symptoms of Fusarium yellows disease and from asymptomatic roots. A total of 199 isolates were classified by colony size, virulence on green celery, and heterokaryon, or vegetative, compatibility. Vegetative compatibility was assessed by pairing nitrate nonutilizing mutants. Eighty-two isolates from California, New York, and Michigan [USA] were virulent on green celery in greenhouse pathogenicity tests. These isolates were also of small colony type and vegetatively compatible with a F. oxysporum f. sp. apii race 2 tester strain. Four isolates were vegetatively compatible with F. o. f. sp. apii race 2 and of small colony type but avirulent in greenhouse pathogenicity tests. The remaining 113 isolates, primarily large colony types and avirulent on green celery, were vegetatively incompatible with F. o. f. sp. apii race 2. These data suggest that colony size and vegetative compatibility may be very useful laboratory tests for identifying F. o. f. sp. apii race 2. In our study, these tests would have correctly identified 197 of the 199 F. oxysporum isolates without recourse to pathogenicity tests. These laboratory tests, in conjunction with virulence tests, should allow faster, more precise identification and characterization of the F. o. f. sp. apii race 2 population.