Vegetative-Compatibility Grouping ofFusarium oxysporumf. sp.vasinfectumfrom Tissue and the Rhizosphere of Cotton Plants

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Abstract
Nitrate-nonutilizing (nit) mutants were produced from 12 isolates of Fusarium oxysporum f.sp. vasirfectum race 3, collected from five sites in two regions in Israel. Complementation (heterokaryon) tests showed that all of the isolates belonged to a single vegetative-compatibility group (VCG), and two mutants were chosen as its tester. Additional isolates of Fusarium from root tissue and the rhizosphere of diseased susceptible (cv. Pima S-5) and healthy resistant (Pima-type cv. F-27) cotton plants (Gossypium barbadense), growing in soils naturally infected with F. o. vasinfectum, were analyzed for pathogenicity and vegetative compatibility with the testers. A total of 631 Fusarium isolates, obtained from three sites in two separate regions, were tested. All the nit mutants of all the pathogenic isolates formed heterokaryons with the testers, indicating that they belonged to the same VCG. None of the nonpathogenic isolates was vegetatively compatible with the testers. Pathogenesis ocurred in 90-94% of isolates from tissues and 3-87% of isolates from the rhizosphere of susceptible plants. Pathogenesis occurred in 1% of isolates from tissues and 4% of isolates from the rhizosphere of the resistant Pima-type cultivar F-27. Population density of F. o. vasinfectum in the tissues of the susceptible plants ranged from 27,600 to 346,000 colony-forming units (cfu) per g of tissue, compared with 33-7,400 cfu per gram of soil in the rhizosphere of the same plants. The nit mutants retained pathogenicity to cotton. The F. o. vasinfectum testers were incompatible with testers of four other formae speciales of F. oxysporum.