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Screening for Resistance to and Pathogenic Specialization of Fusarium oxysporum f. sp. phaseoli, the Causal Agent of Bean Yellows. Raul de L. D. Ribeiro, Research Assistant, Department of Plant Pathology, University of Wisconsin, Madison 53706; D. J. Hagedorn, professor, Department of Plant Pathology, University of Wisconsin, Madison 53706. Phytopathology 69:272-276. Accepted for publication 31 August 1978. Copyright 1979 The American Phytopathological Society. DOI: 10.1094/Phyto-69-272.

Bean cultivars were root-dip inoculated with suspensions of microconidia of Fusarium oxysporum f. sp. phaseoli and placed in chambers at 20, 24, and 28 C. Although optimum temperature for growth of the fungus in vitro was about 28 C, a variable proportion of plants of a susceptible cultivar (Bush Blue Lake 274) did not develop symptoms of wilt when incubated at 28 C after inoculation. All plants developed symptoms at 20 C even with the lowest inoculum level. The cultivar Preto Uberabinha behaved as resistant regardless of the test conditions employed, whereas the cultivar Tenderette was resistant to inoculum containing 104 conidia/ml, but completely susceptible to that containing 106 conidia/ml. The differential response of bean cultivars and lines inoculated with strains from Brazil, the USA, and The Netherlands allowed the characterization of two pathogenic races of F. oxysporum f. sp. phaseoli. The European and North American strains appeared to represent a single race, and the Brazilian strains a different race. The strains from the USA and Europe were highly virulent on Phaseolus coccineus, whereas the Brazilian strains failed to induce wilting symptoms on this species.

Additional keywords: Phaseolus vulgaris, Fusarium wilt, host reaction.