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Races of Fusarium oxysporum f. sp. pisi, Causal Agents of Wilt of Pea. G. M. Armstrong, Research Associate, Department of Plant Pathology, University of Georgia, College of Agriculture Experiment Stations, Georgia Station, Experiment 30212; Joanne K. Armstrong, Research Associate, Department of Plant Pathology, University of Georgia, College of Agriculture Experiment Stations, Georgia Station, Experiment 30212. Phytopathology 64:849-857. Accepted for publication 14 January 1974. DOI: 10.1094/Phyto-64-849.

Nineteen isolates of the pea-wilt Fusarium from four countries and five states of the United States were used to inoculate some or all of 27 cultivars of pea. Plants were grown in sterilized sand supplemented with a nutrient solution. Fungus inoculum was poured around the cut roots of the plants. Wounding was not necessary to obtain wilting, however, wounding increased the rate of symptom development and percentage of wilted plants. Ten races, rather than the four designated by the donors as races 1, 2, 4, and 5, were differentiated with seven cultivars. Race separation and a comparison with the results of others was difficult because some seed with the same cultivar name differed in genes for resistance. In cross-inoculations, two races of f. sp. pisi, formerly thought to be races 1 and 2 but classified herein as races 6 and 5, respectively, were nonpathogenic on plants in 49 different genera, species, or cultivars that have been useful in identifying forms and races of F. oxysporum. Fifty-one f. spp. and races of F. oxysporum pathogenic on their respective hosts were nonpathogenic on the susceptible pea cultivars Thomas Laxton or Little Marvel. When new races arise by mutations or recombinations in pathogenic f. spp. of F. oxysporum, one would expect that they could be found most easily in locations where the crop had been grown intensively for many years. This may explain why the four isolates received indirectly as races 1 and 2 from one location are different from their previous designations.