PATHOGENICITY, MORPHOLOGY AND ISOZYME VARIABILITY AMONG ISOLATES OF APHANOMYCES SPP FROM WEEDS AND VARIOUS CROP PLANTS

In a root disease survey in the west part of Scania in southern Sweden, Aphanomyces spp. were obtained from roots of spinach, sugar beet, red beet, barley, oats, pea, and also from eight weed species found in spinach and pea fields. Morphological studies, pathogenicity tests performed on spinach, sugar beet, pea, barley, oats, tomato and radish, and isozyme analysis revealed four distinct groups or species. A. euteiches was isolated from pea and Lamium amplexicaule. The isolates were highly pathogenic on peas, but also slightly pathogenic on spinach. A. cochlioides was found on roots of sugar beet, red beet, spinach and Chenopodium album. This species was highly pathogenic on sugar beet and somewhat less so on spinach. The Aphanomyces sp. most frequently found on spinach roots, but also isolated from roots of sugar beet, red beet, barley, oats, C. album, Polygonum convolvulus, Senecio vulgaris, Stellaria media, Urtica urens, Veronica agrestis and Viola arvensis was identified as A. cladogamus. These isolates were, however, not pathogenic on tomato. They were moderately to highly pathogenic on spinach and weakly pathogenic on sugar beet. The fourth group of isolates had considerably larger oogonia and oospores than the other three groups. This unidentified Aphanomyces species was isolated from roots of spinach and barley. It was moderately pathogenic on spinach and sugar beet, and weakly pathogenic on barley. Several isolates from each group were compared by isozyme analysis. The intraspecific variation was low, and two enzymes, glucose-6-phosphate dehydrogenase and malate dehydrogenase, clearly differentiated between the four species.