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A Conductivity Assay for Measuring Virulence of Sclerotinia sclerotiorum. H. C. Newton, Department of Plant Pathology, University of Wisconsin, Madison 53706; D. P. Maxwell(2), and L. Sequeira(3). (2)(3)Department of Plant Pathology, University of Wisconsin, Madison 53706. Phytopathology 63:424-428. Accepted for publication 13 October 1972. DOI: 10.1094/Phyto-63-424.

Water-soaking of host tissues is generally associated with infection by Sclerotinia sclerotiorum. The early occurrence of this symptom in advance of the mycelium of certain isolates of the fungus suggested a possible relation of electrolyte loss to virulence. Liquid exuded from damaged tissues accumulated around mycelial inoculum placed on carrot root discs and this infection drop contained electrolytes. Conductivity increased rapidly within the first 10 min after infected discs were placed in distilled water, but the rate of increase declined thereafter. The initial change in conductance was apparently due to electrolytes in the infection drop; subsequent increases in conductance were due to leakage from affected tissues. Electrolyte loss increased as the incubation of infected carrot discs increased from 3 to 24 hr at 20 C; losses were greater from tissues incubated at 20 C than at 16 or 24 C. Five isolates of S. sclerotiorum were rated for virulence in terms of degree of colonization and symptom expression following inoculation of lettuce and bean seedlings, or of excised carrot, turnip, lettuce, and celery tissues. Virulence of most isolates was correlated with the rate and total amount of electrolyte loss from infected carrot discs incubated for 12 hr at 20 C. Total conductance after 10-min immersion in the bathing solution was 40 µmhos for the least virulent isolate vs. 95 µmhos for the most virulent; other isolates had conductance values between these extremes. The procedure appears to provide a reliable estimate of the virulence of different isolates of the fungus.