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First Report of the Pathogenicity of Rhizoctonia solani on Salvinia molesta and S. minima in Florida

Salvinia molesta Mitchell (giant salvinia) and S. minima Baker (common salvinia) are exotic aquatic ferns that have invaded drainage basins in Texas, Louisiana, Alabama, Arizona, California, Florida, Georgia, Hawaii, Mississippi, North Carolina, and Oklahoma (2). These ferns rapidly colonize bodies of water and form thick mats, displace native species, disrupt recreational activities like boating and fishing, block drainage and irrigation intakes, interfere with electricity generation, and degrade water quality (1). Patches of water-soaked lesions were observed on the pinnules and rachises of screenhouse-grown S. molesta plants in Florida. Mycelia spread centrifugally from these patches and caused diseased plants to disintegrate and sink. Brown-to-black sclerotia were formed on and around the disintegrated plants. A fungus was consistently isolated from symptomatic tissues of S. molesta plants. Seven-day-old cultures turned buff-colored and produced sclerotia on potato dextrose agar, while cultures on water agar were hyaline and produced black sclerotia. Both types of sclerotia were not differentiated into rind and medulla. The mycelia branched at right angles from the main hyphae, were constricted at the base of the angle, and had a septum after the constriction. Vegetative cells were multinucleate. The fungus was identified as Rhizoctonia solani Kühn (3,4). Koch's postulates were performed to confirm pathogenicity on S. molesta and S. minima. Seven-day-old cultures of R. solani that were grown in potato dextrose broth were filtered through four layers of cheesecloth and washed with distilled water. Fourteen grams of the mycelial residue was suspended in 28 ml of distilled water and macerated in a small blender for 30 s to obtain a mycelial suspension. Healthy S. molesta and S. minima plants grown in screenhouse-tanks were immersed in tap water supplemented with 1 drop per 4 liters of surfactant (Tween 80), rinsed thoroughly, and approximately 40 g of the plants was floated in plastic jars (18.5 cm diameter × 7.5 cm high) filled to a depth of 5 cm with tap water. Three jars each of S. molesta and S. minima were misted with 1.5 ml of the mycelial suspension. Individual jars were covered with a clear plastic lid with a 2.5-cm-diameter hole in the center for ventilation. These jars were placed in a growth chamber maintained at 28 (+1)°C and 12-h fluorescent light cycles. Typical water-soaked lesions appeared on pinnules within 3 to 7 days, spread rapidly, and resulted in disintegration of pinnules and rachises. R. solani was consistently reisolated from symptomatic tissues of both Salvinia species. To our knowledge, this is the first report confirming pathogenicity of R. solani on S. molesta and S. minima. This fungus should be further evaluated as a potential mycoherbicide for control of Salvinia species.

References: (1) K. L. S. Harley and D. S. Mitchell. J. Aust. Inst. Agric. Sci. 47:67, 1981. (2) C. C. Jacono et al. Castanea 66:214, 2001. (3) B. Sneh et al. Identification of Rhizoctonia Species. The American Phytopathological Society, St. Paul, MN, 1991. (4) C. C. Tu and J. W. Kimbrough. Bot. Gaz. 139:454, 1978.