First Report of Rhizoctonia solani AG-3 on Potato in Catalonia (NE Spain).

During the last 3 years, potatoes (Solanum tuberosum L.) with symptoms typical of Rhizoctonia canker and black scurf have been found in Catalonia; from 21,546 samples submitted to the official diagnostic laboratory (Laboratori de Sanitat Vegetal, DARP, Generalitat de Catalunya), 20% were infected with R. solani. Also, basidia and basidiospores of Thanatephorus cucumeris (A.B. Frank) Donk, the teleomorph of R. solani, have been observed frequently on stems near the soil surface. Previous work (1) has reported R. solani in the same geographic area, but anastomosis groups (AG) were not identified. Isolates (103) were collected from damaged roots, mature or progeny tubers, soil, debris, and weeds (roots and stem of Oxalis latifolia, Diplotaxis eurocoides, Solanum nigrum, Sorghum halepense, and Chenopodium album) in fields where potatoes were cultivated. All isolates were multinucleate, variable in number of nuclei (mean = 12, range = 7 to 19), and with morphological features typical of R. solani (3). Following the procedure of Parmeter et al. (2), all isolates (84) were identified as R. solani AG-3. Thirty-two isolates were evaluated for pathogenicity on potato plants at 20 to 25°C in the greenhouse. Germinated tubers (sprout length 3 cm) free of Rhizoctonia were placed in pea-sand mix (3:1, vol/vol; Compo Sana Universal, Barcelona, Spain) at 5-cm depth and inoculated by placing a 5-mm-diameter plug cut from 1-week-old cultures of each isolate adjacent to the sprouts. Potatoes were harvested 20 days after planting, and both emerged and nonemerged sprouts were examined for lesions and rated using a 0 to 4 scale. The AG-3 isolates caused major damage to roots and shoots (average disease rating 3.4). R. solani isolates were recovered from diseased plants and identified as AG-3. Preliminary field studies in Catalonia (Baix Llobregat and Maresme, unpublished) indicate that disease severity is not dependent on cultivar or related to yield reduction (10 to 65%), but is related to soil and seed contamination with sclerotia and mycelium. This is the first reported occurrence of R. solani AG-3 on potato in Catalonia. References: (1) Cebolla et al. Actas del III Congreso de la SECH, 1989. (2) Parmeter et al. Phytopathology 59:1270, 1969. (3) Sneh et al. Identification of Rhitzoctonia species. American Phytopathological Society, St. Paul, MN, 1991.