First Report of Rhizoctonia solani AG-7 on Potato in Mexico.

Among isolates of Rhizoctonia solani Kühn collected as sclerotia from potato (Solanum tuberosum L.) tubers harvested from the Toluca Valley in 1995 was one isolate (MexND) of anastomosis group 7 (AG-7). Virulence of this and other isolates of R. solani representing AG-3 (four isolates) or AG-7 (four isolates), was determined on developing potato plants (cv. Russet Burbank) in growth chamber studies at 10 and 21°C by methods used previously (1). A control treatment was inoculated with sterile disks of potato dextrose agar (PDA). Seed tubers were placed on a layer of pasturized sand-soil mix near the bottom of 6- × 25-cm black plastic tubes, then covered with 2 to 3 cm of the sand-soil mix. Seed tubers used in virulence tests carried no R. solani as they had been surface disinfested by immersing for 2 min in a 1.85% solution of formaldehyde. Inoculum, in the form of one 7-mm agar disk cut from the growing edge of appropriate colonies growing on PDA, was placed 2 to 3 cm above the seed and covered with 7 to 10 cm of sand-soil mix (1). Each treatment had five replications. Plants were harvested after control plants had emerged, 29 days at 21°C and 36 days at 10°C, and damage to roots and shoots was assessed according to Carling and Leiner (1). All four isolates of AG-3, one from the U.S. and three from Mexico, caused major damage to roots and shoots at both temperatures. The two isolates of AG-7 from the U.S. caused superficial discoloration to shoots at both temperatures but the two isolates of AG-7 from Japan caused no damage to shoots. None of the four isolates of AG-7 damaged roots at either temperature. MexND caused no damage to roots or shoots at 21°C but produced lesions (< 1 mm in diameter) on roots and shoots at 10°C. Thus, MexND caused less damage to potato than AG-3, more damage than other isolates of AG-7, and appears more aggressive at lower temperatures. This is the first reported occurrence of R. solani AG-7 in Mexico and its first reported recovery from a potato tuber-borne sclerotium. Reference: (1) D. E. Carling and R. H. Leiner. Phytopathology 80:930, 1990.