Characterization of a new subgroup of Rhizoctonia solani anastomosis groupI (AG-1-ID), causal agent of a necrotic leaf spot on coffee

A new foliar disease on coffee leaves was observed in Mindanao, Philippines , in 1996., The symptoms appeared as large circular or irregularly shaped n ecrotic areas with small circular necrotic spots (I mm or less in diameter) usually found around the periphery of the large necrotic areas. Rhizoctoni a solani was consistently isolated from these diseased coffee leaves. Isola tes, obtained were multinucleate (3 to 12 nuclei per hyphal cell), had an o ptimum temperature for hyphal growth at 25 degreesC, prototrophic for thiam ine, and anastomosed with tester isolates belonging to R. solani anastomosi s group 1 (AG-1). Mature cultures on potato dextrose agar (PDA) were light to dark brown. Sclerotia, light brown to brown, were formed on the surface of PDA and covered the whole mature colony culture. Individual sclerotia of ten aggregated into large clumps (3 to 8 turn in diameter) and their color was brown to dark brown. In pathogenicity tests, isolates from coffee cause d necrotic symptoms on coffee leaves, whereas isolates of AG-l-IA (not isol ated from coffee), l-IB, and I-IC did not. The results of analyses of restr iction fragment length polymorphism of ribosomal DNA internal transcribed s pacer, random amplified polymorphism DNA, and fatty acid profiles showed th at R. solani isolates from coffee are a population of AG I different from A G-l-IA, 1-IB, and I-IC. These results suggest that R. solani isolates from coffee represent a new subgroup distinct from AG-l-IA, 1-IB, and I-IC. Anew subgroup ID (AG-1-ID) is proposed.