DisplayTitle

Characterization of a New Subgroup of Rhizoctonia solani Anastomosis Group 1 (AG-1-ID), Causal Agent of a Necrotic Leaf Spot on Coffee

First author: The United Graduate School of Agricultural Science, Gifu University, Yanagido 1-1, Gifu 501-1193, Japan; second and third authors: College of Agriculture, University of Southern Mindanao, Kabacan, Cotabato, Philippines; fourth author: Plant Pathology Laboratory, Institute of Plant Breeding, University of the Philippines at Los Banos, College, Laguna 4031, Philippines; fifth author: Molecular Genetics Research Center, Gifu University, Yanagido 1-1, Gifu 501-1193, Japan; and sixth and seventh authors: Laboratory of Plant Disease Science, Faculty of Agriculture, Gifu University, Yanagido 1-1, Gifu 501-1193, Japan

A new foliar disease on coffee leaves was observed in Mindanao, Philippines, in 1996. The symptoms appeared as large circular or irregularly shaped necrotic areas with small circular necrotic spots (1 mm or less in diameter) usually found around the periphery of the large necrotic areas. Rhizoctonia solani was consistently isolated from these diseased coffee leaves. Isolates obtained were multinucleate (3 to 12 nuclei per hyphal cell), had an optimum temperature for hyphal growth at 25°C, prototrophic for thiamine, and anastomosed with tester isolates belonging to R. solani anastomosis 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 often aggregated into large clumps (3 to 8 mm in diameter) and their color was brown to dark brown. In pathogenicity tests, isolates from coffee caused necrotic symptoms on coffee leaves, whereas isolates of AG-1-IA (not isolated from coffee), 1-IB, and 1-IC did not. The results of analyses of restriction fragment length polymorphism of ribosomal DNA internal transcribed spacer, random amplified polymorphism DNA, and fatty acid profiles showed that R. solani isolates from coffee are a population of AG-1 different from AG-1-IA, 1-IB, and 1-IC. These results suggest that R. solani isolates from coffee represent a new subgroup distinct from AG-1-IA, 1-IB, and 1-IC. A new subgroup ID (AG-1-ID) is proposed.

Additional keywords: Coffea arabica , differentiation , fatty acid methyl ester , Thanatephorus cucumeris .