Comparison of the pathogenicity of some of Rhizoctonia solani isolates on some Phaseolus vulgaris cultivars

Rhizoctonia root rot and crown rot of bean, caused by Rhizoctonia solani is one of the most important diseases of this crop in Iran. In this study, during 2004-2005, plant and soil samples were collected from the main bean-growing areas in Tehran province and totally, 81 isolates of the fungus were obtained.The morphological characteristics of the isolates including the color of colony, production of aerial mycelia, shape and size of sclerotia, the diameter of hyphae and the number of nuclei per cells were surveyed. After staning the nuclei, all of the isolates except two, were characterized as multinucleous. Anastomosis studies showed that 81.3% of the isolates belonged to the anastomose group 4 (AG-4), 16.04% to AG-6 and two isolates to AG-2. The two last groups are first reports from common bean in Iran. Pathogenicity tests were carried out on common bean, Naz cltivar. The inoculum consisted of wheat and barly seeds colonized with the fungus in complete randomized blocks design. We identified high variation in the pathogenicity of isolates. All of the isolates, except those belonging to the anastomosis group 6 (AG-6) wer relatively aggressive. The isolates Rs21 and Rs71 showed the highest and the lowest pathogenicity, respctivly. Sixteen improved cultivars of navy bean, kidney bean, French bean, green bean were tested for resistance to the five highely pathogenic isolates Rs7, Rs18. Rs21, Rs62 and Rs74 in greenhouse. Results showed that, all of the cultivars were susceptible or semi-susceptible to R.solani and just two cultivars Goli and Sayyad kidney bean, showing the average disease severity (2.58 and 2.9 using the1-5 scale) were semi-resistant to the disease.