Isolates of Rhizoctonia solani Can Produce both Web Blight and Root Rot Symptoms in Common Bean (Phaseolus vulgaris L.)

In common bean (Phaseolus vulgaris L.), Rhizoctonia solani Kühn is an important pathogen causing web blight (WB) in the tropics, and it is also a soilborne pathogen causing root rot (RR) worldwide. This pathogen is a species complex classified into 14 anastomosis groups (AG). AG 1-IA, AG 1-IB, AG 1-IE, AG 1-IF, AG 2-2, and AG 4 have been reported to cause WB of the aboveground structures of the plant, while AG 4 and AG 2-2 have been associated with RR. There is limited information, however, concerning the ability of particular isolates of specific AG to cause both diseases in common bean. Nine R. solani isolates, including three AG 1 and three AG 4 WB isolates and three AG 4 RR isolates collected from both leaves and roots, respectively, of common bean in Puerto Rico, were used to evaluate the response of 12 common bean genotypes to WB inoculated using a detached-leaf method and to RR inoculated using a solution suspension of R. solani mycelia in the greenhouse. All R. solani isolates were able to induce both RR and WB symptoms. RR readings were generally more severe than the WB readings. The RR isolate RR1 (AG 4) produced the most severe RR scores. A few bean lines had mean RR scores ≤4.4 for specific R. solani isolates on a scale of 1 to 9, with 1 representing resistant and 9 highly susceptible. However, all of the bean lines had mean RR scores ≥5.0 when inoculated with the isolates RR1, RR2, and RR3, which were determined to be AG 4 in this study. Significant line–isolate interactions were observed for the WB and RR inoculations for the three planting dates, suggesting a differential response of the common bean lines to the pathogen. This genotypic interaction may require bean breeders and pathologists to monitor the virulence patterns of R. solani in specific growing environments, while the compatibility of specific R. solani isolates to both aerial and root tissue needs to be considered for disease control strategies.