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Common bean (Phaseolus vulgaris L.) production in the humid tropics would benefit from the selection of cultivars with greater levels of resistance to web blight, a devastating disease caused by Rhizoctonia solani Kuhn. This will require screening techniques that are sensitive enough to detect moderate levels of resistance. Greenhouse experiments evaluated the effectiveness of a droplet inoculation technique for screening five [F.sub.5:6] segregating populations for resistance to web blight. Another objective was to estimate the heritability of resistance to web blight in five common bean populations. Significant differences in lesion size were detected among [F.sub.6] lines within each population. Lines with less web blight infection than the resistant parent MUS83 were observed in each population. This suggests that both parents contributed to the resistance of the progeny. Narrow sense heritability estimates for web blight reaction ranged from 0.61 to 0.79. These relatively high heritabilities suggest that selection for smaller lesion size may be effective in earlier generations when MUS83 is used as a source of resistance to web blight. Physiological resistance to web blight, as measured by lesion size by means of the droplet inoculation technique, will need to be combined with disease avoidance traits such as erect architecture to obtain effective resistance in the field. Five lines from the MUS83 x DOR483 population with smaller lesion sizes in the greenhouse had moderate to low levels of web blight infection in field experiments conducted in Puerto Rico and the Dominican Republic., Web blight is caused by Rhizoctonia solani Kuhn, which is the asexual stage of the basidiomycete fungus Thanatephorus cucumeris (Frank) Donk. In the humid lowland tropics of Latin America and [...]

`CIAS-95' red mottled bean (Phaseolus vulgaris L.) (Reg. no. CV-171, PI 603942) was developed and released in 1988 by Centro de Investigacion Agricola del Suroeste (CIAS), Ministry of Agriculture, Dominican [...]

`PC-50' a red mottled bean (Phaseolus vulgaris L.) (Reg. no. CV-173, PI 603944) was released in 1987 by Centro de Investigacion Agricola del Suroeste (CIAS), Ministry of Agriculture (MA), Dominican [...]

`Arroyo Loro Negro' black bean (Phaseolus vulgaris L.) (Reg. no. CV-170, PI 603941) was developed and released in 1998 by Centro de Investigacion Agricolas del Suroeste (CIAS), Ministry of Agriculture, [...]

`JB-178' red mottled bean (Phaseolus vulgaris L.) (Reg. no. CV-172, PI 603943) was developed and released in 1998 by Centro de Investigacion Agricolas del Suroeste (CIAS), Ministry of Agriculture, Dominican [...]

`Anacaona' white bean (Phaseolus vulgaris L.) (Reg. no. CV-169, PI 603940) was developed and released in 1998 by Centro de Investigacion Agricola del Suroeste (CIAS), Ministry of Agriculture, Dominican Republic [...]

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.