A backcross breeding program between commercial common bean cultivars (VC3 and M20) and sources of resistance (Ex-Rico 23 and G122) was conducted with a view toward selecting carioca (beige with brown stripes) progenies resistant to white mold. Forty-eight progenies (27 F2:6 BC1 and 21 F1:5 BC2) were evaluated for yield, growth habit, grain type and pathogen response by two methodologies for assessment ("straw test" and "oxalic acid"). The methods were effective in discriminating the progenies, showing differing results, for they may assess different resistance mechanisms. Thus, they should be used together. Simultaneous selection for yield, growth habit, grain type and white mold resistance proved to be viable. The most appropriate strategy was to use lower intensities of selection and prioritize traits such as resistance and grain type, which are essential for commercial acceptance. Two progenies proved their superiority for the breeding program for they combine the traits of resistance and favorable grain type.

O presente trabalho teve como objetivo determinar o número mínimo de plantas por plota para avaliar a resistência do feijoeiro ao mofo branco em campo. Treze cultivares foram inoculadas com seis isolados de Sclerotinia sclerotiorum. O delineamento utilizado foi o de blocos casualizados com três repetições, com plota constituída de quinze plantas por metro linear. A inoculação foi realizada utilizando o método do straw test. Oito dias após a inoculação foi realizada a avaliação utilizando uma escala diagramática de 1 (plantas assintomáticas) a 9 (morte da planta). Foram utilizados os métodos: curvatura máxima, modelo segmentado linear, modelo segmentado quadrático e o modelo CV relativo, para determinar o número mínimo de plantas por plota. Houve diferença significativa entre as cultivares, entre os isolados e entre a interação cultivares x isolado. Observou-se que oito plantas por plota é um número adequado para avaliar a reação de feijoeiro ao mofo branco.

ABSTRACT This study aimed to determine the minimum number of plants per plot to assess the field resistance in common bean to white mold. Thirteen cultivars were inoculated with six isolates of Sclerotinia sclerotiorum and evaluated in a randomized block design with three replications and plots consisting of 1-m rows with 15 plants. Plants were inoculated by the straw test as proposed by Petzoldt and Dickson (1996), to evaluate partial resistance in a greenhouse. Eight days after inoculation the disease severity was evaluated on a 1-9 diagrammatic scale, where 1 = asymptomatic plants to 9 = plant death. To determine the minimum number of plants per plot, the following methods were used: maximum curvature, segmented linear model, quadratic segmented model and the relative CV model. There were significant differences among cultivars and isolates and no significant cultivar - isolate interaction. It was observed that eight plants per plot is an adequate number to assess the reaction of common bean to white mold.