A common bean (Phaseolus vulgaris L.) breeding program for resistance to Pseudocercospora griseola (Sacc.) Crous & U. Braun (angular leaf spot) using recurrent selection has been conducted since 1998 in Brazil. The selection has been carried out in the field with high natural occurrence of the pathogen. One recurrent selection cycle has been conducted per year. The efficiency of artificial inoculation of P. griseola and early assessment of angular leaf spot severity in common bean progenies of this program were evaluated. For this, 254 and 166 S 0:1 progenies from Cycles XVI and XVIII, respectively, were assessed for angular leaf spot severity in field, with natural occurrence of the pathogen and in greenhouse with artificial inoculation of P. griseola. Most progenies showed a high level of resistance to angular leaf spot, indicating the efficiency of recurrent selection. Artificial inoculation of P. griseola in the greenhouse showed promise for the selection of resistant plants in a recurrent selection program allowing breeders to carry out two cycles per year. A scheme was proposed that combined the advantages of early-generation selection with artificial inoculation of the pathogen in greenhouse with the advantages of selection in field with natural occurrence of P. griseola in the dry season, increasing genetic gain per unit time.
This study aimed at evaluating the variability in resistance of common bean progenies and plants to diseases caused by Pseudocercospora griseola and Colletotrichum lindemuthianum. It also sought to infer whether S 0 plants were heterozygous for the possible resistance loci and the frequency resistant plants varies with the bulk opening generation. Progenies from the fourteenth yield recurrent selection cycle were used. One hundred S 2:7 and one hundred S 5:9 progenies derived from ten S 0:2 plants were evaluated. Seedlings were inoculated separately with the 63-63 race of P. griseola and race 65 of C.lindemuthianum. Variance analyses were conducted for disease severity scores and both the genotypic and phenotypic parameters were estimated. The progenies showed variable resistance levels to the disease caused by both pathogens. Such variability was detected among plants of the population and among descendants of some S 0 plants, thus suggesting that they were likely heterozygous for the resistance loci. The frequency of resistant progenies/plants in the population was high, even though the selection was not specific these pathogens. The time of bulk opening did not significantly affect the reaction to the disease by both pathogen. This information is useful for future recurrent selection cycles.
The angular leaf spot (ALS), caused by the fungus Pseudocercospora griseola, is one of the most notable diseases of common beans (Phaseolus vulgaris). The most effective strategy to control ALS is quantitative disease resistance provided by major and minor genes. One breeding strategy for obtaining lines with durable resistance to P. griseola is a recurrent selection that aims to gradually assemble favorable alleles. In this study, common beans were artificially inoculated with P. griseola and grown in a greenhouse under a recurrent selection program that aims to accelerate the breeding cycle and assimilate resistant conferring genes per generation. From the initial population, three cycles of evaluation, selection, and intercross were carried out in a greenhouse and the most resistant plants were phenotypically selected. Plants at the V2 stage were inoculated with an isolate of P. griseola, race 63-63. Selected plants were transplanted into pots and intercrossed to reap the next cycle of selection. Progenies obtained from the base population (C0) and subsequent cycles (CI, CII, and CIII) were assessed for the degree of the symptoms for ALS under greenhouse and field conditions. The rate of genetic progress per breeding cycle was - 14.8% and - 5.3% for the plants grown in greenhouse and field, respectively. Artificial inoculation with P. griseola allowed three recurrent selection cycles per year, signifying a promising method to obtain ALS-resistant common bean lines in a short period.
Angular leaf spot (ALS), a typical disease in common beans (Phaseolus vulgaris), is caused by the fungus Pseudocercospora griseola (Singh & Schwartz, 2010). Conidia are mainly spread by wind; however, water droplets, agricultural implements and contaminated seeds also facilitate pathogen transmission. The optimum conditions for infection include humidity and a temperature of 24°C. The conidia can germinate in about 3 h, penetrating the leaves through the stomata within 2 days. The whole tissue is colonized by the pathogen after 3-7 days and lesions can be observed 8-15 days after infection. Symptoms of ALS appear primarily on leaves and pods (Liebenberg & Pretorius, 1997). The lesions may coalesce, causing premature defoliation that compromises pod
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