Ana Lilia Alzate-Marin

Inheritance of anthracnose resistance in common bean differential cultivar AB 136

Inheritance of anthracnose resistance of the common bean (Phaseolus vulgaris L.) differential cultivar AB 136 to races 89, 64, and 73 (binary system designation) was studied in crosses with the susceptible differential cultivars Michelite (race 89), Mexico 222 (race 64), and Cornell 49-242 (race 73). In each cross two progenitors, the F1, F2, and backcross-derived plants were inoculated with the respective race under environmentally controlled greenhouse conditions. The results indicate that single dominant gene(s) control resistance to races 89 and 64, giving a segregation ratio of 3:1 in the F2, 1:0 in the backcrosses to AB 136, and 1:1 in the backcross to Michelite (race 89), and to Mexico 222 (race 64). For race 73, the following segregation ratios between resistant and susceptible plants were observed: 13:3 in the F2, 1:0 in the backcross to AB 136, and 1:1 in the backcross to Cornell 49-242. Such results suggest that two independent genes may determine resistance of AB 136 to race 73, one dominant (Co-6) and one recessive that is proposed to be assigned co-8. Genotypes Co-6_ or co-8 co-8 would condition resistance, whereas susceptibility would be present in genotypes co-6 co-6 Co-8_. Given the dominant nature of anthracnose resistance genes present in line AB 136 and its resistance to 25 races of Colletotrichum lindemuthianum identified in Brazil by other researchers, we included this cultivar as one of the donor parents in our molecular marker-assisted backcross breeding program, to develop common bean cultivars resistant to anthracnose and adapted to Central Brazil.

Characterization of the anthracnose resistance gene present in Ouro Negro (Honduras 35) common bean cultivar

Ouro Negro (Honduras 35) is a highly productive Mesoamerican black seeded bean cultivar that possesses a major dominant gene conferring resistance to anthracnose (causal organism Colletotrichum lindemuthianum). In this work the anthracnose resistance gene present in Ouro Negro was characterized by studying allelic relationships to the following previously characterized anthracnose resistance genes (cultivars): Co-1 (MDRK), Co-12 (Kaboon), Co-13 (Perry Marrow), Co-2 (Cornell 49-242), Co-3 (Mexico 222), Co-4 (TO), Co-42 (SEL 1308), Co-5 (SEL1360), Co-6 (AB 136), and the resistance genes present in PI 207262 and Widusa. In addition, we determined the resistance spectrum of Ouro Negro in relation to 19 pathotypes of C. lindemuthianum. The allelism tests confirmed that the dominant anthracnose resistance gene present in Ouro Negro is positioned at a locus distinct from those with which it was compared. We propose that this new gene be named Co-10. The inoculation of Ouro Negro with the 19 pathotypes of C. lindemuthianum demonstrated that Co-10 confers resistance to pathotypes 23, 64, 67, 73, 81, 83, 87, 89, 95, 102, 117, 119, 343, 453, 1033, 1545 and 1600. The identification of Co-10 is an important contribution to bean breeding programs that are in constant need of new sources of resistance to anthracnose.

Allelic relationships for genes that confer resistance to angular leaf spot in common bean

Angular leaf spot is one of the major diseases of the common bean. The extensive genetic variability of this pathogen requires the constant development of new resistant cultivars. Different sources of resistance have been identified and characterized. For the State of Minas Gerais, Brazil, four main resistance sources were found: Mexico 54, AND 277, MAR 2 and Cornell 49-242. Independent characterization of these genotypes demonstrates that resistance in all four sources is dominant and monogenic. However, there are no studies on the relationship and independence of these genes. In the present work, allelism tests were carried out to understand the relationship among the resistance genes present in these four resistance sources. The data revealed a much higher complexity in the resistance inheritance of these genes than previously reported. It was demonstrated that Cornell 49-242 possesses a dominant gene (Phg-3); Mexico 54 possesses three genes, denominated Phg-2, Phg-5 and Phg-6. In MAR 2, two genes were found, one independent designated Phg-4 and the other, an allelic form of Phg-5, denominated of Phg-52. Allelic forms were also found in AND 277, Phg-22, Phg-32 and Phg-42. These results have special importance for breeding programs aiming to pyramid resistance genes.

Identification of a RAPD marker linked to the Co-6 anthracnose resistant gene in common bean cultivar AB 136

The pathogenic variability of the fungus Colletotrichum lindemuthianum represents an obstacle for the creation of resistant common bean (Phaseolus vulgaris L.) varieties. Gene pyramiding is an alternative strategy for the development of varieties with durable resistance. RAPD markers have been proposed as a means to facilitate pyramiding of resistance genes without the need for multiple inoculations of the pathogens. The main aims of this work were to define the inheritance pattern of resistance present in common bean cultivar AB 136 in segregating populations derived from crosses with cultivar Ruda (susceptible to most C. lindemuthianum races) and to identify RAPD markers linked to anthracnose resistance. The two progenitors, populations F1 and F2, F2:3 families and backcross-derived plants were inoculated with race 89 of C. lindemuthianum under environmentally controlled greenhouse conditions. The results indicate that a single dominant gene, Co-6, controls common bean resistance to this race, giving a segregation ratio between resistant and susceptible plants of 3:1 in the F2, 1:0 in the backcrosses to AB 136 and 1:1 in the backcross to Ruda. The segregation ratio of F2:3 families derived from F2 resistant plants was 1:2 (homozygous to heterozygous resistant). Molecular marker analyses in the F2 population identified a DNA band of approximately 940 base pairs (OPAZ20940), linked in coupling phase at 7.1 cM of the Co-6 gene. This marker is being used in our backcross breeding program to develop Ruda-derived common bean cultivars resistant to anthracnose and adapted to central Brazil.

Use of RAPD-PCR to identify true hybrid plants from crosses between closely related progenitors

RAPD-PCR molecular markers were used to identify common bean and soybean hybrid plants derived from crosses between closely related progenitors, with no apparent phenotypic differences. Primers OP-F12 and OP-0O3 were used to identify true hybrids derived from crosses between common bean cultivars Ruda (A 285) and AN 910408, and soybean cultivars Cristalina and Bossier, respectively. Each primer generated one polymorphic DNA band which was present in the male progenitor and absent in the female progenitor. As RAPD bands are normally inherited as dominant characters, the presence of these bands in the F1 plants confirmed their status.

Co-evolution model of Colletotrichum lindemuthianum (melanconiaceae, melanconiales) races that occur in some Brazilian regions

Colletotrichum lindemuthianum, the causal agent of anthracnose in the common bean (Phaseolus vulgaris L.), displays a high level of virulence diversity, which explains the large number of existing pathotypes. Several lines of evidence indicate that such diversity is, at least in part, due to plant and pathogen co-evolution. A co-evolution model based on the binary classification of 25 races identified in Brazil by inoculation of differential cultivars and random amplified polymorphic DNA (RAPD) data is proposed. In this model, races 8 and 64 that infected bean cultivar Cornell 49-242 (Are gene) and Mexico 222 (Mexico I gene) are considered to be sources of two important evolutionary routes. Inferences about undescribed races from Brazil could be made.