The 2014 ADP nursery, comprised of 410 entries and three replications, was planted under field conditions in Potchefstroom and Cedara, and in the greenhouse in Potchefstroom, by Dr. Deidre Fourie of the South African Agriculture Research Counil (ARC). The ADP nursery in Potchefstroom was inoculated with race 6 of the halo blight (HB) pathogen. HB is a serious disease of common bean in many countries of Eastern and Southern Africa. Race 6 is important because it overcomes all HB monogenic resistance ‘R’ genes. The HB disease in Potchefstroom in 2014 was consistently distributed throughout the entire field with many entries displaying high levels of HB severity (susceptibility) and some entries with resistance to HB. Thus, this was an excellent nursery for HB resistance screening. The most significant finding was the identification of 29 ADP entries with resistance to HB. Any ADP accession with resistance to HB at Potchefstroom is a potential new source of HB resistance to race 6.
These findings form a foundation of genotype response data that can be used for the development of Andean bean cultivars with resistance to HB in Africa and other locations worldwide. These findings also provide a base for additional studies of the HB disease. The ADP entries planted at Cedara exhibited a broad range of response to rust and angular leaf spot (ALS) diseases. Both diseases, that are widespread and economically important in Africa, occur naturally at Cedara. The reaction of known cultivars at Cedara indicated that 2014 was also an excellent year for rust and ALS resistance screening.
A total of 89 ADP entries were identified as resistant to ALS and 84 ADP entries as resistant to rust. Moreover, a total of 42 entries were resistant to both diseases. Equally important, of the 84 entries with resistance to rust at Cedara in 2014, 29 entries were also resistant to rust at Cedara in 2013. It was interesting and important that the six Mesoamerican ALS differential cultivars were highly resistant to rust and ALS at Cedara in 2014. Conversely, all six Andean ALS differential cultivars were either susceptible to rust and ALS in 2014 or had at least intermediate levels of severity to both diseases.
These results, as observed at Cedara in previous years, suggest that Andean cultivars in South Africa and other countries of Eastern Africa, are much more likely to be susceptible to ALS and rust, while the Mesoamerican cultivars are more likely to be resistant to both diseases. These results also suggest that the isolates of the ALS and rust pathogens present in the field at Cedara co-evolved with bean primarily of Andean origin since they generally infect Andean bean cultivars, but not Mesoamerican beans. Similar results are likely to be observed for rust, angular leaf spot, and anthracnose in other countries of Eastern and Southern Africa, where Andean beans predominate.
The ADP cultivars with resistance to rust and ALS are important candidates for further rust evaluation to determine if they harbor new Andean disease resistance genes that can be used to broaden the genetic base of common bean for resistance to highly variable pathogens. Finally, one of the most important discoveries of the evaluation in Potchefstroom and Cedara is the identification of 15 cultivars with resistance to the three diseases, HB, ALS, and rust.