1CIAT, PO Box 6247, Kampala, Uganda; 2CIAT, AA 6713, Cali, Colombia; 3INERA, Mulungu, Democratic Republic of Congo

Background and objectives
Angular leaf spot (ALS) caused by Phaeoisariopsis griseola is one of the most important diseases of common beans in Africa. Host resistance, which is an effective and practical management option to small-scale farmers, is often rendered ineffective due to occurrence of races. Two distinct groups of races; the Andean, associated with large-seeded Andean beans and, the Mesoamerican, associated with small- and medium-seeded Mesoamerican beans have been identified in Latin America. It is suggested that parallel co-evolution of P. griseola with its host in the two centres of origin, the Andes and the Middle America, resulted in the two race groups. The objectives of these studies were to determine pathogen diversity of P. griseola in Africa and how it relates to the genotypes grown. The information is expected to be useful in developing and implementing host resistance strategies to manage the disease.

Material and Methods
. A total of 202 isolates from 10 countries in eastern, central and southern Africa were studied using the RAPD technique. Virulence phenotypes of 90 isolates were characterized on the basis of a set of 12 ALS differential bean cultivars: six large-seeded of Andean origin and, six small- or medium-seeded of Mesoamerican origin. Data analysis matrixes were generated for the presence or absence of bands in RAPDs and compatible or incompatible reactions in virulence. Dendrograms were created using the SAHN program of NTSYS-pc package (version 1.80).


Results and conclusions


Both the molecular technique and virulence analysis grouped isolates of P. griseola into two groups. One group comprised of isolates obtained mainly from small seeded common bean cultivars of the Middle American origin, while the other comprised of isolates obtained mainly from large-seeded cultivars of the Andean origin. Our results indicate that the two groups are similar and correspond to the Mesoamerican and Andean groups identified in Latin America. RAPDs, also classified isolates that were classified as Andean or Mesoamerican by virulence, as Andean and Mesoamerican. Isolates representing both Andean and Mesoamerican groups were shown to occur in six countries and in some cases were obtained from the same site. An Andean sub-group that we have designated as the Afro-Andean could only be identified on the basis of virulence, after giving compatible reactions on some of the Mesoamerican differentials. Occurrence of this sub-group is unique to Africa and probably represents a divergent form of co-evolution with the bean host, influenced by growing together of the two phaseolus bean genepools