Pseudocercospora griseola (angular bean leaf spot)

Spots originating on the lower leaf surface are delimited by the veins and veinlets and develop into grey lesions which later turn light-brown. Lesions may be surrounded by a chlorotic halo but they lack a coloured border. The striking angularity of the spots is a diagnostic feature of P. griseola. The lesions may be so numerous as to cause premature defoliation.Stem lesions caused by P. griseola are elongate and brown.Lesions on pods are less frequent than on leaves. They are oval to circular, superficial at first, with nearly black margins and reddish-brown centres, both of which are sharply defined. The spots vary in size and, ultimately, may become so crowded that they coalesce and occupy the width of the pod.Dark stromata are produced on lesions and, in humid weather, many synnemata may develop bearing conidia. For more information, see Cardona-Alvarez and Walker (1956), Hagedorn and Wade (1974) and Saettler (1991).

Cultural Control

Cultural measures to control P. griseola include crop rotation, possibly with 2 years between bean crops to allow for decomposition of plant residues, and the use of disease-free seeds.

Host-Plant Resistance

Several varieties and lines of beans and cowpeas resistant to P. griseola exist (Schwartz et al., 1982; Ponte and Almeida, 1994; Teverson et al., 1994). However, resistance of leaves and of pods are not always correlated (Correa and Saettler, 1987). Msuku et al. (1990) detected more resistance in indeterminate than determinate bean lines and also in small-seeded versus large-seeded lines. The pathogen exhibits a high degree of pathogenic variation: at least 14 races are known to exist (Saettler, 1991; Lacerda et al., 1994; Guzmán et al., 1995). Therefore, resistance reactions vary between locations.

P. griseola is primarily of importance in the tropics and subtropics, causing premature defoliation and pod spotting.In Central and Eastern Africa, Phaseolus vulgaris is an important protein source and angular leaf spot is considered one of the most important diseases (Trutman et al., 1992). In Ethiopia the disease caused 50-60% crop loss (Golato and Meossi, 1972).Inoculation experiments in Brazil showed that yield losses of 51-70% can occur when P. griseola and Ascochyta sp. build up a disease complex (Mora et al., 1985). Temperatures of 24°C and alternate periods of high and low relative humidity caused epidemic development of the disease and severe yield losses in some regions of Brazil (Sartorato, 1988). While the disease is considered to be of minor importance in bean-producing areas of the Northern USA, it can, nevertheless, cause epidemics when weather conditions are favourable to disease initiation and development. In 1954, losses of 50% or more were reported in several commercial snap bean plantings in central Wisconsin, USA. The disease was not observed again in Wisconsin until 1973 when plants in many fields of red kidney beans were found to be seriously infected late in the season (Hagedorn and Wade, 1974).In the European countries where the disease currently occurs, it was rated of some economic importance in Hungary and Yugoslavia in the 1970s (EPPO, 1997).