Characterization of Pythium species associated with pythium root rot of bean in eastern Africa and identification of resistant genotypes

Pythium root rot, caused by a number of Pythium spp., is the most destructive soil-borne disease of beans (Phaseolus vulgaris L.) in eastern and central Africa. In some seasons it can bring about complete crop failure. Four species (P. aphanidermatum, P. irregulare, P. myriotylum, P. ultimum) are known to cause bean root rot, but their distribution and relative importance in the region are unknown. The objective of this study was to characterize Pythium species, determine their distribution and pathogenicity, and evaluate diverse types of germplasm to identify sources of resistance that may be used directly by farmers or for varietal improvement. Two hundred Pythium isolates obtained from root rot–affected areas in Kenya, Rwanda and Uganda were characterized by sequencing, following amplification of their DNA using universal eukaryotic primers that target the internal transcribed spacer regions. Pathogenicity of some of the characterized Pythium species was determined on susceptible cultivar CAL 96 and resistant cultivar RWR 719. Among the 38 Pythium species identified, 9 were pathogenic to bean. Species distribution maps developed showed that P. ultimum var. ultimum is the most frequently occurring species in the region. Five species (P. nodosum, P. pachycaule, P. salpingophorum, P. spinosum, P. torulosum) were shown for the first time to be pathogenic to bean and possibly contributing to the root rot problem on bean. Among the 39 resistant genotypes evaluated, 30 maintained their resistance against 9 isolates representing 7 Pythium species pathogenic to bean—a demonstration of their potential value in genetic improvement of commercial varieties. There was no differential reaction of the bean genotypes to different Pythium species, implying that the genes responsible for resistance might have a similar mode of action. The large- or medium-seeded Andean varieties (for example, CAL 96, GLP 2, GLP 24, RWR 1058, and RWR 1092) were susceptible, including nearly all 172 medium-altitude climbers, which were largely of the Andean genepool. The varietal improvement program is targeting such materials, particularly the commercial varieties important in eastern and central Africa.