Pathogen variation and quantification of pythium species bean fields in Uganda

In Uganda, a major pathogen genus causing severe phaseolus bean root rot, particularly in South western, is Pythium. But there is no information on the different pythium species associated with bean root rots nor about impact of use of organic amendments on pythium populations in field soils. This study was therefore, undertaken to a) identify pythium species pathogenic to beans and map their distribution in major bean producing areas of Uganda, and b) develop a procedure of quantifying pythium populations in field soils and c) to evaluate the impact of soil organic amendments on inoculum levels of pythium spp. pathogenic on beans (phaseolus vulgaris L.). The first part of the study involved morphological, molecular and pathogenicity analyses of pythium isolates collected from Kisoro and Kabale districts in South-Western Uganda and Mbale in the East. Molecular analyses using PCR-RFLP and sequencing techniques were used in the identification of Pythium species. Eleven pythium species were found to be associated with bean root rots in Uganda. Apart from the traditional species, P. ultimum var ultimum, known to infect beans, other pythium species recovered from infected beans were P. spinosum, P. torulosum, P. salpingophorum, P. vexans, P. dissotocum, P. nodosum, P. echinulatum, P.pachyaule, P. oligandrum and P. deliese. All isolates of P. ultimum var ultimum and P. spinosum caused severe root rot on susceptible bean varieties CAL 96 and K20. The other isolates belonging to P. torulosum, P. nodosum and P. pachycaule were also pathogenic to beans varieties CAL 96 and K20. Pathogenicity tests indicated significantly more disease (8 to 9 score) when highly pathogenic pythium species were used to inoculate susceptible bean varieties CAL 96 and K20 compared to the highly tolerant variety RWR719 (score of 2 to 5). Generally, morphological, molecular and pathogenicity analyses revealed that there is wide diversity within pythium population in Uganda. The results obtained demonstrated that P. ultimum var ultimums is responsible for the increased disease epidemics in kabale and Kisoro districts. But, pathogenicty of P. spinosum, P. nondosum and P. pachycaule isolates suggest that these species also cause bean root rot under field conditions. Isolates of P. torulosum and P. salpingophorum were more variable in their pathogenicity, but the species could probably contribute to bean root rot as part of the complex of minor pathogens.