Occurence and characterisation of root rot fungi infecting common bean (Phaseolus vulgaris L.) in Rubirizi District, South-Western Uganda.

Abstract

Beans are intensively grown in south western Uganda and are the second most important source of dietary proteins. Of the production constraints, Bean root rot is the most serious disease, causing total (100%) field loses in some areas. The study was conducted in Rubirizi district, bordering the southern section of Queen Elizabeth National Park. The specific objectives were: 1) To establish the effect of agronomic practices on the severity of bean root rot. 2) To determine incidence and severity of common fungal species causing bean root rot disease. 3) To characterize the common fungi causing bean root rot disease in Rubirizi district, using cultural, microscopic and pathogenicity methods. A semi-structured questionnaire was used to collect data on agronomic practices. To establish occurrence (incidence and severity) of the disease, a W-shaped pattern was used as transect/quadrat method, on farm size of a minimum of an acre. Symptomatic bean plants were picked randomly from the quadrats, in two farms per sub-county, eight sub-counties were visited. A total of 320 bean samples were obtained. Results showed that beans neighbouring sweet potatoes had the lowest average severity score (F=3.64, P<0.05). There was significant difference in the mean disease severity scores of beans grown in gardens that previously had different crops (F= 35.57, P<0.05). All the eight sub-counties had fields infected with bean root rot. There was a significant difference in the mean severity scores of bean root rot disease among the sub-counties (F=4.87, P<0.05), with Magambo, Ryeru and Katanda having the highest average disease score of 4.0. Using cultural, morphological and molecular techniques, most of the isolates were identified as F. oxysporum (70%) and F. solani (20%). Pathogenicity of 26 fungal isolates was determined using five bean varieties: CAL 96 (K 132), MLB-49-89A, NABE 15, RWR 719 and KWP 9. A significant difference in percentage germination of beans in soil inoculated with the different isolates (F=2.992, P<0.05) was observed, with MLB-49-89A having the highest average percentage germination (82.48%). It was therefore least affected by F. oxysporum and F solani. There was a significant difference in average severity score among the varieties (F=44.481, P<0.05) with NABE 15 variety having the highest average severity score of 4.07. The phylogenetic tree from DNA sequences of the isolates contained three main branching nodes with all the three branches well separated of which two were Fusarium. Bean variety MLB-49-89A is most suitable for planting in Rubirizi district to enable an increase in bean production, and Nambale which is already grown there.