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Occurrence and Distribution in Rwanda of Soilborne Fungi Pathogenic to the Common Bean

The occurrence and distribution of soilborne fungi pathogenic to the common bean were determined by surveys conducted in seven of Rwanda's 10 prefectures during four growing seasons from 1989 to 1990. The pathogens were identified on the basis of symptoms, colony characteristics, reproductive structures, and pathogenicity tests. Of the plants sampled during the March through June 1989 season from four prefectures, 97% exhibited symptoms associated with soilborne pathogens. Root and hypocotyl symptoms appeared in plants sampled during the September through December 1989 season from the prefectures of Kigali (19%), Butare (40%), Gikongoro (33%), and Cyangugu (33%) and during the March through June 1990 season from Gisenyi (60%) and Kibungo (53%). Pythium spp., Macrophomina phaseolina, Rhizoctonia solani, and Fusarium oxysporum f. sp. phaseoli were isolated from all seven prefectures during all four seasons. Sclerotium rolfsii was isolated only during the 1990 September through December season. The frequency of isolation varied by region and season. Pythium spp. were the most frequently isolated fungi from about 40% of the samples, suggesting their potential importance in the root rot complex in Rwanda. The relative importance of the soilborne pathogens and possible relationships with existing cropping systems are discussed. These results will help focus efforts in developing management strategies for common bean diseases caused by soilborne pathogens in Rwanda.