Management of bean root rots in the bean based cropping system in Southwestern Uganda

Abstract

Root rots are the main cause of declining bean (Phaseolus vulgaris L.) production in southwestern Uganda. In this region, beans are often grown in association with other crops, i.e., maize, sorghum, peas, solanum potatoes, sweet potatoes and yams with at least three crops existing in the same field at the same time. The causal agents for bean root rots, have been isolated from the roots of some of these crops, however, the effect of these crops on the severity of root rots in beans is not known. Let alone the root rot burden on these crops in this bean based system is not known. Current management of bean root rots has focused on root rots in beans with no focus on other crops. Yet a more holistic approach to root rot management would not only offer better control of root rots in beans but equally benefit the other crops in the whole bean based cropping system. Root rots were assessed over two rainy seasons in 2004 on a susceptible bean cultivar (K131) as a check, maize, sorghum and peas grown on naturally infested soils in farmer fields in Rubaya Sub County, southwestern Uganda, a hot spot for bean root rots. A randomised complete block design was used for this study. Several root rot management options; farm yard manure, green manure (Crotalaria spp.) and NPK (17:17:17) evaluated on sorghum, maize, peas and beans in naturally infested farmer fields over two seasons in 2004 in a separate experiment. A split plot design was used. Additional treatments included Ridomil Gold MZ 68 WP (4% Metalaxyl-M and 64% Mancozeb) as a positive check and an un-amended control as a negative check. Parallel screen house trials with soils artificially infested with bean pathogenic Pythium spp. and Fusarium spp. root rot pathogens (3:1 ratio) were also established at Kawanda. Root rots in the screen house crops were also evaluated with and without the above soil amendments. In the field, root rots affected all the crops with beans (K132), sorghum and peas severely affected. In the parallel screen- house trial, all the crops had root rot symptoms with the bean, sorghum and pea crop being susceptible while maize was resistant. The soil amendments improved crop tolerance though; their performance was influenced by the disease pressure and environmental condition. Ridomil Gold MZ 68 WP reduced root rot incidence and severity early in the season/trials (i.e. 18 days after planting) though did not significantly improve dry matter yield and plant vigour. Except in beans, FYM also reduced disease incidence and severity early in the season with no significant differences (p<005) observed late in the season. FYM however significantly improved dry matter yield and vigour across all crops. NPK also improved crop tolerance to root rots, ranking second to FYM in plant vigour and dry matter yield. Crotalaria spp. green manure on the contrary increased root rot incidence and severity early in the season, despite improving later in the season. The results of the study revealed that bean root roots are still a major problem in southwestern Uganda. More still, root rots are not only a critical problem to beans but also the other major crops in the system especially sorghum and peas. The study also suggests that sorghum and peas in this region may also play a role as alternative hosts in the perpetuation of the bean root rot problem. It’s therefore not recommended to intercrop or rotate beans with sorghum and beans but with maize so as to reduce root rot pathogen inoculum in the soil. The management options proved to be effective in beans are also effective in managing root rots across all the crops. An IPM package harnessing the various advantages offered by these amendments and serial application would yield much better response. A systems approach is therefore recommended instead of the current commodity approach for the management of root rots in this region.