Influence of cropping systems on Pythium root rot epidemics in a highland agroecology of South Western Uganda
Gichuru, Virginia Gathoni
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The bean crop is one of East Africa’s major legume crops. It is grown primarily by small-scale farmers who are mainly women, for home consumption and any excess is sold (Wortmann et al., 1998). The increase in severity and incidence of bean root rots has been associated with recent changes in farming systems, especially under high demographic pressure and decline in soil fertility (Rusuku et al., 1997). The importance of root rots in causing bean crop failures was recognised in Rwanda in 1988 and subsequently in Burundi, the Democratic Republic of Congo, Kenya and Uganda (CIAT, 1992; Otsyula et al., 1998; Opio, 1998). Although bean root rot is caused by a number of soil borne pathogens depending on environmental conditions, Pythium spp. are the fungal pathogens most frequently associated with severe epidemics in eastern Africa (Rusuku et al., 1997). In south western Uganda, root rot is caused by a number of pathogens, which occur either singly or as complexes. These include Fusarium spp, Rhizoctonia solani and Pythium spp. with the latter being the major pathogen (Opio, 1998). Studies on root rots have indicated that continuous cropping of beans, a common practice in eastern Africa exacerbates the problem (Rusuku et al., 1997). Due to population and land pressure in these high productive areas, beans are commonly cultivated with other crop species. Yet Pythium species attack a number of crop species and other plants (Ampaire, 2003). There is a need therefore to investigate whether root rots occur in other crops in the bean based system and in addition, to characterise Pythium species responsible for these root rots. This information will provide evidence on whether novel Pythium species are implicated in the bean root rot epidemics and whether other crop species are influencing the root rot epidemics in south western Uganda. In the first part of the study, surveys were done in Kabale district so as to characterise root rots of other crop species grown in association with beans. Molecular characteristation using the ITS-DNA sequences was also carried out on these crop species. These crops in the bean pathosystem of south western Uganda were found to be affected by root rots. The crops included potato, sorghum and peas. This implies that beans are not the only crops in the pathosystem to be attacked by the disease. Using ITS-DNA sequences, 142 Pythium species were characterised from crops intercropped with beans. The most abundant of the Pythium species on these crops was Pythium ultimum. Also, complexes of pathogens were isolated from crops intercropped with beans and these included Pythium spp., Fusarium spp. and Verticillium coccosporum. The implication of this is that there is host-pathogen selectivity as some Pythium species were found to affect leguminous crops and other solanaceous crops. In the second part of the study cross pathogenicity was done in the screen house. Bean derived and Pythium spp. derived from other crops were used to test their pathogenicity on resistant and susceptible bean variety, cereals and legumes. Sorghum and peas were found to be susceptible to both bean derived and Pythium species derived from other crops. Maize and millet were found to be resistant to the Pythium spp.These resistant crops are possibly able to produce biochemical reactions in their cells and tissues which inhibit the pathogen. Cereal crops having fibrous roots could counteract infection better than legumes which have tap roots. Hence cereals had a higher root mass compared to legumes. Symptoms characteristic of Pythium infection such as wilting, stunting and chlorosis were observed. This arises due to Pythium species reducing water uptake to leaves therefore resulting in wilting. The third part of the study involved the use of light and electron microscopy techniques to investigate the pattern of infection of bean pathogenic Pythium species on sorghum and maize. Sorghum was found to be susceptible to bean pathogenic Pythium species. The infection pattern in sorghum was similar to susceptible bean variety (CAL 96). Maize was resistant to bean pathogenic Pythium species and the infection pattern was similar to resistant bean variety (AND 1062). This confirms that sorghum is an alternative host of Pythium. Pythium infection in crop species was mediated by the formation of appressoria-bearing hyphae. In the study, there was also evidence of hemibiotrophic infection found with Pythium ultimum possessing two kinds of hyphae. This suggests that virulence of P.ultimum is affected by these two hyphae. This study has therefore found evidence that 1. The cultivation of beans in mixed cropping systems with other crop species may partly contribute to bean root rot epidemics. Sorghum and peas which are popular intercrops were found to be alternative hosts of pathogenic Pythium species implying that they contribute to pathogen inoculum load in the soil hence increased disease outbreaks. 2. Maize and millet were found to be resistant to Pythium species. This implies that these crops are poor hosts of pathogenic Pythium species therefore these crops could be included in bean rotations in south western Uganda so as to reduce Pythium soil inoculum load. 3. Differences in pathogenicity were found to occur within the pathogenic Pythium species. This phenomenon suggests the possibility for directional selection leading to increase in species or even pathotype abundances among Pythium pathogenic species. 4. Of the Pythium species isolated from bean and other crop host species some were pathogenic others were not. Given the multi- pathogenicity capacity of this genus, evolution of novel Pythium pathogens on both beans and other crop species cannot be precluded. 5. Resistant bean varieties (RWR 719 and AND 1062) and other crop species such as maize had similar disease reaction to bean pathogenic Pythium infection.