The Role of mixed cropping systems on bean root rot epidemics in south western Uganda

Research PaperIn south western Uganda, beans are largely grown as intercrops with sorghum, maize, sweet potato and potato . Continuous cropping of beans, has increased bean root rot epidemics. Since some of the root rot causing organisms are known to affect other crops, there was need to investigate the role they may be playing in the current root rot epidemics. Surveys were carried out in Kabale district in order to establish the incidence of root rot on other crops grown in association with beans. Plant samples for isolation of Pythium, the main causative agent of root rot were also collected. Results indicated that potato had a high root rot incidence while maize had a low root rot incidence. Also, sorghum and peas had root rot symptoms. Out of the 142 Pythium isolates collected, 21 different Pythium species were identified by ITS-DNA sequencing. Fifteen new Pythium species not previously identified in the region were found. This study finds evidence that diverse crop species associated with beans may be playing a role in bean root rot epidemics.In south western Uganda, beans are largely grown as intercrops with sorghum, maize, sweet potato and potato . Continuous cropping of beans, has increased bean root rot epidemics. Since some of the root rot causing organisms are known to affect other crops, there was need to investigate the role they may be playing in the current root rot epidemics. Surveys were carried out in Kabale district in order to establish the incidence of root rot on other crops grown in association with beans. Plant samples for isolation of Pythium, the main causative agent of root rot were also collected. Results indicated that potato had a high root rot incidence while maize had a low root rot incidence. Also, sorghum and peas had root rot symptoms. Out of the 142 Pythium isolates collected, 21 different Pythium species were identified by ITS-DNA sequencing. Fifteen new Pythium species not previously identified in the region were found. This study finds evidence that diverse crop species associated with beans may be playing a role in bean root rot epidemics

Pathogenicity of pythium species on hosts associated with bean-based cropping system in south western Uganda

Research PaperA pathosystem is a subsystem of an ecosystem and is characterised by the phenomenon of parasitism. The bean-Pythium pathosystem consists of the host (bean), the pathogen (Pythium) and their host-pathogen relation. Of interest is how the pathogen causes pathogenicity on other crops and beans. To investigate this, screen house experiments were set up to test the pathogenicity of Pythium species derived from bean and other crops grown in association with beans. Pathogenicity was tested on maize (Zea mays), millet (Eleusine corcana), sorghum (Sorghum bicolor), peas (Pisum satium), susceptible bean variety (CAL 96) and resistant bean variety (RWR 719). The results indicated that distinct symptoms were observed in the roots and shoots of test crop species which are characteristic of Pythium infection. For instance peas had brownish watery stems and roots Also bean-derived pathogenic Pythium spp. were found to be more virulent than Pythium spp. derived from other crop species.Sorghum and peas had the highest disease scores upon infection by Pythium spp. We can conclude that there is cross pathogenicity among Pythium spp. especially affecting sorghum and peas. This phenomenon may account for the current root rot epiphytotics in south western Uganda and other similar agroecologies. An integrated disease management strategy that will deploy multi-non hosts to Pythium root rot is recommended.A pathosystem is a subsystem of an ecosystem and is characterised by the phenomenon of parasitism. The bean-Pythium pathosystem consists of the host (bean), the pathogen (Pythium) and their host-pathogen relation. Of interest is how the pathogen causes pathogenicity on other crops and beans. To investigate this, screen house experiments were set up to test the pathogenicity of Pythium species derived from bean and other crops grown in association with beans. Pathogenicity was tested on maize (Zea mays), millet (Eleusine corcana), sorghum (Sorghum bicolor), peas (Pisum satium), susceptible bean variety (CAL 96) and resistant bean variety (RWR 719). The results indicated that distinct symptoms were observed in the roots and shoots of test crop species which are characteristic of Pythium infection. For instance peas had brownish watery stems and roots Also bean-derived pathogenic Pythium spp. were found to be more virulent than Pythium spp. derived from other crop species.Sorghum and peas had the highest disease scores upon infection by Pythium spp. We can conclude that there is cross pathogenicity among Pythium spp. especially affecting sorghum and peas. This phenomenon may account for the current root rot epiphytotics in south western Uganda and other similar agroecologies. An integrated disease management strategy that will deploy multi-non hosts to Pythium root rot is recommended

Ultrastructure of the infection of sorghum bicolor and zea mays by pythium species

Research PaperThe practice of cultivating mixed crops is common in Tropical Africa and elsewhere especially in areas of high population density. The practice of mixed cropping can promote disease spread especially in multi-host pathosystems. Pythium is a soil borne oomycete with a wide host range affecting both cereal and legumes. In this paper we examine pathogenesis by Pythium species in maize and sorghum, crops commonly included as intercrops in south western Uganda. In this study, both electron and light microscopy were used to study infection process using bean derived Pythium species-Pythium ultimum (MS 61) and Pythium irregulare (DFD 47) on beans, maize and sorghum. Electron microscopy revealed that on maize P.irregulare hyphae remained extracellular while P. ultimum hyphae in epidermis underwent necrosis after 9 days. In sorghum on the contrary, P. ultimum and P. irregulare extensively colonised both the epidermis and endodermis. In this study, P. ultimum also had two types of hyphae which mediated infection thus making it more virulent than P. irregulare. The results of this study confirm that Pythium spp. are pathogenic on sorghum and therefore the role of sorghum in Pythium inoculum build-up in bean fields cannot be precluded.The practice of cultivating mixed crops is common in Tropical Africa and elsewhere especially in areas of high population density. The practice of mixed cropping can promote disease spread especially in multi-host pathosystems. Pythium is a soil borne oomycete with a wide host range affecting both cereal and legumes. In this paper we examine pathogenesis by Pythium species in maize and sorghum, crops commonly included as intercrops in south western Uganda. In this study, both electron and light microscopy were used to study infection process using bean derived Pythium species-Pythium ultimum (MS 61) and Pythium irregulare (DFD 47) on beans, maize and sorghum. Electron microscopy revealed that on maize P.irregulare hyphae remained extracellular while P. ultimum hyphae in epidermis underwent necrosis after 9 days. In sorghum on the contrary, P. ultimum and P. irregulare extensively colonised both the epidermis and endodermis. In this study, P. ultimum also had two types of hyphae which mediated infection thus making it more virulent than P. irregulare. The results of this study confirm that Pythium spp. are pathogenic on sorghum and therefore the role of sorghum in Pythium inoculum build-up in bean fields cannot be precluded

Influence of farming systems and crop host varieties on pythium root rot epidemics in a highland agroecology of South Western Uganda

A Thesis submitted to the school of graduate studies in fulfillment for the requirements of the award of a Doctor of Philosophy of Makerere University. Full thesis.The bean crop is one of East African’s principal 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 etal., 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 solanii 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 plants. 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 non bean crops grown in association with beans. Molecular characteristation using the ITS-DNA sequences was also carried out on these crop species. Non bean crops in bean pathosystem of south western Uganda were found to be affected by root rots. The crops included Irish potato, sorghum and peas. This implys 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 non bean crops. The most abundant of the Pythium species on these crops was Pythium ultimum. Also, a complex of pathogens were isolated from non bean crops and these included Pythium, Fusarium species and Veriticillium. The implication of this is that there is a 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 non bean derived Pythium species were use 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 non bean derived Pythium species. Maize and millet were found to be resistant. These resistant crops may be able to produce biochemical reactions in their cells and tissues which are toxic to 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 charactersistic 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 th cultivation of beans in mixed cropping systems with non bean 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 non bean hosts some were pathogenic others were not. Given the mulit- pathogenicity capacity of this genus, evolution of novel Pythium strains/ pathogens on both beans and non-bean hosts cannot be precluded. 5. Resistant bean varieties ( RWR 719 & AND1062) and non bean crops such as maize had similar disease reaction to bean pathogenic Pythium infection ..The bean crop is one of East African’s principal 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 etal., 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 solanii 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 plants. 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 non bean crops grown in association with beans. Molecular characteristation using the ITS-DNA sequences was also carried out on these crop species. Non bean crops in bean pathosystem of south western Uganda were found to be affected by root rots. The crops included Irish potato, sorghum and peas. This implys 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 non bean crops. The most abundant of the Pythium species on these crops was Pythium ultimum. Also, a complex of pathogens were isolated from non bean crops and these included Pythium, Fusarium species and Veriticillium. The implication of this is that there is a 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 non bean derived Pythium species were use 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 non bean derived Pythium species. Maize and millet were found to be resistant. These resistant crops may be able to produce biochemical reactions in their cells and tissues which are toxic to 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 charactersistic 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 th cultivation of beans in mixed cropping systems with non bean 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 non bean hosts some were pathogenic others were not. Given the mulit- pathogenicity capacity of this genus, evolution of novel Pythium strains/ pathogens on both beans and non-bean hosts cannot be precluded. 5. Resistant bean varieties ( RWR 719 & AND1062) and non bean crops such as maize had similar disease reaction to bean pathogenic Pythium infection .