Effect of Cassava Production Practices on Intensity of Bacterial Blight in Busia County and Resistance Reaction of Cultivars Grown in Kenya

Abstract

Cassava (Manihot esculenta) is an essential crop to many farming households in Kenya. However, diseases continue to disrupt its huge potential as a food security and economic crop. Among the most devastating diseases is cassava bacterial blight caused by Xanthomonas phaseoli pv manihotis (syn. Xanthomonas axonopodis pv manihotis) and Xanthomonas axonopodis pv cassavae. Therefore, this study was conducted to determine the distribution of cassava bacterial blight in Western Kenya a key cassava belt in Kenya. Multistage sampling was used to select 193 farms from Nambale and Teso south sub counties in Busia county Western Kenya. Information on cassava production practices was obtained through questionnaires. From each of these farms 30 plants were assessed for cassava bacterial blight and symptomatic leaves collected for isolation and conformation of the CBB pathogens. GPS coordinates were also collected from the farms which were used to make distribution maps. Analysis of the questionnaire data was done through descriptive statistics and chi-square tests. The causal agents were isolated aseptically from the diseased leaves using yeast peptone glucose agar media in a laminar hood. The plates were incubated for 24hrs after which biochemical tests and pathogenicity test were done to confirm the causal agents. This aided the generation of distribution maps of the two causal agents across Western Kenya using the previously collected coordinates. A greenhouse experiment was set up to evaluate seven varieties for cassava bacterial blight resistance in a factorial treatment structure within a randomized complete block design. All varieties were inoculated with 106CFU/ml of the cassava bacterial blight causal agents on the leaves and stem. Observations were recorded at an interval of six days’ post inoculation using a severity scale of 1-5. After 24hrs the cultured plates had white and yellow colonies which are traits of bacterial blight pathogens. Bacteria from both colonies could utilize various sugars, but none could use either lactose or cellobiose. The pathogenicity tests indicated that the white bacterial isolate is Xanthomonas phaseoli pv manihotis as it caused both foliar and systemic disease while the yellow one is Xanthomonas axonopodis pv cassavae as it only caused foliar diseases. Among the samples collected over 90% of the farms had Xanthomonas phaseoli pv manihotis from both sub counties visited while less than 10% had Xanthomonas axonopodis pv cassavae and a combination of both causal agents. Majority of the farms (89%) had a moderate severity score of 3. There was no association between cassava bacterial blight incidence and training, seed source, and intercropping suggesting that other factors like ignorance could be contributing to the high incidence of the disease. Since 85% of the farmers interviewed were unaware of the disease thus spreading the disease unknowingly. From the greenhouse experiments Xanthomonas phaseoli pv manihotis inoculated varieties had the highest incidences compared to those inoculated with Xanthomonas axonopodis pv cassavae or both bacteria combined. The varieties also registered high severity score with the highest scores being of Xanthomonas phaseoli pv manihotis inoculated varieties. A similar case was registered for the area under disease progress curve values as all of the varieties inoculated with Xanthomonas phaseoli pv manihotis had values over 50%. The most affected varieties included mm 96/2480, Naro 56, and mm 96/1871. Xanthomonas phaseoli pv manihotis was more severe compared to Xanthomonas axonopodis pv cassavae. The study concluded that the high prevalence of cassava bacterial blight in western Kenya might be due to ignorance of the disease among farmers, lack of application of existing cassava bacterial blight mitigation measure, and reliance of informal seed systems which are characterized by recycled cuttings. Furthermore, Xanthomonas phaseoli pv manihotis was found to be the more severe of the two causal agents and the most prevalent indicating the existence of factors that may be affecting the survival of Xanthomonas axonopodis pv cassavae. From the greenhouse experiments none of the varieties was resistant to cassava bacterial blight indicating that more varieties currently ought to be assessed for cassava bacterial blight resistance. The following recommendations were made based on the study findings promotion of existing cassava bacterial blight management practices to farmers through elaborate extension services, encourage use of certified cassava cuttings among farmers when establishing their plantations, improve the capacity of community based organizations, nongovernmental organizations and Agricultural institutes through training and infrastructural development to produce enough disease free cuttings for farmers, and study the underlying differences in virulence and pathogenicity by the two cassava bacterial blight causal agents Xanthomonas phaseoli pv manihotis and Xanthomona saxonopodis pv cassavae, evaluate more cassava germplasm for resistance against cassava bacterial blight in the field and green house for better deployment of the germplasms.