Studies on epiphytic population, interactions of xanthomonas campestris qy manihotis, pv cassavae and evaluation of available cassava germplasm against cassava bacterial blight in Kenya
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The susceptibility of cassava cultivars to disease is one of the most important factors that limit cassava production. Cassava bacterial blight caused by Xanthomonas campestris Pv manihotis often results in considerable, sometimes total losses in tuber yield, leaves and planting material. Therefore this study was designed to survey the distribution of cassava bacterial blight and its causal agents in different agro-ecological zones of Kenya, interactions between the two strains of the pathogen, K. campestris QY manihotis and K. campestris QY cassavae and to screen cassava cultivars for resistance against the above disease. A survey was undertaken to map the distribution of the disease and its causal agents in different agro-ecological zones of Kenya. The survey revealed the disease and its causal agents as not being spread allover cassava growing areas but being confined to Western province and lake region areas of Kenya. Disease severity and incidence differed with locality depending on the amount of rainfall. Areas with higher rainfall had higher disease severity and incidence ratings. Disease incidence was also influenced by the method of farming being practiced. Monocrop showed more severe symptom development as compared to mixed cropping where cassava was intercropped with maize and bananas. Physiological and biochemical tests indicated that the two organisms are similar in most characteristics except in the utilization of sucrose and production of hydrogen sulphide from peptone. Xanthomonas campestris QY cassavae produced acid from sucrose and hydrogen sulphide from peptone, which was not the case with Xanthomonas campestris QY manihotis. In-vitro and in-vivo studies of the two strains of the pathogen and epiphytic micro flora found on cassava leaves were carried out in the laboratory and in the greenhouse respectively. Results showed that Xanthomonas campestris QY cassavae partially inhibits Xanthomonas campestris QY manihotis. Epiphytic microflora had no effect on the progress of Xanthomonas campestris QY manihotis wi thin leaves and stems. For cassava plants separately inoculated with the two pathogens, Xanthomonas campestris QY manihotis was more virulent and invaded stems faster as compared to QY cassavae. It also grew faster on nutrient agar. Twelve selected cassava cultivars were screened against cassava bacterial blight disease in the green house using three isolates identified from the survey. Leaf clipping and stem puncture methods of inoculation developed by CIAT were used . Leaf clipping method was rapid in operation and showed higher susceptibility of varieties than the stem puncture method. Isolates were significantly different in their reaction to the cultivars. Of the twelve cultivars tested, 5543/156, Mwakazanga, Kasimbiji green were resistant and 46106/27, 4670/37, 4763, 47234/114, 5543/19, 5043/11, Kibandameno and Guzo were susceptible to cassava bacterial blight. A local variety Kasimbiji green showed consistent resistance against the disease in all the methods and isolates used in these study.