Comparative studies of different isolates of colletotrichum Coffeanum noack - the causal agent of Coffee berry disease
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This study was undertaken to investigate the type of variation in Colletotr·i·chumcoffean:um, Noack the. causal agent of coffee berry disease in Kenya. Samples of infected berries were collected from different parts of Kenya where coffee is widely grown. It was found that C.coffeah:um has a wide range of variation with respect to spore sizes, spore shapes, sporulation capacity, growth and pathogenicity. In terms of colour, some grey isolates mutated to produce light coloured mutants. These mutants retained their light colours. Spore shapes ranged from cylindrical to oval shapes. Every isolate produced a mixture of spores of various shapes. However, over 95% of the spores were cylindrical. Spores did not have a significant difference in diameter measurements. The average width was 4.lp. Spore lengths however varied from isolate to isolate. There was also some variation within each of the isolate as shown by the coefficient of variation (3-33%). Mutants had spores similar to grey isolates in terms of shapes and sizes. When grey isolates from each district were considered, it was found that all the districts - xi - except one could be grouped together in terms of .their sporulation capacity.· The mutants, however had a higher sporulation capacity than most of the grey isolates. Colour mutants tended to grow faster than the grey isolate. Grey isolates from Trans Nzoia district had a slower growth than those from Nandi district. Pathogenicity varied from isolate to isolate. From each district grey isolates and mutants that were weakly pat~!ogenic and others which were highly pathogenic were obtained. \,Pathogenicity was shown by reactions of six week old seedlings as well as berries that were artificially inoculated. Some of the isolates interacted significantly with coffee varieties while others interacted negatively. Four out of twenty isolates interacted positively with HibrLdo de Timor. Two others interacted positively with Rume Sudan. Two others interacted negatively with SL 28. When the reactions of coffee seedling infection was classified as resistant (R), medium resistant (MR), medium susceptible (MS) and susceptibe (8) the isolates fell into different groups. However, 32 isolates out of 52 isolates fell into four main groups. Isolates from different districts responded ( -:'xii r- 'to fungicides in a similar way. Most isolates could not germinate in fungicides when concentrations were above 25 ppm except in copper oxide. Dithane M-45 which had not been used in controlling coffee berry disease proved to be as effective as the others in inhibiting germination of spores.