Characterization Of Resistance To Coffee Leaf Rust And Coffee Berry Disease Among Rwandese And Associated Coffee Germplasm
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Coffee leaf rust (CLR) and coffee berry disease (CBD) are the most economically important diseases affecting coffee production in Africa. A study was undertaken to evaluate the available coffee germplasm for resistance to CLR and CBD and to determine the genetic diversity between resistant lines and commercial cultivars in Rwanda. Germplasm evaluation was done through surveys conducted in March and August 2010 when climatic conditions were favorable to CBD and CLR epidemics respectively. Screening for resistance to CBD was done using the hypocotyls inoculation test while selection for resistance to CLR was carried out using the leaf disc inoculation method. The genetic diversity between CLR and CBD resistant varieties and commercial coffee cultivars in Rwanda was assessed using both Random Amplified Polymorphic DNA (RAPD) and Simple Sequence Repeats (SSR). Differences among genotypes in the coffee germplasm evaluated over two survey periods were determined on the basis of analysis of variance for repeated measurements while genetic distances generated for each pair of varieties were subjected to cluster analysis. Results showed highly significant differences between introductions (P < 0.001), time of scoring (P < 0.001) and their interaction (P < 0.001) for both CLR and CBD resistance indicating that genetic diversity exists and could be exploited in breeding for resistance to the above diseases. Introductions from Mulungu such as Locale Bronze, Locale Bronze 11-2066, and BM 139 did not show any CBD infection along with seven Ethiopian introductions (Ainamba Babaca Kaffa, Babaca Kaffa, Debie Sciable Kaffa, Irgalem Kella Sidamo, Wondo Sidamo, Harar Dugda Lemita Arussi and Teffari Kella Sidamo). In contrast to CBD, all introductions from Mulungu and Ethiopia were susceptible to CLR. However, five introductions from Portugal namely Pop 4/91, Pop 2/91, Pop 1/91, CIFC 15706, and CIFC 8224 did not show any CLR infection. Screening for CBD resistance also indicated highly significant differences (P < 0.001) between inoculated genotypes confirming the genetic diversity in the germplasm collection. In addition, all Ethiopian introductions were significantly different from the check. Moreover, introductions from DRC that included 6 varieties that are commercial cultivars in Rwanda, among which BM 139 was highly resistant. In relation to CLR, 19% of inoculated introductions were highly resistant (Catimor T8663, Selection 5A, Selection 6 and CIFC 8224). All parameters of resistance to CLR were positively correlated except for days to sporulation and the percentage sporulating discs where the correlation was low and non significant. Finally, two introductions (CIFC 8224 and Catimor T8663) were resistant to both CBD and CLR. Commercial coffee cultivars in Rwanda and CLR and CBD resistant varieties were genetically different. The highest genetic distance was recorded between BM 139 and HDT and between Mibilizi and Rume Sudan indicating wide genetic differences. However, the lowest values were obtained between BM 71 and Mibilizi and BM 139 and BM 71 indicating that these varieties were genetically similar. It was recommended from the study that BM 139 should be improved for resistance to CLR through backcross breeding as it showed high resistance to CBD in addition to its adaptability to local conditions, acceptable yield, good bean size and liquor quality. Moreover, both Catimor T 8663 and CIFC 8224 should be tested for yield, cup quality and adaptability in varying environmental conditions and be released if they are superior or similar to existing cultivated varieties. Lastly, crosses should be made between genetically distant susceptible and resistant varieties to derive hybrids that combine resistance with marked levels of heterosis particularly for yield.