CHARACTERIZATION OF CANKER-CAUSING FUNGI ON MELIA VOLKENSII AND AZADIRACHTA INDICA TREES IN THE DRYLANDS OF KENYA.

Abstract

About 80% of Kenya’s land area has been classified as Arid and Semi-Arid lands, and experiences high temperatures and low erratic rainfall throughout the year which leads to water scarcity, degraded and poor soils leading to resource conflicts. In the search for suitable tree species for agroforestry and landscape restoration in Kenya’s ASALs two Meliaceae tree species, indigenous Melia volkensii (Geurke) and exotic Azadirachta indica (A. Juss.) were selected for health status research. Six (6) counties from Coastal and Eastern regions of Kenya were identified for the study. Systematic sampling was undertaken through a survey of the health status of the two tree species to identify fungi associated in the wild and on farms. In the laboratory, standard fungal isolation procedures were employed in culturing the emerging fungi on 2% Malt Extract agar (MEA) and incubated at 25 o C for 14 days. Emerging cultures were sub-cultured into pure cultures through single-spore isolation. Eighty-six (86) isolates were selected from the common fungal groups from the two species for Deoxyribonucleic acid extraction and amplification of the internal transcribed spacer (ITS) and translocation elongation factor (Tef) - 1-alpha gene regions Nine (9) species isolated from the Botryosphaeriaceae, were selected for pathogenicity tests under glass house conditions with average temperature of 28 o C. Statistical analyses used were analysis of variance and phylogenetic analyses. Symptoms found in the field associated with Melia volkensii were dieback and canker with resin flow while symptoms on Azadirachta indica were dieback with dry cankers. The combined percentages of canker and dieback were 32%, cankers only 13%, shoot die-back 10%, while 42% of the trees sampled were categorized as healthy and less than 2.5 % were dead or dying. The disease severity was higher in Melia volkensii >45 % than in Azadirachta indica >40 -<65 %. In total 484 pieces of diseased Melia were cultured using standard laboratory fungal isolation procedures out of which 1452 isolates were realized. Likewise, 694 isolates were made for A. indica. Morphological identification of the fungi isolated revealed 8 fungal groups from M. volkensii and 6 groups from A. indica. Molecular identification grouped the majority of isolates to the family Botryosphaeriaceae (51%). Three (3) of the Botryosphaeriaceae species belonged to the genus Lasiodiplodia namely L. pseudotheobromae, L. theobromae and L. parva. Phylogenetic analysis showed that most of the fungal groups identified occurred on both tree species and grouped into same clades. Pathogenicity tests showed that the L. pseudotheobromae species was most virulent to both M. volkensii and A. indica while L. theobromae was least virulent to both tree species. Wilting and necrosis was recorded within 7 days of inoculation, but wound healing occurred on both species after 12 weeks. A. indica had an average lesion of 8cm compared to M. volkensii with an average lesion of 14cm. The study gives insight into the two Meliaceae species and fungal attacks and could be suitable for dryland agroforestry. Management of the disease and plants in agroforestry in the drylands will further enhance the resilience of dryland agroforestry systems in Kenya.