Identification of Stem End Rot Fungal Pathogens on Avocado Fruits and Efficiency of Fungicides and Trichoderma spp. in Murang’a County, kenya
Wanjiku, Mwaura Elizabeth
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Avocado (Persea americana Mill.) is a commercially important fruit cultivated in tropical and sub-tropical climates. In Kenya the fruit is cultivated for both local and export market, significantly contributing to the country’s GDP. The production of avocado fruits in Kenya is limited by fruit diseases among others. Stem end rot (SER) is an economically important post-harvest disease of avocado fruits contributing to losses of the fruits at the farm level, during storage and on the shelves. The causal agents of the disease in Kenya are not clear, but Botryosphaeriaceae spp. are most important causal agents of avocado SER in other parts of the world where the fruit is grown. This study was conducted to evaluate avocado production and SER disease management; determine the incidence of SER on avocado fruits in Murang’a County; identify the SER fungal pathogens, determine the efficacy of selected fungicides and the inhibitory effect of selected Trichoderma species against the SER fungal pathogens of avocado fruits. A structured questionnaire was administered by face-to-face interview with farmers while colony growth and conidia characteristics were used in morphological identification of the pathogens. Two fungicides, (Green Cop 500WP (Copper oxychloride 500g/Kg) and Milraz 76 WP (Propineb 70% + Cymoxanil 6%) were used in the in vitro inhibitory study while four Trichoderma spp. (T. asperellum, T. harzianum, T. atroviride and T. virens) were tested against the most isolated SER pathogens on post-harvest ‘Hass’ avocado fruits. The survey revealed that 84% of the farmers preferred ‘Hass’ avocado fruits, 85% intercropped the avocado fruits with food crops, while 88% of the farmers used organic manure only to replenish the soils. Majority of the farmers (62%) used grafted seedlings while 27% of the farmers top worked the old trees. Most farmers (84.6%) did not adopt the recommended field management practices, and none of the farmers used fungicide in the management of SER. Avocado fruits from the market and orchard had SER disease incidence of 52.08 % and 45.70 % respectively. Lasiodiplodia theobromae, Neofusicoccum parvum, Nectria pseudotrichia, Fusarium solani, Fusarium oxysporum, Fusarium equiseti and Geotrichum candidum were identified as the causal agents of SER on avocado fruits. Lasiodiplodia theobromae was the most frequently isolated pathogen followed by, N. parvum, N. pseudotrichia and F. solani respectively. All the fungicides significantly (P≤0.05) inhibited mycelial growth of the four pathogens. In both in vitro study and on post-harvest avocado fruits T. atroviride had highest efficacy against N. parvum, N. pseudotrichia and F. solani while T. harzianum had the highest efficacy against L. theobromae. This study has identified the causal agents of stem end rot disease of avocado fruits in Kenya. Trichoderma atroviride and T. harzianum present a potential alternative to synthetic fungicides against SER diseases of avocado fruits, and the possibility of using them at a commercial level should be explored.