Investigating of defence mechanisms in relation to stem-end rot disease of mango

Abstract

Mango stem-end rot (SER) disease is caused by a number of fungi including Lasiodiplodia theobromae (Synonym Botryodiplodia theobromae), Dothiore/la dominicana, D. mangiferae, Cytosphaera mangiferae, Phomopsis mangiferae, Pestalotiopsis mangiferae and Nattrassia mangiferae. This disease is more of a problem when anthracnose levels are reduced by climatic factors or preharvest control. Unripe mango fruits are resistant to SER where as ripe fruits are prone to attack. A study was carried out to isolate the SER causing pathogens from local cultivars and the response of different mango cultivars to SER pathogens. The host pH variation during infection of selected cultivars and the sensitivity of SER pathogen to host antifungal activity was also investigated. Naturally infected mangoes were used to isolate the stem-end rot pathogens and their morphological characters were observed. For cultivar evaluation, fruits were inoculated with L. theobromae and diseased area was measured 3, 5, 7, 9 and 11 days after inoculation. Fruits from two cultivars {Karathacolombon (KC) and Kohu}, were inoculated with L theobromae and the tissue pH variation during SER development was determined using a flat pH electrode. Data was analysed using Statistical Analysis System soft ware (SAS). Peel tissues were obtained from healthy and diseased fruits of cultivar "KC", to detect the antifungal activity, TLC bioassay was carried out using Cladosporium cladosporioides and L theobromae as test fungi. L. theobromae, Dothiorella and Pestalotiopsis were isolated from stem-end rots collected during the study, of them L. theobromae was frequently isolated from all the cultivars, whereas Dothiorella was isolated only from cultivar "KC". On inoculation L. theobromae or Dothiorella alone' would develop typical SER symptoms. Artificially inoculated fruits with L. theobromae and Dothiore/la sp. produced similar symptoms. It was observed that cv. "KC" was more susceptible to SER than other cultivars tested. In cv. "KC" and "Kohu" the peel tissue pH was high in unripe fruit but decreased progressively during SER development. Bioassay of peel extracts with Cladosporium sp. showed the presence of preformed antifungal activity however, no inhibition zones were visible when the TLC plate was sprayed with L. theobromae spores. This indicates that L. theobromae is not sensitive to preformed antifungal compounds present in mango peel. This clearly supports the fact that certain pathogens still manage to invade plants, though plants contain preformed defences.