Causes and Control of Postharvest Fruit Rot of Some Neglected Fruit Trees in the Greater Accra and Eastern Regions of Ghana

Abstract

There are a wide range of fruits found in Ghana, both indigenous and introduced. Some of these fruits have been neglected over time and therefore suffer high postharvest losses due mainly to rots caused by fungal pathogens. A study was therefore undertaken to: assess the knowledge, perception and experience of farmers and retailers of soursop, sweetsop and African star apple on harvest and postharvest losses and their causal factors; describe the symptoms of postharvest rot diseases associated with the fruits and identify the microorganisms causing the rot diseases; determine the frequency of occurrence of the fungal pathogens associated with the diseased fruits and evaluate the efficacy of neem seed extract, pawpaw seed extract, soursop seed extract and Zamir 40 EW (prochloraz + tebuconazole) for control of postharvest rots in the fruits. A questionnaire survey was carried out at the farmer level in the Eastern region and retailer level (selected markets and grocery shops) in the Greater Accra region to obtain information on postharvest losses incurred in the harvest and handling of the fruits. Using snowball sampling, 90 farmers were interviewed while 90 retailers were selected at random (30 within each of three authoritatively sampled markets) and grocery shops. Data from the survey were analyzed using IBM Statistical Package for Social Sciences (SPSS) software version 24. Description of disease symptoms on fruits were done by visual observation of colour and feeling of diseased tissue with fingers for texture, while noting signs of the fungus/fungi associated with the disease. Diseased soursop, sweetsop and African star apple fruits (25 each) were collected at random from different retail points and sent to the Plant pathology laboratory for isolation and identification of fungi. This was firstly carried out on water agar and sub-cultured on PDA for seven days. Fungi were identified under a compound microscope based on their morphological and cultural characteristics: colour, growth rate, mycelial and sporulating structures. Healthy fruits each of soursop, sweetsop and African star fruits were surface sterilized and punctured with sterilized cork borer. Pure cultures of isolated fungi were inoculated into fruits for test of pathogenicity with plain PDA as control and incubated for one week in the laboratory. The frequency of isolations of fungi associated with rot diseases from 25 samples each of soursops, sweetsops and African star apple fruits were determined by counting the number of times each fungus was isolated from each type fruit. The inhibitory effect of seed extracts from pawpaw, neem and soursop (10 g/100 ml of water) and Zamir fungicide (prochloraz + tebuconazole) amended with PDA (15 ml/150 ml PDA for the plant extracts and 0.45 ml/150 ml PDA for Zamir) on mycelial radial growth of Aspergillus niger, Colletotrichum sp., Lasiodiplioda theobromae, and Rhizopus stonolifer, was assessed. Plain PDA was used as control. Six-millimeter mycelial plugs from five-day-old actively growing cultures of the fungi were placed singly on the fungicide amended media in petri dishes. The treatments were arranged in a completely randomized design and incubated at 23 – 25oC and 70 – 80% RH in the laboratory till the control plate was full. Radial mycelial growth of the fungi was measured daily with a 30 cm ruler and used to calculate percentage inhibition of mycelial growth. Only a few (20%) farmers had knowledge on the extent and causes of postharvest losses associated with the fruit. However (94.4%) retailers had fair knowledge since they encounter most of these losses at their point of sale and during storage. Fruit losses were mainly associated with microbial infections (78.9%) amongst other rot causing factors such as physiological rot due to heat and bruises (66.6%) and wilting (33.3%). In the laboratory studies, soursop and sweetsop fruits were observed to be inflicted by two types of fungal rots (dry and wet) caused by Lasiodipliodia theobromae, Colletotrichum spp, Aspergilus niger and Rhizopus stonolifer. Infection by these organisms on the fruits resulted in colour change from green to colours ranging from brown to dark-brown at any point on the epidermis. Rots associated with African star apple fruits were caused by Lasiodipliodia theobromae and Aspergilus niger. resulting in dry and shrunken tissues with the colour of diseased tissue changing from the usual pink to brown and dark patches. Lasiodipliodia theobromae had the highest frequency of occurrence (40%) on soursop fruits, followed by Colletotrichum sp. (23.3%), Rhizopus stonolifer (23.3%) and Aspergilus niger (13.4%). Lasiodipliodia theobromae again recorded the highest frequency of occurrence (53.6%) on sweetsop fruits, followed by Colletotrichum sp. (28.5%), Aspergilus niger (10.7%) and Rhizopus stonolifer (7.2%). However, Aspergilus niger had the highest frequency of occurrence (56%) as compared to Lasiodipliodia theobromae (44%) on African star apple fruits. Soursop seed extract showed the highest efficacy (in-vitro) with more than 90% growth inhibition rate on all the tested four fungi, followed by neem seed extract (87.75%), and pawpaw seed extract (83.25%).