Agronomic practices and postharvest management of anthracnose in avocado

Abstract

Avocado is a valued fruit in the Kenyan horticultural industry for local and export market and it contributes about 17% of the horticulture's foreign exchange earnings in Kenya. In spite of this, diseases and poor quality fruits limit access of Kenyan avocado to export market as well as reducing on the profitability of the crop to farmers. The current study was conducted to establish the agronomic and handling practices in avocado production so as to identify those processes influencing post-harvest losses. A questionnaire was used to conduct a survey in three avocado producing districts; Ernbu, Muranga South and Kiambu and 60 farmers, 12 bulk traders and 2 processors were randomly selected as respondents. Laboratory investigations were done to establish the pathogenic factors contributing to post-harvest losses in avocado. During the survey five mature diseased fruit samples were collected from every farm for identification of pathogens responsible for postharvest losses. Anthracnose is considered the most important disease contributing to post-harvest losses in avocado. Four commonly grown avocado cultivars; Fuerte, Pinkerton, Hass and Puebla were evaluated for their susceptibility to Colletotrichum gloeosporioides. Susceptibility was assessed through surface necrosis (SN) diameter, lesion depth (LD) and compound aggressiveness index (CAI). The susceptible cultivar Fuerte was used to evaluate the efficacy of three fungicides (Iprodione, Fenhexamid, Thiophanate methyl) and hot water as post-harvest management strategies for anthracnose. Results of the study indicated that post-harvest losses are contributed by improper management of avocado trees and poor post-harvest handling of fruits. In brief, pre-harvest pest and disease control, pruning, methods of harvesting and handling are improperly done. There were also significant differences in spraying practices and farmer-training status amongst districts surveyed. Colletotrichum gloeosporioides was the most frequently isolated post-harvest pathogen (53.3%) followed by Fusarium spp (16.4%) with significant (P<O.OI) association between pathogen incidence and locality. Other pathogens were Cladosporium spp (9.0 %), Penicillium spp (8.6%), Botrysphaeria spp (8.5%), Pestalotiopsis spp and (2.2 %) Rhizopus spp (2.2%). The pathogens were isolated from both body and stem end rots. All the four avocado cultivars tested were susceptible to anthracnose. Fruit maturity did not affect susceptibility but there were significant (P<0.05) differences in their susceptibility. Cultivar 'Fuerte ' was the most susceptible. Strong positive correlations were revealed between lesion diameter (Surface Necrosis) and lesion depth in cultivars; Pinkerton (R2 = 0.660), Fuerte (R2 = 0.554) and Puebla (R2 = 0.599). There was a weak correlation in cv Hass (R2 = 0.242). Thiophanate methyl significantly (P<O.O1) reduced disease progression when compared to the other three fungicides tested and hot water, with the lowest mean Surface Necrosis (SN), Lesion Diameter (LD) and Complex Aggressiveness Index (CAI). It was evident that there was need to train farmers on all aspects of avocado production and post-harvest handling. Further studies on post-harvest management of anthracnose in avocado are necessary. In addition further comparative studies with different cultivars, varying chemical concentrations and hot water at varying temperatures as well as postharvest disease management under controlled atmosphere are necessary. Studies aimed at establishing effects of cultivar and temperature on post-harvest disease management is also required.