Characterization and Management of Pathogenic Fungi Contributing to Bulb Onion (Allium cepa L.) Postharvest Rot in Kenya

Abstract

Bulb onion (Allium cepa L.) is an excellent source of vitamin C, Vitamin B6, potassium, magnesium, polyphenols and phytonutrients. Among the aromatic and medicinal crops grown in Kenya bulb onion production was ranked first in 2020. Despite economic importance of bulb onion production in the country, its productivity is low which is attributed to pre- and postharvest handling factors. Furthermore, Kenya’s produced bulb onion have poor keeping quality which necessitates the country to import from Tanzania and other neighbouring countries to meet its demands. Currently, scanty information on postharvest handling practices, extent of losses and causes in bulb onions, limits the development of postharvest reduction strategies in Kenya. Postharvest rot in bulb onion has mainly been attributed to losses during handling. However, in Kenya information on fungi contributing to postharvest rot in bulb onions is scanty, thus restricting the development of suitable handling techniques to manage the rot. Curing is one of the recommended practices that can minimize the postharvest losses of bulb onions. Therefore, the purpose of this study was to assess postharvest losses in bulb onions, isolate and characterize fungi that cause postharvest rot in bulb onions, and to determine optimal curing temperature to manage the rot and maintain or improve quality. In both field and market surveys questionnaires were administered through face-to-face interviews with farmers and traders respectively. Data collected during the field survey included postharvest handling techniques, postharvest loss quantity, and their causes at both farm and market levels in Bungoma, Meru, and Kajiado counties which were major growing regions of Kenya. Additionally, from seven major open air markets namely; Nkubu and Meru (Meru County), Kitengela and Kajiado (Kajiado County) and; Chwele, Kimilili and Cheptais (Bungoma County), bulb onions were collected to isolate and identify fungi that cause postharvest rot. Fungal isolates were grown in Potato Dextrose Agar (PDA) and identified using both morphological and molecular methods. Appropriate curing temperature to manage bulb onion postharvest rot was determined by inoculating bulb onions with specific fungal pathogens. Some bulb onions were placed at room temperature and others incubated at different temperatures of 30, 35, 40°C for 14 days and rot severity scores recorded. Effect of different curing temperatures on bulb onions physicochemical and phytochemical quality attributes were assessed. The results on assessment of bulb onion postharvest loss levels indicated that 66% of respondents reported that their farms incurred losses ranging from 5 to 30 percent. Fourty percent of the respondents indicated that bulb onion rot was the main cause of postharvest losses at farm level. Occurrence of postharvest losses in bulb onions at farm level were significantly influenced by harvesting tools and transportation mode. Furthermore, secondary and tertiary education and socio-economic factors showed a significant impact on bulb onion postharvest loss. The study revealed that rot was the primary cause of postharvest losses in bulb onions at both farm (10%) and market (14%) levels. Eighteen fungal pathogens isolated from bulb onions collected from different markets were identified through morphological and molecular methods. Morphologically the pathogenic fungal isolates were categorised into three clusters and identified as Fusarium spp. The three Fusarium spp. clusters were identified as Fusarium oxysporum f.sp. cepae (55%), F. acutatum (17%), and F. solani (28%) through molecular analysis. Mainly, F. oxysporum f.sp. cepae was obtained from bulb onions collected in Bungoma County, while F. solani and F. acutatum were mostly found in samples from Kajiado and Meru County, respectively. Bulb onions inoculated with F. solani and cured at 24±2°C (room temperature) had significantly (P≤0.05) higher (2.3±0.08) score in rot severity compared to those cured at 30℃, 35℃ and 40℃, 1.7± 0.41, 1.7± 0.35 and 1.3 ±0.13 respectively. Increase in the curing temperature decreased the severity of Fusarium rot in bulb onions. Therefore, the study established that curing bulb onions at 30 to 35°C for 14 days reduced severity of postharvest Fusarium rot. Bulb onions cured at 40°C had significantly higher weight loss (3.9±0.53%) and lowest (2.8±0.26%) at 35°C. The reduction in neck thickness was not significantly different (P>0.05) among bulb onions that were cured at 24±2°C, 30°C, 35°C, and 40°C. Total Soluble Solids (TSS) level in bulb onion was not significantly (P>0.05) influenced by different curing temperatures. Curing improved physical attributes in bulb onion which enhances its marketability and shelf-life. The lowest level (97.5±3.32mg/100g) of total flavonoids content (TFC) was observed after curing bulb onion at 30°C whereas the highest (248.6±2.85mg/100g) level was obtained after curing at 24±2°C. This study concludes that improper postharvest handling of bulb onions in Kenya causes losses ranging from 5 to 30%, with rot accounting for the majority of these losses (10 to 14%) at both the farm and market levels. Fusarium oxysporum f.sp. cepae, Fusarium solani and Fusarium acutatum are the causes of postharvest rot, and these pathogenic fungi can be controlled by curing bulb onions for 14 days at 30-35°C. The physical and phytochemical qualities of bulb onions were enhanced after curing for 14 days at 30–40°C. Though, bulb onions cured at 40°C developed black sooty rot. Consequently, it is recommended that, to manage postharvest Fusarium rot bulb onions can be cured for 14 days at 30-35°C, while also preserving the quality. In order to reduce postharvest losses, the study further recommends capacity building of farmers and traders on appropriate postharvest management techniques for bulb onions.