Abstract
Dieback is the most important fungal disease of passion fruits causing over 70% of crop loss. The disease is difficult and complex to manage due to the complexity of symptoms exhibited diversity of causal organisms and the dissemination pathways. The high preference of a highly susceptible variety by farmers aggravates the problem resulting in use of pesticides which lead to high cost of production and residues in the produce. Therefore, this study was carried out to determine the occurrence and distribution of dieback in Uasin-Gishu County and to evaluate passion fruit varieties for resistance to the disease.
A survey was carried out in 2019 after the short rain season to determine the occurrence and distribution of dieback of passion fruit in Uasin-Gishu County. A sample of 107 large and small scale farmers was taken in three agro-ecological zones (AEZs).A semi - structured questionnaire was administered to obtain information on socio-economic, passion fruit production, incidence, severity and prevalence of passion fruit diseases and their management. Disease incidence was determined as the percentage of plants with dieback symptoms in 25m2quadrants, while severity was determined as the proportion of diseased plant area using a visual rating score chart. Twenty- five leaves and five stems of passion fruit with typical dieback symptoms were collected from five farms per AEZ for isolation of dieback causal agents. Pathogenicity of the isolated fungi was confirmed in vitro using detached passion fruit parts and on seedlings in a greenhouse. Evaluation of passion fruit varieties for resistance to dieback was carried out by inoculating three month old seedlings with four fungi isolated from passion fruit parts with dieback symptoms. The four fungal pathogens were inoculated singly and in combination into six passion fruit genotypes. Data on disease incidence and development was collected at a seven-day interval for nine weeks. Descriptive data was analyzed using SPSS version 20, while quantitative data was subjected to analysis of variance (ANOVA) using Genstat 15th Edition and the means separated using Tukey’s test at P≤0.05.
The survey showed that purple passion fruit grafted on yellow variety was the most preferred by 94% of the farmers. Diseases (92%) and insect pests (71%) were the major constraints to passion fruit production. Majority (80%) of the diseases were fungal while 20% were viral. Dieback was the most prevalent fungal disease at 66%, while the least pervasive was Phytophthora blight with 15%. The disease distribution varied considerably across the ecological zones but was more
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severe in LH3 at 29% and lowest in LH2 at 8.6%. Majority (82%) of the respondents used fungicides as disease management intervention. Fungal pathogens isolated from the passion fruit dieback complex were Fusarium oxysporum, Fusarium semitectum, Phytophthora nicotianae and Alternaria passiflorae. Phytophthora nicotianae and Alternaria passiflorae were isolated in higher frequencies from the leaves as compared to Fusarium oxysporum and Fusarium semitectum, whose isolation frequency was higher in the stems. Inoculation experiments on purple passion fruit showed that Alternaria passiflorae was the most aggressive pathogen causing higher disease severity and incidence compared to Fusarium oxysporum, Phytophthora nicotianae and Fusarium semitectum. The six passion fruit genotypes inoculated with combined inoculum of the four dieback pathogens showed significant variation in the level of resistance with the mean AUDPC values ranging from 863 to 2683. Ester variety was the most resistant with the lowest AUDPC value of 863, while ordinary purple was the most susceptible with the highest AUDPC value. Based on the disease incidence and AUDPC values, the genotypes were grouped into four categories as resistant, moderately resistant, tolerant and susceptible.
The results indicate that passion fruit dieback is prevalent in Uasin-Gishu County and is widely distributed across all the agro-ecological zones but more severe in LH3. To reduce the spread and distribution, there is need to strengthen the capacity of farmers and nursery operators on best planting material propagation protocols. Farmers prefer to grow the ordinary purple passion fruit variety that is highly susceptible to the disease. The disease is caused by different fungal pathogens with Alternaria passiflorae being the most virulent. Farmers are therefore advised to adopt an integrated disease management approach in the management of dieback. Ester genotype, a purple variety with superior fruit characteristics than the ordinary purple, showed remarkable resistance to dieback under controlled conditions. Therefore, further field evaluation of ester passion fruit variety for reaction to dieback under varying climatic conditions and for acceptability by farmers is recommended.
