Effect of Arbuscular Mycorrhizal Fungi on Growth, Water Stress, Tolerance to Phytophthora Blight and Nutritional Quality of Selected Papaya Hybrids in Kenya

Abstract

Papayas are major tropical fruits grown globally and locally due to their nutritional benefit and as a source of income from local and export markets. However, increased pest and disease pressure, low uptake of new technologies to improve growth and maturation has led to low production and poor quality of papaya fruits. Mycorrhizal fungi has potential application in plant water and nutrient uptake, growth and development and consequently enhancing plant’s resilience to biotic and abiotic stress. This could be exploited for enhancing plant adaptation to stress and for improving overall crop performance. The aim of this study was to evaluate the effectiveness of indigenous arbuscular mycorrhizal fungi (AMF) on growth, water stress, tolerance to phytophthora blight and nutritional quality of JKUAT and Malkia F1 papaya hybrids. Soil samples were obtained from Mwea, Mitunguu and Juja areas from grass, banana and papaya plants’ rhizosphere at a depth of 0-20 cm. AMF spores were isolated from the collected soil samples using sucrose method. Papaya seeds were sown on sterile coarse sand and transplanted to sterile soil media at 2 and 3 leaves seedling stage. The treatments (AMF inoculum, AMF inoculum and composted manure, composted manure and control) were introduced to the soil media at 4 weeks after transplanting at a ratio of 1:9 (treatments: soil media). Plant growth parameters were assessed every 4 weeks for a period of 20 weeks. Nutritional analysis of the fruits obtained included selected minerals, proximates, ascorbic acid, total polyphenols and total carotenoids. The collected data was subjected to analysis of variance (ANOVA) at p≤0.05 level of significant and significant means separated using Tukey’s HSD test in Genstat statistical package 15th edition. The highest number of spores were isolated from the grass plants’ rhizosphere. The results showed that combining manure and inoculum had highly significant effect (p≤0.05) on the performance of papaya plants compared to other treatments. Plant growth significantly differed (p≤0.05) between treatments with the highest root mass recorded in plants treated with a combination of AMF and manure (49.00g and 58.45g for JKUAT and Malkia F1 hybrids, respectively) and the least (16.11g and 18.24g, for JKUAT and Malkia F1 hybrids, respectively) recorded in sole compost manure treated plants at 20 weeks after transplanting. Root colonization differed significantly (p≤0.05) among the treatments and the hybrids at 20 weeks. The JKUAT hybrid treated with both AMF and manure had the highest root colonization of 78% while manure only treated plants had the least % colonization (41%). The stem girth of JKUAT hybrid with AMF and manure treatment and with pathogen infection increased from 0.8 cm to 3.9 cm while pathogen infected JKUAT hybrid with manure treatment only increased from 0.5 cm to 2.9 cm from 4 weeks to 20 weeks. The carotenoids contents of pathogen infected Malkia F1 hybrid with manure treatments only and a combination of AMF and manure treatments at 2 weeks after infection was 0.6mg/100g and 0.8mg/100g respectively while the non infected ones had 1.8 mg/100g and 2.4mg/100g respectively. At 20 weeks, Malkia F1 hybrid subjected to water stress and with inoculum treatment only had a stem girth of 3 cm while the control treatment had 0.32cm. Crude fibre content for Malkia F1 hybrid with both AMF and manure treatments was 5.7% while manure treatment only had 2.0% and 1.41% for the controls. Increased root biomass and high root colonization in AMF treated plants led to vigorous growth in papaya plants and accumulation of more nutrients in the fruits.The grass rhizospheric soils had the most abundant spores and could be isolated and bulked to boost on soil media for papaya establishment. Treating papaya plants with AMF combined with manure from seedling stage is recommended for plants’ tolerance to phytophthora blight disease, vigorous growth of papaya plants and accumulation of nutrients such as ascorbic acid in the fruits.