Detection and Management of Viruses Associated With Passion Fruit Woodiness in Rwanda

Abstract

Passion fruit (Passiflora edulis [SIMs] is an important food and cash crop in Rwanda. However, its production is low due to pests and diseases, poor disease management and lack of clean planting materials. Passion fruit woodiness disease (PWD) is among the diseases that reduce the production of passion fruit in the world including Rwanda. It is caused by one or mixed infections by three potyviruses, Passion fruit woodiness virus (PWV); Cowpea aphid-borne mosaic virus (CABMV) and East Asian Passiflora virus (EAPV). This study was conducted to: (i) determine relative sensitivity and effectiveness of ELISA and RT-PCR diagnostic methods for screening viral infections in passion fruit seedlings and (ii) determine the potential of combining sticky traps and orchard fertilization in managing viruses associated with passion fruit woodiness disease in Rwanda. Sensitivity and effectiveness of ELISA and RT-PCR techniques for detection of CABMV was compared by testing passion fruit plants grown from seeds saved by farmers, from private nurseries and government sources and serial dilutions over a period two seasons. After germination and growing for two months (four leaf stage), 25 leaf samples from each seed source were collected and tested at three different time periods, 30, 60 and 90 days. Confirmed disease free passion fruit seedlings were inoculated with CABMV and allowed to develop symptoms for two months. Using one confirmed CABMV infected leaf extract seven serial dilutions up to 10,000,000 times were done. The results showed that leaf samples tested at 30 and 60 days using both ELISA and RT-PCR tested negative for CABMV for all three seed sources. However, at 90 days, 32% of samples from farmer’s saved seed tested positive for CABMV using RT-PCR method but was negative with the ELISA test. There was a significant difference (p≤ 0.005) between the two diagnostic methods in detecting CABMV presence. ELISA detected CABMV

from 32% and 58% of the leaf samples in season 1 and 2, respectively while RT-PCR detected CABMV in 60% and 62% of the leaf samples in season 1 and 2, respectively. Sixty one and 45% of samples tested positive for CABMV using RT-PCR and ELISA respectively. In the dilution experiment, ELISA detected the presence of CABMV up to the forth serial dilutions (104) while RT-PCR diagnostic method detected CABMV up to seventh serial dilutions (107). For the second objective, six vector management options namely:1) yellow sticky trap deployment, 2) Inorganic fertilizer for rapid plant growth, 3) yellow sticky traps + inorganic fertilizer, 4) pesticide application, 5) yellow sticky traps + inorganic fertilizer and pesticide application, 6) untreated control were investigated during two cropping cycles of 2017 and 2018. The treatments were replicated three times. Aphid vector populations were recorded once a week for duration of 29 weeks. The incidence of passion fruit woodiness virus disease (%) was assessed during each season. Disease severity was also recorded based on severity scores of 1-5. At harvest, passion fruits were harvested and evaluated on six plants per treatment. The results indicated that yellow sticky traps, yellow sticky traps and inorganic fertilizer, and a combination of yellow sticky traps, inorganic fertilizer and pesticide application recorded the highest number of aphid populations trapped compared to inorganic fertilizer, pesticide applications and the untreated control during both cropping seasons. There were no significant differences among aphid vector management options during the long rains cropping cycle. However, there was a significant (p<0.05) difference during the short rain period. There were negative correlations between the numbers of aphids, disease incidence and passion fruit woodiness virus severity. The application of pesticides, yellow sticky trap and a combination of yellow sticky + inorganic fertilizer trap resulted in numerically higher marketable yield, as compared to the other treatments. These results indicate that both ELISA and RT-PR are able to index for CABMV

presence on passion fruit seedlings, however, RT-PCR is more sensitive. Therefore RT- PCR could be used in certification program for screening passion fruit viral diseases in nurseries of planting materials while ELISA could be used for disease management. Yellow sticky traps were the most effective strategy for monitoring and management of aphid vectors in passion fruit orchard in the tropical highlands of Rwanda.