Characterization and Distribution of a Potyvirus

Ochwo-Ssemakula, M., Sengooba, T., Hakiza, J. J., Adipala, E., Edema, R., Redinbaugh, M. G., Aritua, V., and Winter, S. 2012. Characterization and distribution of a Potyvirus associated with passion fruit woodiness disease in Uganda. Plant Dis. 96:659-665. This article describes the incidence and etiology of a viral disease of passion fruit in Uganda. Symptoms, including those characteristic of passion fruit woodiness disease (PWD), were observed on 32% of plants in producing areas. Electron microscopic observations of infected tissues revealed flexuous filaments of ca. 780 nm. Enzymelinked immunosorbent assays indicated a serological relationship with Cowpea aphid-borne mosaic virus (CABMV) and Passion fruit ringspot virus (PFRSV). In host range studies, only species in the families Solanaceae and Chenopodiaceae were susceptible, and neither Vigna unguiculata nor Phaseolus vulgaris became infected. Coat protein (CP) gene sequences of eight isolates exhibited features typical of potyviruses and were highly similar (88 to 100% identity). However, the sequences had limited sequence identity with CP genes of two of the three potyviruses reported to cause PWD: East Asian Passiflora virus and Passion fruit woodiness virus (PWV). Deduced amino acid sequences for the CP of isolates from Uganda had highest identity with Bean common mosaic necrosis virus (BCMNV) (72 to 79%, with evolutionary divergence values between 0.17 and 0.19) and CABMV (73 to 76%, with divergence values between 0.21 and 0.25). Based on these results and in accordance with International Committee for Taxonomy of Viruses criteria for species demarcation in the family Potyviridae, we conclude that a previously unreported virus causes viral diseases on passion fruit in Uganda. The name “Ugandan Passiflora virus” is proposed for this virus. Commercial production of passion fruit in Uganda began around 1960, following its introduction from Kenya (25). Shortly afterward, the newly established large plantations were devastated several times by pests and infectious diseases. As a result, several large farming enterprises were either abandoned or downsized in favor of small and isolated plantings that could be better protected, resulting in the subsistence level of production (26) that is still prevalent in Uganda today (36). In the late 1980s, the Government of Uganda initiated an export diversification program that targeted horticultural crops, including passion fruit, as a means of boosting national revenue (16). Dominance of the passion fruit production chain by small-scale farmers (36) enhanced the potential of this crop to affect the socio-economics of rural development (27). Clones of a new variety, the Kawanda Hybrid, which had just been released by the National Agricultural Research Organisation of Uganda (NARO), were introduced. This cultivar had high levels of disease resistance as well as superior agronomic traits compared to other cultivars in the East African region, and thus gave Uganda a competitive advantage for the European market (8). Thus, several commercial ventures were established and export trade began (39). Shortly after commercial production started, pilot field studies revealed extensive symptoms indicative of several fungal and virus-like diseases characterized by foliar mosaics, vein clearing, fruit hardening, and malformation (15,19). Up to 90% of passion fruit fields in areas of central Uganda had plants with symptoms characteristic of virus infection (2), and almost 50% marketable yield loss was recorded (28). The losses attributed to viral diseases have continued to increase and remain a challenge to passion fruit production (36). The situation is further compounded by limited knowledge on the identity of the causal viral agent(s), extent of infection, and associated damage levels in the various regions of Uganda. This paucity of information has rendered it impossible to develop an effective disease management strategy. A number of control measures, such as use of disease-free planting material, were attempted but without much success (3). The characteristic hardening and malformation of fruits found on virus-infected plants have been related to the passion fruit woodiness disease (PWD) attributed elsewhere to several viruses belonging to the genus Potyvirus, including Passion fruit woodiness virus (PWV) (34) and relatives of Cowpea aphid-borne mosaic virus (CABMV) (9,24). More recently, the East Asian Passiflora virus (EAPV) has been confirmed to cause a similar disease in Japan (22). In this study, we present results of intensive surveys in Uganda for viruses known to infect passion fruit worldwide. Biological and molecular techniques were also used to characterize a potyvirus previously reported to cause leaf mottling and distortion and fruit woodiness in this horticultural crop in Uganda (29). The findings of this research are considered important for a better understanding of the viral disease situation on passion fruit in Uganda and for the development of effective disease management strategies. Corresponding author: M. Ochwo-Ssemakula, E-mail: [email protected] *The e-Xtra logo stands for “electronic extra” and indicates that Figure 1 appears in color in the online edition. Accepted for publication 15 September 2011. http://dx.doi.org/10.1094 / PDIS-03-11-0263 © 2012 The American Phytopathological Society e-Xtra*