Characterization and Distribution of a Potyvirus Associated with Passion Fruit Woodiness Disease in Uganda.

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.

Host Range, Purification, and Genetic Variability in Sweet potato chlorotic fleck virus.

Sweet potato chlorotic fleck virus (SPCFV) has recently been classified as a putative new member of the genus Carlavirus (family Flexiviridae) on the basis of its molecular properties. In this study, SPCFV was characterized in terms of host range, physical and biological characteristics, and genetic variability. In addition to sweet potato, SPCFV infected some plant species in the families Convolvulaceae, Chenopodiaceae, and Solanaceae. Limited numbers of virus particles were observed in the assimilation parenchyma cells of infected plant tissues; some cells had a distorted and enlarged endoplasmic reticulum though without any cytoplasmic and amorphous inclusions. The normal length of SPCFV particles was determined to be approximately 800 nm. In enzyme-linked immunosorbent assays, polyclonal antibodies raised against purified SPCFV virions were able to detect the virus in infected sweet potato and indicator plant tissues. In immunoelectron microscopy, SPCFV particles were all strongly decorated when reacted with homologous antiserum. Comparison of the 3' terminal part of the genome of a range of geographically diverse isolates revealed a high level of genetic diversity. The amino acid sequence identity in the coat protein and the nucleic acid binding protein ranged from 89 to 99.7% and from 75.9 to 99.2%, respectively. Phylogenetic analysis of both proteins showed a geographically associated clustering into two genogroups.

Sequence analysis of the entire RNA genome of a sweet potato chlorotic fleck virus isolate reveals that it belongs to a distinct carlavirus species.

Since the paucity of information on sweet potato chlorotic fleck virus (SPCFV) had precluded its classification, we have determined the complete nucleotide sequence of the single-stranded RNA genome of a Ugandan isolate of SPCFV. The genome is 9104 nucleotides long (excluding the poly(A) tail) and potentially includes six open reading frames (ORFs). Based on genomic organisation and sequence similarity, SPCFV appears to be a member of the genus Carlavirus (family Flexiviridae). However, SPCFV is distantly related to typical carlaviruses, as most of its putative gene products share amino acid sequence identities of <40% with those of typical carlaviruses. Its closest relative is melon yellowing-associated virus, a proposed carlavirus from Brazil, with which it shares ORF5 and ORF6 amino acid sequence identities of 61 and 46%, respectively.

Occurrence of Peronosclerospora sorghi in Uganda.

A study was conducted to determine the occurrence and severity of sorghum downy mildew on three major hosts (maize, sorghum, and Johnson-grass) in Uganda. Five surveys were conducted in four growing seasons, between 1994 and 1995. The disease was encountered in 11 of the 22 districts surveyed. In the majority of the areas, incidence was less than 10%, but it was significantly higher (P < 0.05) in the second season of 1994 than in the first season of 1995. Soil type significantly (P = 0.001) influenced the incidence of sorghum downy mildew, with high incidence being associated with sandy soils, followed by loamy soils, and lowest incidence in clay soils. Incidence, shredding, and oospore production were comparatively higher on sorghum and Johnson-grass than on maize.

Identification of Sources and Inheritance of Resistance to Sphaceloma Scab in Cowpea.

Resistance in cowpea to Sphaceloma scab was characterized based on foliar and pod infection. Disease severity on cowpea plants was rated on a scale of 1 to 5, as follows: 1 = no symptoms, 2 = less than 10% infection, 3 = 10 to 20% infection, 4 = 20 to 50% infection, and 5 = more than 50% infection. Mean severity scores were used to calculate areas under the disease progress curve. Of the 75 cowpea lines evaluated, 10 were resistant, 30 were moderately resistant, and 35 were susceptible based on foliar infection. For pod infection, 24 lines were resistant, 40 were either moderately resistant or susceptible, and 11 were very susceptible. Local lines were less infected than plant introductions. In a separate experiment, 25 lines previously considered resistant were evaluated in the field with infested cowpea crop debris and susceptible rows as sources of inocula. Only 3 lines (39, KVu/175, and 46) maintained their high levels of resistance to Sphaceloma scab. Five cowpea lines were subsequently diallel-crossed, and parents and F2 were evaluated in the field to determine the nature of inheritance of resistance to Sphaceloma scab. The majority of the crosses between the resistant and susceptible lines showed intermediate reaction to scab, and high variation was obtained due to genotype and combining abilities. Partitioning of the variance into components due to general and specific combining ability revealed that additive genetic variation constituted the major portion of the total genetic variance for resistance to scab in cowpea.

Influence of Season and Cropping System on Occurrence of Cowpea Diseases in Uganda.

An on-farm survey was conducted for two seasons (first and second rains of 1994) to determine influence of season and cropping systems on occurrence of cowpea (Vigna unguiculata) diseases in Uganda. Viral diseases were widespread in both seasons but were more severe in the intercropped than in the nonintercropped cowpea. In contrast, scab, rust, and powdery mildew were less common and less severe when cowpea was grown in intercrops. High plant populations favored scab and anthracnose, but viral diseases were more severe under low plant populations. Higher incidence and severity of viral diseases, anthracnose, and scab were recorded during the first (wetter) season than during the second (drier) season. Disease level varied considerably among the common cultivars, but these differences may have been influenced by environmental differences.