RESUMEN

We analysed the sequence diversity in the reverse transcriptase (RT)/ribonuclease H (RNaseH) coding region of 19 badnavirus isolates infecting yam (Dioscorea spp.) in Ghana, Togo, Benin, and Nigeria. Phylogenetic analysis of the deduced amino acid sequences revealed that the isolates are broadly divided into two distinct species, each clustering with Dioscorea alata bacilliform virus (DaBV) and Dioscorea sansibarensis bacilliform virus (DsBV). Fourteen isolates had 90-96% amino acid identity with DaBV, while four isolates had 83-84% amino acid identity with DsBV. One isolate from Benin, BN4Dr, was distinct and had 77 and 75% amino acid identity with DaBV and DsBV, respectively, and may be a member of a new badnavirus species infecting yam in West Africa. Viruses of the two main species were present in Ghana, Togo and Benin and were observed to infect both D. alata and D. rotundata indiscriminately. This is the first confirmed report of DsBV infection in yam in Ghana and Togo. The results of this study demonstrate that members of two distinct species of badnaviruses infect yam in the West African yam zone and suggest a putative new species, BN4Dr. We also conclude that these species are not confined to limited geographic regions or specific for yam host species. However, the three badnavirus species are serologically related. The sequence information obtained from this study can be used to develop PCR-based diagnostics to detect members of the various species and/or strains of badnaviruses infecting yam in West Africa.

Asunto(s)

Badnavirus/genética , Dioscorea/virología , Enfermedades de las Plantas/virología , Ribonucleasa H/genética , Secuencia de Aminoácidos , Badnavirus/clasificación , Badnavirus/aislamiento & purificación , Secuencia de Bases , Benin , Variación Genética , Ghana , Datos de Secuencia Molecular , Nigeria , Filogenia , Ribonucleasa H/química , Alineación de Secuencia , Togo

RESUMEN

A diagnostic survey was conducted in 2002-03 to determine the status of cassava mosaic begomoviruses in Nigeria and to ascertain if the virulent Ugandan variant of East African cassava mosaic virus (EACMV-Ug2) was present. Of the 418 farms visited, 48% had cassava with moderately severe or severe symptoms, whereas 52% had cassava with mild symptoms. These distributions were at random. Of the 1,397 cassava leaf samples examined, 1,106 had symptoms. In polymerase chain reaction tests, 74.1% of the symptom-bearing samples tested positive for African cassava mosaic virus (ACMV) alone, 0.3% for EACMV alone, 24.4% for mixed infections by the two viruses, and 1.2% did not react with any of the primers used. The two viruses also were detected in 32% of the 291 symptomless plants and in the whitefly vector samples. EACMV-Ug2, Indian cassava mosaic virus, and South African cassava mosaic virus were not detected in any of the whitefly or leaf samples. Most farms had ACMV in single infection as well as in mixed infections with EACMV. Most doubly infected plants showed severe symptoms. Two biological variants of ACMV were identified based on symptom expression on cassava in the field. ACMV and EACMV were detected in the leguminous plant Senna occidentalis (L.) Link and the weed Combretum confertum Lams.; these are new natural hosts of the viruses. Although the virulent EACMV-Ug2 was not detected, the occurrence of variants of ACMV and a high proportion of mixed infections by ACMV and EACMV, which could result in recombination events such as the one that produced EACMV-Ug2, demands appropriate measures to safeguard cassava production in Nigeria.

RESUMEN

Despite the development and deployment of maize streak-resistant (SR) germ plasm, virus-induced symptoms are still commonly observed on maize in Lagos, Nigeria. Therefore, surveys were conducted between April 2001 and February 2002 to determine the identity, prevalence, and incidence of maize viruses in 18 local government areas (LGAs) in and around Lagos by visual examination and serodiagnostic screening of symptomatic plants. All 112 fields surveyed during the dry season (September to December) and 18 fields surveyed during the late dry season (December to February) had plants infected by Maize streak virus (MSV), whereas 97.1% of the 170 fields surveyed during the wet season (April to August) had plants infected by MSV. Maize mottle/chlorotic stunt virus (MMCSV) was prevalent in 99.1, 88.9, and 67.4% of the fields surveyed during the dry, late dry, and wet seasons, respectively. The incidence of MSV was higher in 16 of the LGAs. The highest incidence of MSV was 18.9%, whereas that of MMCSV was 7.4%. Serodiagnostic screening of leaf samples showing virus-induced symptoms, using antigen-coated plate enzyme-linked immunosorbent assay, indicated that 1,192/1,475 (80.8%) and 949/1,210 (78.4%) of the samples were positive for MSV and MMCSV, respectively. Vector transmission and host range studies confirmed the identity of the viruses. The results confirm the presence of MSV and MMCSV in Lagos and suggest that the use of MSV-susceptible cultivars is still widespread. Methods of ensuring effective utilization of existing SR germ plasm and controlling maize viruses in general are discussed.

RESUMEN

Symptoms resembling those of viral leaf streak disease, caused by banana streak badnavirus (BSV), were observed in May 1998 on two banana (Musa spp.) landraces grown from farmer-collected propagules in a farmer's field at Kiboje Uchukuni, Zanzibar. Those showing symptoms were "French plantain" cv. Mzuzu and "Cavendish" banana cv. Mtwike. Leaf symptoms were expressed as chlorotic streaks and blotches. Leaf samples were indexed by immunosorbent electron microscopy with BSV and cucumber mosaic cucumovirus (CMV) antibodies, using partially purified preparations (2). The two landraces tested positive for BSV, corroborating the occurrence of BSV in Zanzibar. In addition, cv. Mtwike was found to be coinfected with CMV. No other viruslike particles were seen by electron microscopy. Although BSV has been reported in Zanzibar (1), it was only from symptoms in the Musa field genebank at Kizimbani Research Station. BSV has been found in many Musa collections worldwide, particularly in the widely grown cv. Mysore. This report confirms the presence of BSV in farmers' fields and is also the first report of CMV infecting banana in Zanzibar. References: (1) A. J. Dabek and J. M. Waller. Trop. Pest Management 36:157, 1990. (2) M. Diekmann and C. A. J. Putter. Musa spp. FAO/IPGRI Technical Guidelines for the Safe Movement of Germplasm No. 15. FAO/IPGRI, Rome, Italy, 1996.

RESUMEN

Two viruses naturally infect Musa in Nigeria: banana streak badnavirus (BSV) and cucumber mosaic cucumovirus (CMV). During a recent field survey at Ibadan (Nigeria), some severely stunted banana plants (cv. Valery) were found that tested negative for CMV, banana bunchy-top virus, and BSV. The plants had symptoms of leaf crinkling, leaf necrosis, and cigar-leaf die-back. Subsequent suckers from the same mats were progressively more stunted. A 28- to 30-nm isometric virus was purified, and used for the production of antibodies, from the affected plants with (NH4)2SO4 to precipitate the virus. The antiserum (titer of 1:10,000) was used in enzyme-linked immunosorbent assay and immunosorbent electron microscopy to detect the virus. Mechanical inoculation with partially purified virus preparations resulted in stunting and development of pinpoint chlorotic lesions on Vigna unguiculata TVu-76 and symptomless systemic infection of Nicotiana occidentalis. The virus was not mechanically transmissible from N. occidentalis to banana. A serological relationship between this virus, banana die-back virus (BDBV), and tobacco ringspot, tomato ringspot, and cacao necrosis nepoviruses was found. The nematode species around the affected banana plants were isolated: Helicotylenchus multicinctus (Cobb) Golden was the dominant species, low numbers of H. dihystera (Cobb) Sher were present, but no virustransmitting nematodes were found in soil or banana roots. Further studies are needed to determine the mode of spread of BDBV, the implications for banana/plantain production in sub-Saharan Africa, and the safe international movement of germplasm.

RESUMEN

The effect of temperature on symptom expression and detection of banana streak badnavirus (BSV) by immunosorbent electronmicroscopy (ISEM) and enzyme-linked immunosorbent assay of 12 in vitro-propagated plantain hybrids (genome AAB × AA), 3 ABB cooking banana, and 3 AAB plantain landraces was studied. Experiments were done for 2 years under two temperature regimes, 28 to 35°C in a screenhouse and 22°C in a temperature-controlled room. Most BSV-infected plants of plantain hybrids expressed symptoms under both conditions. Symptom expression was enhanced when plants were continuously grown at 22°C, but later became indiscernible when plants were continuously grown at 28 to 35°C. Plants grown at 22°C and showing severe symptoms contained significantly higher virus titer than plants grown at 28 to 35°C. When asymptomatic plants with very low virus titer at 28 to 35°C were transferred back to 22°C, there was a significant increase in both symptom severity and concentration of virus (greater than 3 to 5 times) in leaf tissues after 9 months. In contrast, the concentration of virus and symptom severity decreased in plants after transfer from 22°C to 28 to 35°C. Micropropagated plants of AAB plantain landrace cv. Mimi Abue and ABB cooking bananas (cvs. Bluggoe, Cardaba, and Pelipita) did not express visible symptoms under either temperature regime, but BSV was detected by ISEM in 23% of the plants. After 2 years at 22°C, virus was detected in 64% of the plants, but the concentration of virus remained low. Implications of these results on quarantine screening of in vitro plants and virus diagnosis are discussed.

RESUMEN

Banana streak badnavirus (BSV) has been reported from Musa spp. in many parts of West Africa, including Benin, Côte d'Ivoire, Ghana, and Nigeria (1). Symptoms of BSV infection in Musa spp. are sometimes similar to and confused with those caused by cucumber mosaic virus (CMV). BSV is prevalent in areas of southern Nigeria bordering Cameroon, and the disease may also be present in other Central African countries. In June 1996, six leaf samples with viruslike yellow/chlorotic streak symptoms were collected from plantain in the four villages, Awae, M'Balmayo, Nkolfep, and Nkolfoulou, within a 60-km radius of Yaoundé, Cameroon's capital. The samples were indexed for BSV and CMV by both triple antibody sandwich indirect enzyme-linked immunosorbent assay (TAS-ELISA) and immunosorbent electron microscopy (ISEM) to ascertain the presence of these two viruses. The TAS-ELISA was performed with rabbit polyclonal antiserum (obtained from B. E. L. Lockhart, University of Minnesota) for trapping and mouse polyclonal antiserum (obtained from G. Thottappilly, IITA) for detection. Out of the six samples, one tested strongly positive (>×2 A405 of the healthy control) and four were weakly positive (<×2 but >×1.5 A405 of the healthy control) for BSV by TAS-ELISA. However, all six samples contained BSV particles when examined by ISEM with rabbit polyclonal antiserum (from B. E. L. Lockhart) for trapping. None of the samples tested positive for CMV. These results confirm that BSV is present in Cameroon and that BSV is likely to be the causal agent of the symptoms. Reference: (1) C. Pasberg-Gauhl et al. Plant Dis. 80:224, 1996.