The tospoviruses Tomato spotted wilt virus (TSWV), Tomato chlorotic spot virus (TCSV), Groundnut ringspot virus (GRSV) and Chrysanthemum stem necrosis virus (CSNV) are well-known pathogens on tomato in Brazil. The thrips species Frankliniella occidentalis , F. schultzei , Thrips tabaci and T. palmi were studied for their competence to transmit these tospoviruses. Frankliniella occidentalis transmitted all four tospoviruses with different efficiencies. Frankliniella schultzei transmitted TCSV, GRSV and CSNV. Although F. schultzei has been reported as a vector of TSWV, the F. schultzei population in the present study did not transmit the TSWV isolate used. A population of T. tabaci known to transmit Iris yellow spot virus (onion isolate) did not transmit any of the studied tospoviruses, and nor did T. palmi . Replication of these tospoviruses could be demonstrated by ELISA, not only in the thrips species that could transmit them, but also in those that could not. The results strongly suggest that competence to transmit is regulated not only by the initial amount of virus acquired and replication, but also by possible barriers to virus circulation inside the thrip's body.

In Brazil, tospoviruses have been reported in several horticultural and ornamental plants. In the northeast region of Brazil, a tospovirus has emerged as a devastating virus on onion cultures. Based on serology and the sequence of nucleocapsid (N) protein gene, this pathogen was identified as a strain of iris yellow spot tospovirus (IYSV) (1). This virus was first identified on iris and leek in The Netherlands and later on onion in Israel. For an effective integrated management of tospoviruses in Brazil, identification of IYSV vector is essential. Three thrips species, Thrips tabaci, Frankliniella schultzei, and F. occidentalis, that are major vegetable and floral crop pests in the Federal District, Brazil, were tested for their ability to transmit the virus by leaf disk assay (2). All thrips, up to 8 h old, were given an acquisition access period of 48 h at 25°C on IYSV-infected Nicotiana benthamiana plants in Tashiro-cages. Thrips were then reared on uninfected Datura stramonium detached leaves until the adult stage. These adults were transferred individually to microcentrifuge tubes containing an N. benthamiana leaf disk and were incubated for 48 h for virus inoculation. The leaf disks were then incubated 4 more days to allow development of the virus infection, and the presence of virus was evaluated by Dot-enzyme-linked immunosorbent assay (Dot-ELISA) with polyclonal antibodies against N protein of IYSV. Adult thrips were also used for direct inoculation to N. benthamiana plants, three thrips per plant. By the leaf disk assay, 45.8% (22 out of 48) of T. tabaci transmitted the virus, but F. schultzei (n = 48) and F. occidentalis (n = 32) did not transmit it. All plants (4 out of 4) directly inoculated by T. tabaci showed symptoms and infection by Dot-ELISA, while no plants inoculated with F. schultzei (n = 5) and F. occidentalis (n = 3) were positive, either by symptom observation or by Dot-ELISA. Only T. tabaci showed potential for a high capacity to transmit the IYSV onion isolate. In the field, considering the host preference of thrips, T. tabaci was considered the most important vector species of IYSV on onion. References: (1) L. Pozzer et al. Plant Dis. (In press.) (2) I. Wijkamp and D. Peters. Phytopathology 83:986, 1993.

O mosaico comum do milho (Zea mays) destaca-se, atualmente, entre as doenças mais importantes dessa cultura, sendo causada por um complexo de potyvirus transmitido por afídeos. Esse trabalho teve por objetivo a identificação de fatores que podem contribuir para a incidência dessa virose e disseminação do seu agente causal. Plântulas de 115 cultivares dos Ensaios Nacionais de Milho-Centro foram submetidas a quatro inoculações com o complexo viral, utilizando-se, em cada uma, parcelas com cinco plantas de cada cultivar. A maioria das cultivares mostrou-se suscetível, apresentando sintomas da virose, aos 15 dias após a inoculação. Espécies de gramíneas também foram inoculadas e mostraram-se hospedeiras desses vírus. Através do teste dot-ELISA, a presença dos vírus do mosaico comum foi detectada em folhas de milho provenientes de vários municípios dos estados de São Paulo, Minas Gerais e Goiás. No período de mar/97 a fev/98, dois híbridos de milho foram plantados mensalmente, e semanalmente avaliados quanto à incidência do mosaico comum. Observou-se tendência de maior incidência nos plantios de novembro, dezembro e janeiro, coincidindo com as elevadas temperaturas e precipitações pluviométricas do verão. Os resultados obtidos contribuem para a recomendação de medidas para o controle dessa virose.