Mechanical Transmission of Citrus Viroids

Abstract

Preliminary transmission assays conducted under greenhouse conditions demonstrated that Citrus exocortis viroid (CEVd), Citrus bent leaf viroid (CBLVd), Hop stunt viroid (HSVd), Citrus viroid III (CVd-III), and Citrus viroid IV (CVd-IV) can be mechanically transmitted from citron to citron (Citrus medica) by a single slash with a knife blade. The impact of mechanical transmission of viroids by pruning and harvesting operations was also demonstrated in experimental and commercial field plots. Transmission efficiency under field conditions ranged from 4% in ‘Nules’ clementine to 10% in ‘Navelina’ sweet orange and 21% in ‘Verna’ lemon. Transmission efficiency varied only slightly with viroid and donor hosts. The impact of viroid transmission on tree height, canopy volume, and crop harvest was minimal. When the donor host was coinfected with several viroids, the viroids were not necessarily cotransmitted. Considerations regarding viroid transmission in other climates are discussed. Measures to control viroid spread in nurseries should be mandatory in certification programs.

Viroides em Citros

“…Plantas de 'Cravo' inoculadas com CEVd apresentavam zonas amareladas na casca dos ramos secundários e as inoculadas com CVd-IV induziram o aparecimento de rachaduras na casca do ramo principal e secundários. As plantas inoculadas com a mistura de viroides apresentaram enfezamento, ramos amarelados, com rachaduras e folhas pequenas (Barbosa & Duran-Vila, 2005;Barbosa et al, 2005a). O amarelecimento de ramos, observado como resposta à inoculação com o CEVd, já havia sido relatado anteriormente como "yellow blotching".…”

Phloem restriction of viroids in three citrus hosts is overcome by grafting with Etrog citron: potential involvement of a translocatable factor

“…These viroids become systemic upon inoculation to their citrus hosts, which may express disease symptoms (sensitive hosts) or not (tolerant hosts). Most citrus species are tolerant, thus acting as reservoirs of viroids, which when transmitted either mechanically to sensitive species (Barbosa et al, 2005) or by grafting infected tolerant scions onto sensitive rootstocks, incite symptoms. Among the sensitive hosts, the selection 861-S1 of Etrog citron (Citrus medica) has been widely used in bioassays because it expresses viroid-specific symptoms and accumulates easily detectable viroid titres.…”

Obtenção de plantas autotetraplóides de variedades de porta-enxertos de citros

“…As principais seriam a indução de maior susceptibilidade das frutas ao armazenamento ao frio, uma potencial redução da produtividade por planta; a falta de conhecimento sobre a patogenicidade dos viróides em outras variedades cítricas; a possível associação de viróides com doenças ainda não identificadas, o risco da distribuição a outras culturas, de viróides com potencial mais destrutivo (ROISTACHER, 1994); e a distribuição dos viróides em outros pomares devido à transmissão mecânica por meio de podas e colheitas (BARBOSA et al, 2005).…”