PRESENCE OF SOME VIROIDS IN SWEET ORANGE TREES INFECTED WITH THE GUMMY BARK DISEASE

Gummy bark disease of sweet orange (Citrus sinensis) trees has been reported from a few citrus growing regions of the world. The disease symptoms consist of mild to severe stem pitting and brown gummy flecks in the bark of sweet orange trees on sour orange ( C. aurantium) rootstock, being conspicuous several centimeter above the bud union on the trunk. The pits were usually impregnated with brown gummy deposits. In surveys conducted in 2005, several sweet orange trees, cv. Moro Blood, with symptoms of gummy bark were encountered in Mahdasht groves in Sari. One year old sweet orange budlings on Troyer citrange (Poncirus trifoliate × C. sinensis) rootstock and sour orange seedlings were graft-inoculated with buds from two symptomatic sweet orange trees. The budded plants were cut back to force growth of new flushes one month after budding. Gum impregnation of the bark of sweet orange on Troyer citrange rootstock was observed three to five years after graft-inoculation. RNA was extracted from graft-inoculated symptomatic sweet orange and symptomless sour orange plants and healthy seedlings of both species, as controls and a sweet orange budling previously proved to harbour several citrus viroids (Alavi et al., Iranian J. plant path. 41: 65-66) using TRIZOL (Invitrogen, Carlsbade. CA). RT-PCR was conducted using specific primers for citrus viroids and for the major coat protein gene of citrus tristeza virus. Electrophoresis of PCR products revealed presence of hop stunt viroid, citrus bent leaf viroid, citrus viroids III and IV and citrus exocortis viroid in the samples. This is the first report on the infection of the gummy bark diseased oranges with five different citrus viroids. The role of these viroids , if any, in gummy bark disease of sweet orange needs to be determined.