DETECTION OF Hop stunt viroid IN LEMON, SWEET LIME, CLEMENTINE AND SATSUMA MANDARINS AND GRAPEFRUIT TREES IN MAZANDARAN

Hop stunt viroid (HSVd) of the genus Hostuvirod, is mostly known around the world as the agent of cachexia-xyloporosis disease of mandarins (Citrus reticulata Blanco, C. unshiu Marco., and C. nobilis Lour.) and their hybrids, tangelos (C. reticulata × C. paradise Macf.) and tangors [C. reticulata × C. sinensis (L.) Osb.]. Sweet lime (C. limettioides Hort. ex Tanaka) is also susceptible, displaying stunted growth and stem pitting. Symptoms on mandarins and their hybrids include overall chlorosis of the trees, gum impregnation of the bark, formation of pits in the xylem and pegs on the cambial surface of the bark which fit in the pits on the wood. The severity of symptoms varies with the host species and cultivar and strain of the viroid. To estimate the prevalence of the viroid in some symptomatic and symptomless species of citrus commonly grown in Mazandaran province, samples were taken randomly from symptomatic Clementine mandarin (C. reticulata, Blanco), old line Wase Satsuma (C. unshiu) and sweet lime all of which displayed stem pitting and impregnation of gum in the bark tissue at and above the bud union and symptomless trees of lemon [(C. limon (L.) Burm. f., local cultivar)] and Duncan grapefruit (C. paradisi Macf.). RNA was extracted from the bark of one year old stems using TRIzol (Invitroge, Carlsbade, CA). Reverse transcription (RT) polymerase chain reaction (PCR) was performed using specific primers for HSVd and the products were electrophoresed on 2% agarose gels. The expected fragments of 300bp typical of HSVd, were detected in all Satsuma (20 out of 20) and 77% Clementine mandarin (19 out of 25), whereas 63% (14 out of 22), 100%(12 out of 12) and 33% (5 out of 15) of trees of sweet lime, lemon and grapefruit, respectively, were found to be infected with the HSVd. These symptomless trees are hidden sources of the viroid, contributing to maintenance and spread of HSVd and necessitate the use of sensitive molecular detection assays wherever eradication programs are in practice. This is the first report on infection of lemon and grapefruit with the HSVd in Iran.