: A Symptomless Carrier of Satsuma Dwarf Virus which Accelerates Natural Transmission in the Fields Permalink

Natural infection of satsuma mandarin by satsuma dwarf virus (SDV) occurred in a field where China laurestine, Viburnum odoratissimum Ker. , was planted as windbreaks. A high percentage of the windbreak plants were infected. Virus-free nursery trees of China laurestine became infected with SDV 2-8 months after planting in potted soil collected from SDV-infested fields. Infected trees were free of symptoms. These results indicate that SDV-transmission is enhanced by the presence of China laurestine and that it may be an important inoculum source for citrus trees. Index words. soil transmission, ELISA. Satsuma dwarf (SDV) was proven to be a graft-transmissible virus disease in 1952 (ll), but had been noticed in Japan for about 20 years previously as an unknown disorder. The natural spread of SDV in the field has been noted since its discovery. Izawa (4) investigated several affected fields and reported that the disease was present in only one tree in 1933, and extended to 48 trees by 1953, and to 153 trees by 1965. Disease spread was in a concentrated circle, and trees replanted after removal of the original diseased trees became infected within several years. Based on these observations SDV has been considered soil-transmissible. Trials to reproduce soiltransmission in pot tests and to find the vector have been unsuccessful. Recently, one of the authors observed rapid spread of SDV in a field where China laurestine, Viburnum odoratissimum Ker. (syn. V. awabuki K. Koch), was planted as a windbreak (7). These windbreak plants indexed positively for SDV by ELISA. This paper reports the presence of SDV in China laurestine and the rapid acquisition of SDV by China laurestine trees from infested soil. MATERIALS AND METHODS Indexing. China laurestine and satsuma mandarin trees were indexed for SDV by ELISA (1) using antiSDV serum. Tender, immature shoots of citrus plants were macerated with 10 volumes ( V N ) of phosphate-buffered saline which contained 0.05% Tween-20 (PBS-tween) plus 2% polyvinylp yrrolidone (PVP) and 0.05% 2-mercaptoacetic acid. Immature leaves of China laurestine and rootlets of these plants and of trifoliate orange rootstock, were macerated in PBS-Tween plus 2% PVP and 0.05% L-cysteine. A microplate filled with sample extracts was incubated at 28 C for two h and then transferred to a refrigerator overnight. Alkaline phosphatase-antibody conjugates were diluted with PBStween plus 0.1% bovine serum albumin, which prevented nonspecific reactions. Incubation conditions for coating plates and application of conjugates was 28 C for 4 h. Other procedures were according to Clark and Adams (1). Back inoculation. Two-week-old plants of white sesame were mechanically inoculated with extracts from the SDV-ELISA-positive China laurestine. Inoculated plants were kept at 22 C in a greenhouse. After development of typical symptoms, mechanical transmission of SDV from diseased sesame to Etrog citron seedlings were made by slashing the receptor plant 60 times with a knife contaminated with sap from sesame. Five to six months after inoculation the citrons were assayed by budding to a virus-free rough lemon rootstock top-grafted with virus-free satsuma mandarin. Other Virus Diseases 349 Acquisition of SDV by China laurestine from soil. China laurestine trees proved SDV-free by ELlSA were propagated as rooted cuttings in autoclaved soil. Propagated plants were potted in soil collected from the 5-10 cm deep zones of the rhizosphere under SDV-affected satsuma mandarin trees. These plants were placed outdoors at the research station for five months and then transferred into a glasshouse. New sprouts were periodically indexed for SDV by ELISA.