SUPERFICIAL RINGSPOT NECROSIS OF POTATO-TUBERS - A RECENT DISEASE CAUSED BY POTATO VIRUS-Y

A disease observed for the first time in 1988 and still present in 1989-1990 occurred in Lebanon on seed potatoes of the Lola cultivar. Tuber surface showed severely necrotic areas which varied in shape. Generally these had the appearance of fairly regular bows and rings (figs 1, 4), firstly protruding, then later becoming sunken and dark brown (fig 3). In some cases the skins were cracked. Under these superficial necroses, the adjacent flesh was brown, but there was no deep abnormality inside the tuber (figs 2, 6). The first symptoms appeared rather suddenly at the harvest period in a large number of tubers; in the others, they occurred during the first weeks of storage. The disease was tuber- and sap-transmitted. In tubers or leaves from infected plants, 2 viruses, potato virus Y (PVY) and potato virus X (PVX) were identified by ELISA and IEM, the former virus being systemtically present. These serological detection methods, biological indexing, bait plants, and dip-method did not reveal any other viruses such as those which normally induce tuber necrosis, like tobacco rattle virus, potato mop-top and tobacco necrosis virus. PVY and PVX isolated from potato plants were multiplied on tobacco and back-inoculated onto potatoes; the same necrotic symptoms on tubers were obtained with either PVY alone or associated with PVX. PVX alone did not provoke necrosis symptoms, nor did it modify the reaction of the plants when present in the inoculum with PVY. So it was clearly demonstrated that PVY was alone responsible for the necrotic reactions in the tubers. This PVY isolate was characterized as PVY(N), first serologically by using Mabs specific to this PVY subgroup, and also from the reactions that it induced on Nicotiana tabacum L Xanthi, Samsum or White Burley. However, when its host range and its effect on potato foliage were taken into consideration, it appeared distinct from our standard PVY(N) strain (tobacco veinal necrosis strain). Our results were also different from those of other authors who previously isolated PVY(N) from tubers showing identical necrosis. Indeed, such tuber necrotic ringspot diseases, consistently associated with PVY, have previously been reported in several European countries, including Hungary and Yugoslavia.