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Potato virus X abd Y on potato crops in Mendoza. The presence of potato viruses X (solanum virus 1 SMITH; Annulus dubius HOLMES) and Y (Solanum virus 2 SMITH; Marmor upsilon HOLMES) on potato crops (Solanum tuberosum L. var. White Rose and var. Up-to-Date) is determined. The virus Y had not been reported up to the present in any host in Mendoza, while the virus X was reported on tomato (Lycopersicon esculentum MILL.), and, exclusively on sintomatological observations bases, on potato crops. The methods followed are described and the results are analized. Se determina la presencia de los virus X (Solanum virus 1 SMITH; Annulus dubius HOLMES) e Y (Solanum virus 2 SMITH; Marmor upsilon HOLMES) en los cultivos de papa (Solanum tuberosum L. var. White Rose and var. Up-to-Date) de Mendoza. El virus Y no habías sido señalado hasta ahora para Mendoza en ningún hospedante; el virus X, en cambio, se encontraba señalado en tomate (Lycopersicon esculentum MILL.) y en base exclusivamente a observaciones sintomatológicas, en papa. Se indican los métodos empleados y se analizan los resultados obtenidos. Fil: Pontis, Rafael E.. Universidad Nacional de Cuyo. Facultad de Ciencias Agrarias Fil: Feldman, José M.. Universidad Nacional de Cuyo. Facultad de Ciencias Agrarias

Potato virus Y (PVY) is the prototype of the potyvirus group (Harrison et al. 1971). The group is characterized by particles of diameter about 15 nm. and a length of 720 to 800 nm. and by the presence of pin-wheel inclusion bodies in infected tissues. Because of their low concentration in infected sap and tendency to aggregate during purification, few viruses of this group have been obtained in sufficient quantity for physical or chemical characterization. Tobacco etch virus (TEV) is the most extensively studied virus of the group. PVY has been recently purified (Delgado-Sanchez & Grogan, 1966; Damirdagh & Shepherd, 1970a). Its amino acid composition has been determined and compared with that of TEV (Stace-Smith & Tremaine, 1970). The present investigation was undertaken to determine the molecular size of protein subunits of PVY. A combined method of amino acid analysis and cyanogen bromide cleavage was used for this purpose.

Potato virus Y was obtained from field crops of potatoes in many strains which differed widely in virulence and caused diseases in the variety Majestic ranging from severe leaf-drop streak to mild mosaic. The symptoms caused by these strains in seven potato varieties and tobacco are described and compared with those caused by the serologically related potato virus C. No changes were noted in the behaviour of any of the strains over three years, during which they were transmitted to many different plants. Potato virus C was not transmitted by Myzus persicae, the most efficient vector of other strains of virus Y. Nor was virus C transmitted by eleven other species of aphides, eight of which transmitted virus Y. The efficiency with which different species acted as vectors of virus Y varied greatly, and it is suggested that in some species only occasional individuals can transmit. Possible mechanisms for the evolution of viruses C and Y are indicated, and the effects of changes in virus, vector, and host on the prevalence of insect-transmitted viruses are discussed.

When potato virus Y (PVY), which is aphid-transmitted, and a related virus, potato virus C (PVC), which is not aphid-transmitted, were jointly inoculated to Nicotiana glutinosa plants, mixed isolates, transmitted by aphids from systemically infected leaves had some of the properties of both. Like PVY they were transmitted by Myzus persicae, but unlike PVY they caused black necrotic lesions on inoculated leaves of Majestic, President, and Craig's Snow-White potatoes (Solanum tuberosum L.). Like PVY and unlike PVC they caused systemic infection when manually inoculated to the potato varieties, but the symptoms that developed resembled those caused by PVC when it is transmitted to the same varieties by grafting. Many isolates from local lesions caused by the mixed virus isolates on potato were like PVC, and few resembled PVY. Isolates from systemically infected parts of potato less often gave symptoms like those caused by PVC, and many gave mild mottle with no necrosis. Three PVC-like viruses that resemble the isolates from mixed infection are: PVCn, an aphid-transmissible PVC-like virus described previously (Watson, 1956); PVCniab, found naturally occurring in International Kidney potatoes in Jersey, and a virus from India found in Solanum jasminoides (Badami and Kassanis, 1959). These viruses do not usually invade the potato varieties systemically but occasionally yield isolates that do so. The symptoms when they do invade resemble those caused by the isolates from mixed infections in N. glutinosa and often appear to be genetically unstable although the viruses from which they were derived either are stable in potato or, as with PVCn, revert to a form even more like PVC. The nature of the interaction between PVC and PVY is unknown, but a likely explanation is that particles are formed that carry genetic determinants from both viruses.

WATSON showed that fasted Myzus persicœ (Sulz.) were more than 60 per cent infective after 5 min. on tobacco infected with potato virus Y, but after 4 hr. on infected plants only about 10 per cent of the aphids were infective1. During investigations on the aphid transmission of potato virus Y, it was observed that, after several hours on tobacco, some aphids appeared to be feeding, others were just commencing to insert their stylets, and still others were wandering about the plant. The present communication reports which aphids transmit potato virus Y after a group has spent several hours on an infected plant.

In addition to giving different kinds of symptoms when infected with potato virus Y, individual potato varieties also differ in their susceptibility to infection, in the concentration of virus attained in their sap, and in their efficiency as sources of virus for aphides. Their relative susceptibility in the open when exposed to equal chances of infection is correlated with the ease with which they become infected when colonized with infective aphides, and can be assessed from tests made under glass. Methods for making such tests are described; these need few tubers and give reproducible results. It is considered that they could be applied in studying the inheritance of this type of resistance and to test the behaviour of new seedlings. The American variety Katahdin was the most resistant of those tested, but there were significant differences between commercial British varieties. In the open, all varieties were equally colonized by aphides and resistance to infection with virus Y was not correlated with resistance to leaf roll.

SINCE many workers in widely divergent fields are interested in the biological effects of ionizing radiations, new methods of approach to this study may have wider application than is at first realized. This communication reports that ultra-violet irradiation of tobacco infected with potato virus Y can reduce the availability of virus to aphids without causing apparent injury to the plants or greatly reducing the infectivity of sap extracts as measured by starch – iodine lesions.