Potato virus X infection of Netted Gem potato

When Netted Gem potato plants were inoculated with potato virus X 7 to 31 days before defoliation the tubers were not uniformly infected. Testing of a single germinating eye chosen at random from each tuber detected fewer than half of the tubers from which stem-end, mid, or apical-end sections produced infected plants. The number of completely and partially infected tubers increased as the length of time between inoculation and defoliation was increased. Infection was not correlated with tuber size. Partially infected tubers produced equal numbers of infected plants from stem-end and apical-end sections.     

Extreme resistance to infection by potato virus X in genotypes of wild tuber-bearingSolanum species

Summary Clones derived from thirty-one different accessions (nineteen of Argentine origin) belonging to eightSolanum species were screened for resistance to infection by potato virus X strain cp (PVX cp) by mechanical inoculation of plantlets that had been micropropagated in vitro. Estimates of PVX multiplication obtained by enzyme linked immunosorbent assay and slot blot nucleic acid hybridization allowed the identification of resistant clones derived from five accessions belonging toS. commersonii S. oplocense, S. sparsipilum andS. tuberosum andigena. Resistant genotypes supported PVX concentrations 5 to 15 times smaller than did the susceptible control cultivar Spunta. Graft inoculation test confirmed the presence of extreme resistance similar to that conferred by the ‘immunity’ gene X¹ (also called RX_act).     

Resistance to virus S in the potato

On the basis of data from 18 crosses, resistance to potato virus S of the type encountered in Saco potato, segregated as an homozygous recessive. Neither Saco nor seedlings found to be resistant in greenhouse tests became infected when grown in the field and brushed with PVS-infected haulms. Meanwhile, susceptible seedlings from a cross segregating resistant and susceptible became infected in both greenhouse and field as readily as did seedlings from a cross producing only susceptibles. Resistance to PVS segregated independently of resistance to PVX.     

Resistance to potato virus M in certain wild potato species

Summary Resistance to PVM has been found in some progenies ofSolanum gourlayi Haw., andS. spegazzinii Bitt. Clones from two progenies ofS. gourlayi, INTA 7330 and 7356, remained uninfected even after repeated graft inoculation.     

Eradication of potato virus X from potato by ribavirin treatment of cultured potato shoot tips

Ribavirin treatment of cultured potato shoot tips was tested as a means of eradicating potato virus X (PVX) from two potato cultivars. Shoot tips were cultured on liquid medium containing 0, 1, 10 or 100μg/ml ribavirin. Cultures were evaluated periodically for viability, and scored for vigor on a relative growth scale. Developed plantlets were assayed for PVX by transmission tests toGomphrena globosa. Ribavirin treatment was phytotoxic at all tested concentrations, and lethal to all cultivars treated at 100 μg/ml. Treatment also delayed plantlet development by up to 2 months at 1 and 10 μg/ml as compared with nontreated controls. PVX assays indicated that 80 and 83% of the plantlets were free of PVX following treatment with 10 μg/ml for cultivars Russet Burbank and Red McClure, respectively. Five and 6% of the plantlets developed from the 1 μg/ml treatment were PVX-free, whereas 0 and 2% of the controls were PVX-free for the same cultivars. Six to 8 months were required to develop plants from shoot-tip cultures treated with 10 μg/ml ribavirin.     

Ultrastructure of potato tubers infected with potato virus X

The ultrastructure of Katahdin tubers infected with potato virus X (PVX) is compared with PVX-free tubers by electron microscopy. Electron-dense globules surrounding the inner periphery of the tonoplast were observed in PVX-infected tubers, while PVX-free tubers did not show such bodies. Other organelles were comparable in PVX-infected and PVX-free tubers     

Resistance to potato virus a in theKatahdin variety

Summary Katahdin potatoes did not become infected with virus A during 2 years field exposure in which 49% ofGreen Mountain were infected. In the greenhouse,Katahdin plants were not infected by aphids at any stage of plant development, and they were resistant but not immune to virus A from sap inoculation. Katahdin plants were readily infected with virus A by grafting, but unlike the hypersensitiveIrish Cobbler, Katahdin reacted like those of susceptible varieties — practically symptomless to virus A alone, but with a mosaic when virus X was also present. Despite the apparent superticial nature of the resistance, several tests failed to show any “inhibitor” in the surface tissues ofKatahdin. Aphids feeding on chlorophyll-free sprouts developed in the dark or on stem tissues beneath stripped-off epidermis did not cause infections. Aphids already carrying virus A probed intoKatahdin leaves without losing the ability to infectNicandra physaloides. And virus free aphids could acquire virus A from leaves of graft-in-fectedKatahdin as readily as from “susceptible” varieties.

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Zusammenfassung Die KartoffelsorteKatahdin war nach einem 2-j?hrigen Feldanbau nicht mit A-Virus infiziert, w?hrend 49% der SorteGreen Mountain erkrankte. In Gew?chshausversuchen konnten Pflanzen der SorteKatahdin in keinem Entwicklungsstadium durch Blattl?use infiziert werden. Diese Sorte ist resistent aber nicht immun gegenüber mechanischer Safteinreibung. Pfropfungsübertragung war stets erfolgreich; in Gegensatz zur überempfindlichen SorteIrish Cobbler reagierteKatahdin jedoch auf die Infektion mit A-Virus ebenso wie anf?llige Sorten, d.h. praktisch ohne Symptome, jedoch mit Mosaiksymptomen in der Gegenwart von X-Virus. Trotz des oberfl?chlichen Charakters dieser Resistenz konnte in keinem Versuch ein “Hemmstoff” in den oberen Zelllagen der Pflanzen nachgewiesen werden. Blattl?use, die auf im Dunkeln gewachsenen Chlorophyll-freien Sprossen oder auf Stengeln deren Epidermis entfernt war saugten, konnten keine Pflanzen infizieren. A-Virus tragende Blattl?use bohrtenKatahdin Bl?tter an ohne das Verm?gen zu verlieren anschliessendNicandra physaloides zu infizieren. Gleichfalls, konnten virusfreie Blattl?use A-Virus aus durch Pfropfung infiziertenKatahdin Pflanzen ebenso aufnehmen wie aus “anf?lligen” Sorten.

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Résumé Dans une culture de la variétéKatahdin effectuéc pendant deux ans d'exposition en plein champ à l'infection, aucune infection par le virus A ne fut observée, tandis que l'infection était de 49% dans la variétéGreen Mountain cultivéc dans les mêmes conditions. En serre, les plantesKatahdin ne furent infectées par les pucerons à aucun stade de développement des plantes et il fut observé qu'elles étaient résistantes, mais non immunes à l'inoculation par le jus infectieux au virus A. Les plantesKatahdin étaient aisément infectables par le virus A par greffe, mais leur réaction n'était pas comparable à celle des plantes de la variété hypersensitiveIrish Cobbler. La réaction des plantesKatahdin était égale à celle des variétés susceptibles. c'est-à-dire qu'elles ne présentaient pratiquement aucun sympt?me après infection par le virus A seul, mais si celui-ci était accompagné du virus X, il apparaissait une mosa?que. Bien que la résistance semble être superficielle, les résultats de plusieurs tests ne révèlent aucun effet d'un élément “inhibiteur” dans les tissus superficiels. Des pucerons porteurs du virus A, su?ant les tiges dont l'épiderme était ou les germes de tubercules germés à l'obscurité, et de ce fait ne contenant pas de chlorophylle, ne causaient aucune infection. Des pucerons porteurs du virus sondaient les feuilles deKatahdin sans perdre leur faculté d'infecter plus tard des plantes deNicandra physaloides. Les pucerons non-porteurs du virus pouvaient être infectés par le virus A en su?ant les feuilles d'uneKatahdin infectéc par greffe aussi bien qu'à partir de variétés “susceptibles”.

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Contribution No. 40 Research Station Fredericton, New Brunswick.     

Resistance to potato leaf roll virus inSolanum brevidens

Summary Plants ofSolanum brevidens graft-inoculated with potato leaf roll virus (PLRV) grew vigorously and had a normal healthy appearance. Although presence of the virus was confirmed in all inoculated plants by graft tests to potato and/orDatura stramonium, recovery of PLRV fromS. brevidens PI 218228 usingPhysalis floridana and the aphidMyzus persicae was erratic and only few test seedlings became infected. Ease of recovery of the virus using aphids was not influenced by presence of a continuous graft union with infected potato. Testing ofS. brevidens PI 245763 withM. persicae was not possible due to poor aphid survival on plants of this accession.     

Reinfection of potato seed stocks with potato virus S and potato virus X in Wisconsin

Reinfection of potato seed stocks with the potato viruses S (PVS) and X (PVX) varied with cultivar, virus, and grower. Rapid recontamination was observed for the cultivars Norgold Russet and Ontario with PVS and for the cultivars LaChipper, Norchip, and Norgold Russet with PVX. Recontamination was low for the cultivars LaChipper and Monona with PVS and for the cultivars Kennebec, Monona, Norland, and Superior with PVX. Survey results suggest that PVS and/or PVX can be eliminated from cultivars which appear to possess field resistance to infection, but that further evaluation of cultivars which are very susceptible to reinfection will be necessary.     

Resistance to potato viruses M,S, X and the spindle tuber virus in tuber-bearingSolanum species

On two separate occasions, some available accessions of tuber-bearingSolanum species were tested for resistance to potato viruses. Challenge inoculations were made mechanically with infective sap; and assessment was by sub-inoculation to differential host plants, supplemented by serological tests. In 1956–57, four accessions remained free from potato virus S (PVS), and nine remained free from PVM. In 1968–70, one or more clones of 11 accessions showed resistance to PVS—one of them,S. megistacrolobum, a diploid, apparently being hypersensitive. Resistance to PVX was found in seven accessions, and to PVM in two. There appeared to be resistance to the potato spindley tuber virus in five accessions.     

Volunteer potatoes as a source of potato leafroll virus and potato virus X

Volunteer potatoes were investigated as infection sources for potato leafroll virus (PLRV) and potato virus X (PVX) in a high elevation seed potato growing area of eastern Idaho. Population densities ofMyzus persicae were assessed. Percentage of PLRV and PVX infection of the volunteers and seed potato crops was determined, as well as density of volunteers and certain parameters of volunteer growth and reproduction. Volunteers apparently harbored no more PLRV than the potato crop from which they originated. But they were found to be an important reservoir of PVX with the infection increasing as much as 12.43% in one year. No aphids capable of transmitting PLRV were found although one species that can transmit potato virus Y was recorded. The mean density of volunteers varied from 0 to 84,880 stems/ha. The number of tubers remaining in the field after harvest and winter weather conditions appeared to be the only factors affecting volunteer density. Volunteer plants arising from seed pieces at an average depth of 6.1 cm were found to set an average of 2.1 new tubers per plant at an average depth of 4.0 cm. These results suggest that volunteer potatoes are a significant source of PVX infection in subsequent seed potato crops.     

Resistance of potato clones to the green peach aphid and potato leafroll virus

During 1980 and 1981 potato cultivars and breeding selections, including cultivated species and their hybrid derivatives, were evaluated for resistance to the green peach aphid (GPA),Myzus persicae (Sulzer), and to potato leafroll virus (PLRV). Criteria used were the number of aphids which colonized the clones in free choice field experiments and the number of plants derived from these experiments which showed symptoms of PLRV infection. Generally, greater resistance to GPA was found inSolarium tuberosum gp.andigena selections and hybrids than in gp.tuberosum cultivars. There were approximately fourfold differences in season-mean GPA levels among the clones tested each year. Forty-two families, representing a cross-section of the USDA breeding populations at the University of Idaho Research and Extension Center, Aberdeen, showed a similar range in colonization levels. Resistance to GPA colonization appeared to be more prevalent in gp.andigena, gp.phureja, and gp.stenotonum derivatives. There was a weak positive correlation (r² = .34, P = .01) between foliar total glycoalkaloids and season-mean GPA colonization levels for six clones representing the range of observed resistance to GPA. Resistance to GPA colonization was apparently not directly related to resistance to PLRV infection. Katahdin, for example, was relatively susceptible to GPA colonization but very resistant to PLRV infection whereas selection A69657-4 (gp.andigena) was among the most resistant to GPA colonization but among the more susceptible to PLRV infection. Breeding for resistance to GPA colonization therefore may not be as promising for PLRV control as developing PLRV resistant cultivars.     

Resistance to potato virus Y inSolanum tuberosum spp.andigena

Tests were made on lines ofSolanum tuberosum spp.andigena to determine 1) whether resistance to potato virus Y exists in the Andigena germplasm, 2) the nature of its inheritance, and 3) the type of resistance. Results of the isolation and identification studies indicated that the pathogen involved was a common strain of potato virus Y. Under field conditions susceptible plants frequently escaped infection. However, field exposures over two seasons resulted in the same ratios as tests in which the same progenies were mechanically inoculated. Mechanical inoculation at the seedling stage proved to be a reliable means of transmission and resulted in accurate screening for resistance. Of 641 tub × tub clones tested, all were susceptible to the virus. Of 366 tub × adg clones tested, 170 were resistant and 196 were susceptible. This fits a 13:15 ratio, assuming random chromatid segregation and a single dominant gene conferring resistance. Plants that were resistant following mechanical transmission were also resistant when inoculated by aphids, indicating the reliability of mechanical transmission as a means of screening for resistance. To determine the type of resistance, top-graft and approach-graft tests were made. Failure to recover the virus from resistant plants inoculated by either grafting method suggests that immunity is the type of resistance involved.     

An enhanced multiplication of potato virus X is not related to a synergistic reaction between the potato viruses X and Y

Summary Greenhouse experiments demonstrated a differential interaction between potato viruses X and Y in two potato cultivars. Enhancement of PVX synthesis in doubly infected plants occurred only in the PVY-susceptible potato cv. Ulla, whereas PVX multiplication was almost completely inhibited in the doubly infected PVY-resistant cv. Franzi. However, a synergistic effect was also evident in the latter cultivar in the form of a growth inhibition of the plants. An increased multiplication of one or both of the viruses is, therefore, not related to a synergistic reaction.