Thermotherapy and tissue culture for elimination of potato virus X (PVX) in Mexican potato cultivars resistant to late blight

Plants from 6 potato cultivars uniformly infected with potato virus X (PVX) were exposed to darkness and gradual temperature increases from 37 to 40 C, during a two-week period. This was followed by shoot tip culture to obtain virus free individuals. The procedure was effective in obtaining PVX-free propagation material from 4 of the 6 varieties. Differences were observed in the response of the cultivars to the heat and to thein vitro culture. At least one plant from every cultivar showed repeated negative serological reaction against PVX after the tissue culture regeneration, except Atzimba and Montsama. Tollocan and Murca were the most resistant to heat, developed better duringin vitro culture and resulted in the largest number of PVX-negative plants. Rosita was intermediate, and Juanita did not adapt to heat stress.     

Salicylic acid enhances heat tolerance and potato virus X (PVX) elimination during thermotherapy of potato microplants

A modification of the standard thermotherapy used to eliminate virus from potato virus X (PVX)-infectedSolanum tuberosum microplants of the Mexican National Potato Program is described here. Microplants were cultured with or without 10^-5 M salicylic acid (SA) for 4 wk, then subcultured without SA and exposed to 42 C for 30 days. Survival was more consistent in SA-treated microplants: among seven genotypes, 40% to 100% of SA-treated microplants and 0% to 96% of controls survived at the end of thermotherapy. SA also improved survival of the post-thermotherapy subculture. Among surviving microplants, SA increased the virus-free yield to 100% from 40% to 65% in the controls. In an additional 30 PVX(+) genotypes, 98% of surviving SA-treated microplants were virus-free after thermotherapy, compared to 75% of controls. SA-treated microplants had lower catalase activity and higher hydrogen peroxide (H₂O₂) levels.     

Electrotherapy and shoot tip culture eliminate potato virus X in potatoes

Potato stems infected with potato virus X (PVX) were exposed to either 5, 10, or 15 miliampers (mA), for 5 or 10 minutes, followed by immediate planting the axillary buds tipin vitro. Temperature increased from 4 to 10 C in the tissues during the exposure to the electricity. After a 60 days growing period, therapy efficiency (TE, TE = % plant regeneration X% virusfree resulting plants) was influenced by the severity of treatment, since organogenesis and virus elimination were both stimulated by the electricity. The highest TE values were obtained at 15 mA for 5 minutes. Under these conditions, 40% to 80% of the buds regenerated, and 60% to 100% of the regenerated plantlets tested virus negative.     

Effects of primary and secondary infection with potato virus X (PVX) on yield,size, chemical composition,blight resistance and cooking quality of potato varietyKerr's Pink

Summary Trials with hand-inoculated and virus-free seed were set out from 1966–1970 to determine the effects of primary and secondary infection with potato virus X (PVX) on yield, tuber size, chemical composition, blight resistance and cooking quality of potato varietyKerr's Pink. Results of these trials show that primary infection with PVX increased the proportion of blighted tubers, reduced the percentage dry matter and dry matter yield. There was no effect on marketable yield, size or cooking quality. Secondary infection with PVX reduced the marketable yield and the yield of dry matter but increased the yield of potatoes over 6 cm in diameter as a percentage of the total yield. There was no effect on percentage dry matter or cooking quality.

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Zusammenfassung In den Jahren 1966 bis 1970 wurden 9 Versuche durchgeführt, um den Einfluss der Prim?r- (5 Versuche) und der Sekund?r- (4 Versuche)_Infektion mit PVX auf Ertrag, Gr?sse, chemische Zusammensetzung, Knollenf?ule und Kochqualit?t der SorteKerr's Pink unter Feldbedingungen festzustellen. Jeder Versuch umfasste als randomized block 2 Verfahren mit 6 Wiederholunge. Die Verfahren bestanden im Falle der Prim?rinfektion aus gesundem und infiziertem Pflanzgut, und im Falle der Sekund?rinfektionen aus gesundem Pflanzgut sowie aus Pflanzmaterial, das im vorhergehenden Jahr infiziert worden ist. Die Infektion wurde von Hand vorgenommen, indem jeder Stengel jeder Pflanze mit Karborundumpuder (400 Mesh) verletzt und mit infiziertem Tabaksaft inokuliert wurde. Das Inokulum wurde hergestellt, indem gleiche Anteile von Virusst?mmen, die auf weissem Burley-Tabak gew?hnlich vorkommende Symptome hervorrufen, erzeugt wurden. Die Ergebnisse der Versuche zeigen, dass bei Sekund?rinfektionen mit PVX der Gesamtertrag signifikant abnimmt (Tabelle 1). Der Einfluss des PVX auf die Gr?sse (Tabelle 2) ist im Falle der Prim?rinfektion nicht signifikant, aber das Gewicht von Knollen über 6 cm Durchmesser nach der Sekund?rinkektion nimmt um 15% zu. DiePhytophthora-Knollenf?ule (Tabelle 3) nahm bei Verwendung von gesundem Pflanzgut in allen F?llen ab, aber signifikante Ergebnisse wurden nur im Vergleich mit der Prim?rinfektion erreicht. Bei Prim?rinfektion mit PVX trat eine signifikante Reduktion in Prozent der Trockensubstanz ein (Tabelle 4), w?hrend die Trockensubstanzertr?ge pro Hektare (Tabelle 5) sowohl bei Prim?r-als auch bei Sekund?rinfektion signifikant abnahmen. Es bestand kein signifikanter Einfluss auf die Kochqualit?t.

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Résumé Les auteurs ont effectué 9 essais durant la période 1966–1970 afin de déterminer l'effet des infections primaire (5 essais) et secondaire (4 essais) avec le PVX sur la production, le calibre, la composition chimique, la résistance auPhytophthora et la qualité culinaire chez la variétéKerr's Pink dans des conditions de plein champ. Chaque essai comportait deux traitements en six répétitions réparties au hasard dans un bloc. Dans le cas d'infection primaire, les traitements consistaient en plants sains et plants infectés, et dans le cas d'infection secondaire en plants sains et en plants provenant du matériel infecté l'année précédente. L'infection a été réalisée à la main par inoculation de chaque tige de chaque plante avec du jus de tabac infecté après étant endommagé par la poudre de carborundum de tamis 400. L'inoculum provenait de la multiplication de proportions égales de strains de virus produisant les sympt?mes communs sur le tabacWhite Burlley. Les résultats des essais montrent que, dans le cas d'infection secondaire de PVX, la production globale est significativement diminuée (tableau 1). L'effet du PVX sur le calibre (tableau 2) n'est pas significatif dans le cas d'infection primaire, mais à la suite de l'infection secondaire, il se révèle un accroissement de 15% dans le poids des tubercules supérieurs à 6 cm. Dans tous les cas, on constate une diminution de l'attaque des tubercules par le mildiou lors de l'utilisation de plants sains mais les résultats ne sont significattifs que lors de l'infection primaire. On observe une réduction significative du pourcentage de matière sèche (tableau 4) dans le cas d'infection primaire de PVX, tandis que les productions de matière sèche par hectare (tableau 5) sont significativement réduites à la fois par les infections primaire et secondaire. Il n'appara?t aucun effet significatif sur la qualité culinaire.

     

Eradication of potato viruses A and S by thermotherapy and sprout tip culture

Summary Potato viruses A and S have been eradicated from varietiesJara andRajka by applying thermotherapy combined with nutrient culture of excised sprout tips.     

Use of the virus strain group concept to characterize the resistance to PVX and PVYo in the potato cv Allegany

Resistance to the potato viruses X (PVX), Y (PVY) and A (PVA) was examined in the potato cv Allegany released by Cornell University in 1990. Standard potato cultivars from North America and Europe were included for characterization of the resistance response. Allegany was extremely resistant to a strain group 3 isolate of PVX and reacted with hypersensitivity to an ordinary strain isolate of PVY (PVY^o). However, Allegany was susceptible to an isolate of PVY causing necrosis in tobacco (PVY^N), and to an isolate of PVA. No symptoms appeared following infection with PVY^N and PVA. Identification of existing virus strain groups in a geographic area is an important aspect of predicting cultivar response to inoculation in the field.     

Multiplex detection ofPotato virus S, Potato virus X, andPotato virus Y by non-radioactive nucleic acid spot hybridization in potato tissue culture plantlets

Potato plantlets initiated into tissue culture must be tested for numerous viruses prior to propagation for seed potato production. Ideally, one plantlet is tested for all pathogens of concern and, if found pathogen-free, this plantlet is propagated for production of seed potatoes. Commercially available ELISA kits are generally used for the pathogen tests, but the commercial kits have some limitations. For example, the protocols differ for different viruses, so multiple extractions must be completed, increasing the time and expense of testing. This is a significant problem with tissue culture plantlets, for which there is limited material available to test and an ever-increasing number of pathogens that must be tested for, including viruses in the potyvirus, carlavirus, potexvirus, luteovirus, pomovirus, tobravirus, tospovirus, alfamovirus, and tymovirus groups. We have optimized a non-radioactive nucleic acid hybridization (NASH) assay for the simultaneous detection of carlavirusPotato virus S (PVS), potexvirusPotato virus X (PVX) and potyvirusPotato virus Y (PVY) in potato tissue culture plantlets. This assay requires a single extraction from a small portion of a tissue culture plantlet for the detection of viruses from three different families.     

Elimination of viruses and hypersensitivity to potato virus Y (PVYo) in an important sudanese potato stock (Zalinge)

Viruses that infect naturally an important Sudanese potato stock Zalinge were detected using enzyme-linked immunosorbent assay, immunosorbent electron microscopy and sap-inoculation to test plants. All of the 19 plants of Zalinge tested were infected with potato leaf roll virus (PLRV) and potato virus S (P VS), and five plants also with potato virus X (PVX). No potato virus Y (PVY), A (PVA) nor M (PVM) were found. The viruses were eradicated with thermo and chemotherapy using standard procedures. The combination of both therapies did not result in any virus-free plants, but resulted in poor plant survival. Thermotherapy reduced the incidence of PLRV and PVS by 45% and 50%, respectively, and one virus-free plant was obtained. It grew vigorously in the greenhouse, was symptomless and had a significantly increased tuber yield compared to the virus-infected plants. Following sap-inoculation with PVY^O, Zalinge showed mosaic symptoms, developed necrosis in the leaves and stem and died 14 days post-inoculation. However, the plants of Zalinge infected with PVY^N remained symptomless, which suggested that hypersensitivity was specific to PVY^O. The fast development of lethal necrosis following infection with PVY^O may contribute to the low incidence of PVY in Zalinge in the field in Sudan.     

Shoot formation in potato tuber discs in tissue culture

Tuber discs of potato (Solanum tuberosum) growing on a modified Murashige and Skoog medium produced callus and embryoid bodies but failed to form shoots. However, if 0.4 ppm of 6-benzylaminopurine was added to the basal medium, shoots became visible seven weeks after inoculation.     

Relationships of defined PVX infection levels to verticillium wilt,yield, and quality of the Russet Burbank potato

Field studies were conducted over a three-year period to investigate effects of potato virus X (PVX) on the Russet Burbank potato cultivar. Seed (PVX-free and PVX-infected) used for this three-year study originated from the same PVX-free source and was grown and stored under similar conditions. PVX-free seed was found to increase yield over PVX-infected plots by 9 to 32%. With these yield benefits, effects of PVX on either the incidence of verticillium wilt or plant nutrition were not significant. There was a trend (P = 0.10) for a reduction of mean tuber weight when the PVX infection level exceeded 19%. Because of increases of undersized potatoes, the yields of U.S. #1 potatoes were reduced as levels of PVX infection were increased. With a PVX infection level of 36%, the yield of U.S. #1 tubers was reduced by 21% when compared with plots having 0% PVX. At an 88% infection level, the yield of U.S. #l’s was reduced still further (36% lower than plots with 0% PVX). Results demonstrate the importance of the level of PVX infection to potato production.     

Reaction to the potato leafroll virus (PLRV) in diploid potatoes

Summary Diploid parents with some resistance to PLRV, were intercrossed to give 3 families with 191 clones which were evaluated for reaction to PLRV and yielding ability. After inoculation with PLRV the clones could be separated into those: 1) resistant, 2) susceptible, 3) intolerant, reacting with low virus concentration, 4) tolerant and 5) intermediate in reaction. Both the ELISA test and the evaluation of external disease symptoms were necessary to separate the clones. No correlation was found between resistance to PLRV and tuber yielding ability.     

Importance and control of potato virus YN (PVYN) in seed potato production

Summary The tobacco veinal necrosis strain of potato virus Y (PVY^N) first became epidemic in several European countries in the 1950s. To prevent PVY^N-infections, account must be taken of virus sources both within and without potato fields; for forecasting, the aphid vector species, their vector efficiency, and their flight activity must all be known. Well-timed haulm destruction needs knowledges of PVY^N-translocation in potato plants and of the development of mature plant resistance. For successful virus control, also reliable methods for virus detection are needed, and for this ELISA has been well established for many years in seed potato certification schemes. A combined treatment of mineral oil and pyrethroids reduces aphid populations and the spread of stylet-borne viruses in the fields. In breeding for resistance, the major gene Ry induces complete protection against infections of all known PVY-strains.     

Maintenance,symptoms and distribution of potato viruses X,S, Y,A, M and leafroll in potato tissue culture plantlets

Maintaining potato viruses X, S, Y, A, M and leafroll in tissue culture plantlets is a convenient, cost and space effective alternative to the use of greenhouse plants. Of these six viruses, only certain strains of PVX induced symptoms in tissue culture plantlets. Nevertheless, all infected tissue culture plants were found to be more reliable than greenhouse grown plants as virus-infected controls in enzyme-linked immunosorbent assay (ELISA) testing. Another advantage of maintaining viruses in tissue culture plantlets was the elimination of contamination by other viruses or other pathogens. Leaves, stems, and roots of virus infected plantlets were tested separately for antigen levels by ELISA. In these tests, the stems and leaves of all but PVA infected tissue culture plants consistently gave positive ELISA values. In contrast, root tissue from PVY infected tissue culture plantlets was not reliable for PVY detection. In all cases, the viruses detected in the original source material were detected in the resulting tissue culture plantlets.     

Detection of potato viruses X and S in tissue culture plantlets

The latex agglutination test (LAT) and enzyme-linked immunosorbent assay (ELISA) were evaluated in separate studies for their ability to detect potato virus X (PVX) and potato virus S (PVS) in tissue culture plantlets. Healthy and infected clonal lines of several potato cultivars were used. LAT was unsatisfactory because only low levels of agglutination were obtained with infected samples, and because variable and inconsistent results were obtained with both healthy and infected clones. ELISA, however, consistently gave high spectrophotometric readings and intense visual reactions for infected but not for healthy clones. The results indicate that ELISA can be used to detect PVX and PVS in tissue culture plantlets, and in programs where tissue culture is employed, early detection and elimination of infected plantlets is possible.     

Factors influencing time to breakdown (TTB) of cooked potato tissue

Cooking ability or time-to-breakdown (TTB) of different portions of potato tubers was determined by taking cores 1.27 cm (1/2inch) in diameter and 0.95 cm (3/8 inch) long from 12 locations in the tubers and cooking until penetration of a weighted rod pressing on the cores. Cores from the stem portion, on the average, required over twice as long for penetration of the rod during cooking as compared to cores from the bud section. The cores from the middle longitudinal portion of the tuber took longer to cook than cores from either side. Low specific gravity (1.070) tubers took significantly longer to cook than cores from high specific gravity (1.090) potatoes. However, when specific gravity of individual cores was determined with a pycnometer and TTB determined for the same samples, no relationships could be established. Cores from the stem portion took longer for TTB regardless of specific gravity or starch content. The data suggested much greater influence of cell wall structure as would be obtained from different portions of the tuber on TTB than starch content of the cells. No differences in TTB were obtained from tubers of high or low fertility or between Norgold Russet and Russet Burbank varieties, except Norgold was more uniform in cooking time between the stem and bud portion than Russet Burbank.     

Mature-plant resistance of potato plants against potato virus YO (PVYO)

Summary Mature-plant resistance of potato plants against potato virus Y^O (PVY^O) was studied in field experiments in 1976–1979 in southern Sweden. At intervals of 7 days, from mid June until mid August, PVY^O was mechanically inoculated to one well-developed upper leaf per stem of each plant. Each week, a new group of plants were inoculated. In 1976–1977, the natural spread of PVY^O by aphids was studied in relation to three different stages of potato plant development. The results of mechanical inoculation demonstrated that mature-plant resistance increases during July in southern Sweden but there were great differences between developmental stages. Early planting with sprouted seed resulted in plants with a considerable mature-plant resistance in mid July when late developed potato plants still were very susceptible.

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Zusammenfassung In Südschweden wurden zwischen 1976 und 1979 Untersuchungen durchgeführt, um zu prüfen, wann und in welchem Mass die Sorte Bintje im Sommer eine Altersresistenz gegen das Kartoffelvirus Y^O (PVY^O) ausbildet. 1977 wurden drei Gruppen von Kartoffelpflanzen unterschiedlicher Entwicklungsstadien mit PVY^O mechanisch inokuliert und von Mitte Juni bis fast Mitte August wurden jede Woche neue Gruppen von Pflanzen inokuliert. Nach der Ernte Anfang September wurden die Knollen auf PVY^O geprüft. Ebenso wurde die natürliche übertragung von PVY^O durch L?use von PVY^O-Infektorpflanzen auf benachbarte gesunde Pflanzen in drei verschiedenen Entwicklungsstadien untersucht. Die Ergebnisse der mechanischen Inokulation von Kartoffelpflanzen-früh und normal entwickelt-zeigen, dass die Altersresistenz der Sorte Bintje im Juli anstieg und Anfang August fast vollst?ndig ausgebildet war. Wurden die Kartoffeln sp?t gepflanzt, waren die Pflanzen Ende Juli noch sehr anf?llig für eine mechanische Inokulation und ein grosser Teil der Knollen war PVY^O-infiziert (Abb. 1, Tab. 2). Die natürliche übertragung von PVY^O bei verschiedenen Entwicklungsstadien der Pflanzen zeigte, dass die Zahl PVY^O-infizierter Knollen anstieg, wenn die Kartoffeln sp?t im Vergleich zur normalen Pflanzzeit gelegt wurden, mit und ohne Vorkeimung (Tab. 3). Die st?rkere Ausbreitung von PVY^O im Jahr 1976 gegenüber 1977 (Tab. 3) k?nnte mit dem st?rkeren Auftreten und der früheren Ausbreitung der L?use, vor allem vonRhopalosiphum padi (Tab. 4), im Jahr 1976 in Beziehung stehen.

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Résumé Des expérimentations sont menées dans le sud de la Suède entre 1976–1979; elles ont pour but de déterminer à quel moment et jusqu'à quelle limite des plantes de la variété Bintje présentent la résistance à maturité vis-à-vis du virus Y^O (PVY^O) au cours de l'été. En 1977, trois groupes de plantes à différents stades de développement sont inoculés mécaniquement avec PVY^O et chaque semaine, de la mi-juin à la mi-ao?t, de nouveaux groupes sont inoculés. Après la récolte, début septembre, des tubercules sont indexés pour déterminer PVY^O. La dissémination naturelle de PVY^O par les aphides, est également étudiée à partir de plantes-sources de PVY^O vers des plantes voisines saines, présentant trois différents stades de développement. Les résultats de l'inoculation mécanique de plantes à deux stades de développment précoce et normal, montrent que la résistance à maturité augmente pendant le mois de juillet pour la variété Bintje et est pratiquement totale vers le début du mois d'ao?t. Lorsque la plantation est tardive, les plantes sont encore très sensibles à l'inoculation mécanique et une forte proportion de tubercules est contaminée par PVY^O (figure 1, tableau 2). La dissémination naturelle de PVY^O pour des plantes à différents stades, montre que la proportion de tubercules atteints de PVY^O augmente lorsque les tubercules sont plantés tardivement par rapport à la date normale, avec ou sans prégermination (tableau 3). La dissémination plus importante de PVY^O en 1976 par rapport à 1977 (tableau 3) pourrait être correlée à celle des aphides également plus importante et plus précoce en 1976, notamment celle deRhopalosiphum padi (tableau 4).

     

Spread of potato virus Y (PVY) in relation to aphid population trends in potato fields in Israel

Summary Test of field spread of potato virus Y (PVY) were carried out in the coastal plain of Israel during the autumn growing seasons of 1964 and 1965. These included twice-weekly samplings of the winged aphid population. PVY-infected tubers of the varietyUp-to-Date, were planted at predetermined points in a trial plot, simultaneously with PVY-free ones. Virus spread to progeny was determined by indexing plants grown from tubers that had been collected at different distances from the infector plants. In 1965, a fair correlation was established between the incidence of naturally-spread PVY and distance from the source of inoculum. In 1964, however, the number of aphids trapped were exceptionally high, resulting in a PVY incidence more than double that of 1965 but with practically no apparent relation to the distance from the infector plants. This can be partly explained by the additional infection arising from sources outside the trial plot through the activity of an increased number of vector individuals.

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Zusammenfassung Im Anschluss an Studien über die Feldausbreitung von PVY, die vor 7 Jahren begannen, wurden in der Küstenzone von Israel w?hrend der zwei Herbstanbauperioden 1964 und 1965 besondere Untersuchungen angestellt. Halbw?chentlich wurden die geflügelten Individuen der Familie Aphididae in der Versuchsparzelle mit Hilfe der Moericke-Falle gesammelt. Vom gesamten Fang wurde nurMyzus persicae identifiziert, da diese Art der Hauptübertr?ger des PVY in Israel ist. Knollen der SorteUp-to-Date, mit dem gew?hnlichen Isolat von PVY infiziert, wurden an vorher bestimmten Stellen der Versuchsparzelle gleichzeitig mit PVY-freien Knollen gepflanzt (Abb. 1) Das Auftreten des Virus auf Knollen der neuen Generation wurde auf folgende Weise festgestellt: Knollen, die in verschiedenen Entfernungen von der Inokulumquelle gesammelt wurden, wurden in der Augenstecklingsprüfung nicht nur visuell beurteilt, sondern auch im A6-Test (Einreiben von Blattsaft) und mittels verschiedenen Testpflanzen überprüft. Zu diesem Zweeke wurden Stecklinge unter insektenfreien Bedingungen angezogen. Im Herbst 1965, wie auch in den Frühjahren 1960 und 1961 und im Winter 1961 (Zimmerman-Gries undNitzany, 1964), wurde eine ziemlich gute Korrelation zwischen Feldauftreten des PVY und der Entfernung von der Inokulumquelle beobachtet (Abb. 2). Im Jahre 1964 waren die Fallenergebnisse aussergew?hnlich gross. Dies nun verursachte ein mehr als doppelt so starkes Auftreten von PVY wie im Jahre 1965, und dies stand praktisch in keiner augenscheinlichen Wechselbeziehung zur Entfernung von der ausgepflanzten Inokulumquelle (Abb. 2). Das Verwischen eines solchen Korrelationsbildes im Jahre 1964 wird teilweise durch die weitere Ansteckung erkl?rt, die, von Quellen ausserhalb der Versuchsparzellen kommend, durch die vermehrte Zahl von übertr?gerindividuen (Abb. 3), die im Felde vorhanden waren, verursacht wurde.

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Résumé Des recherches particulières ont été effectuées dans la zone c?tière d'Isra?l durant les deux saisons automnales de culture 1964 et 1965, en jonction à des études sur la propagation au champ du PVY qui débutèrent il y a sept ans. Les individus ailés de la famille des Aphides étaient récoltés dans les parcelles d'essais deux fois par semaine, au moyen de pièges de Moericke. Parmi toutes les captures seuls les puceronsMyzus persicae étaient identifiés, puisque cette espèce est le principal transmetteur du PVY en Isra?l. Des tubercules de la variétéUp-to-Date, infectés avec l'isolat ordinaire de PVY, ont été plantés à des emplacements déterminés à l'avance dans la parcelle d'essais en même temps que les tubercules libres de PVY (Fig. 1). La présence du virus sur les tubercules de la génération suivante fut déterminée de la fa?on suivante: aux tubercules récoltes à diverses distances de la source d'infection était appliqué le test de bouture d'oeil de même que le test sur folioles détachées de l'hybride pomme de terre A6, comme aussi sur une séric de plantes-test. Pour la réussite de l'épreuve les pousses de tubercules étaient élevées dans des conditions libres de pucerons. En automne 1965, comme au printemps 1960 et 1961, et en hiver 1961 (Zimmerman-Gries etNitzany, 1964), on a observé une corrélation passablement bonne entre l'apparition au champ du PVY et l'eloignement de la source d'inoculum (Fig. 2). En 1964, les produits des pièges étaient exceptionellement élevés. Ce qui, dès lors, occasionna une apparition de PVY à un degré plus que double de celui de 1965, et engendra aussi ce fait que pratiquement il n'y avait aucune corrélation avec l'éloignement des sources d'inoculum plantées (Fig. 2). Cet effacement d'une telle image de corrélation en 1964 sera partiellement expliquée par une contagion plus grande causée par le nombre accru d'individus vecteurs (Fig. 3) qui étaient présents dans les champs, à partir de sources extérieures aux parcelles d'essais.

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Contribution from the National and University Institute of Agriculture, Rehovot, Israel. 1966 Series, No. 1051-E.

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Paper read to the Virology Section of the Third Triennial Conference of the European Association for Potato Research, Zürich, September 1966.