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.     

Reevaluation of the potato aphid,Macrosiphum euphorbiae (Thomas), as vector of potato virus Y

The effectiveness of the potato aphid,Macrosiphum euphorbiae (Thomas), to transmit potato virus Y (PVY) to potato has generally been overestimated because tobacco has been used as the indicator host. Our results demonstrate that, although apterousM. euphorbiae can acquire PVY from potato and tobacco plants and transmit it to tobacco plants, it does not readily transmit it to potato plants. Alatae only transmitted the virus to 4.5% of potato plants. This relative inability to transmit the virus to potato seems independent of potato cultivar. Results suggest that the role of the potato aphid in the spread of PVY in potatoes may be negligible.     

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 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.     

Spread of potato virus S

Summary An experiment withEersteling (originally freed by meristem culture from PVX and PVS) andAlpha plants has been carried out in the field to study the spread of potato virus S. It was shown that depending on virus isolate, in a crop containing approximately 10% of plants with secondary infection, the primary infection inEersteling could rise to 56–76%, whereas that percentage inAlpha plants rose to 2–28 only. From the pattern of infected plants in was concluded that under field conditions in the Netherlands potato virus S is probably transmitted by contact.     

The infection pressure of potato leafroll virus and potato virus Y in relation to aphid populations in Cyprus

Summary The infection pressure of two viruses, potato leafroll (PLRV) and potato virus Y (PVY), both common in seed potatoes grown in Cyprus, was determined in three experiments in 1982–83. Virus-free bait plants, of potato and four other species, were exposed weekly to field infection during the growing season (March–June), and then returned to an aphid-free glasshouse for symptom expression. Only tobacco plants produced clear symptoms enabling reliable assessment of PVY infection pressure. When assessed with ELISA or by tuber indexing, the potato plants were efficient baits for both viruses whose infection period commenced at emergence (mid March to early April) and ended within 6–7 weeks. The seasonal trend of aphid populations, determined with Moericke traps or 100-leaf counts, correspond to that of virus spread. Correlation and regression analysis of aphid and virus data implicated the alate form ofMyzus persicae as the principal vector of both viruses.     

Seed transmission of potato spindle tuber virus

Seed transmission of potato spindle tuber virus (PSTV) has special significance for potato breeding programs. Transmission of PSTV has been reported to occur via true seed. Test families in the Frito-Lay breeding program were obtained from self- and cross-pollinations to determine the incidence of infected seedlings from healthy and diseased parents. Diseased seedlings from PSTV-infected Katahdin selfed were scored by visual and tomato index procedures as 100% and 87% infected, respectively. A similar result was obtained among seedlings from the cross of PSTV-infected Sebago x PSTV-infected Katahdin. Symptoms of PSTV infection were not detected by either method among seedlings from healthy Katahdin, selfed. The Frito-Lay, Inc., potato breeding program employs a number of phyto-sanitary procedures to exclude or reduce the possibility of PSTV contamination and spread.     

Inheritance of extreme resistance to potato virus Y in potato

Summary Segregation for extreme resistance to PVY was evaluated in progenies derived from crossing two extremely resistant potato clones with parents differing in resistance. Resistance was evaluated after mechanical inoculation with PVY^O and PVY^N, and after graft inoculation with PVY^O. Biological and serological tests (ELISA) were used for virus detection. The extreme resistance is governed by a single dominant gene, but observed segregations deviated from the expected ratios. Considerable modifying effects were detectable, depending on the potato genotype and virus isolate, for a significant excess of susceptible genotypes was observed in some progenies. Moreover, genotypes with non-parental types of resistance to PVY were observed.     

Acquisition of potato leafroll virus byMyzus persicae from secondarily-infected potato plants of different genotypes

Summary The acquisition of potato leafroll virus (PLRV) byMyzus persicae nymphs from the top leaves of potato plants was studied throughout a growing season in relation to the antigen titre in those leaves and the feeding behaviour of the aphid. Secondarily-infected plants of eight potato genotypes with different levels of field resistance served as virus sources. Early in the growing season, plants were efficient sources for virus acquisition. The amount of viral antigen detected inM. persicae nymphs fed on the top leaves was strongly correlated with the titres of viral antigen in these leaves. Virus acquisition from the top leaves of older potato plants was markedly impaired and could not be correlated with their virus titre. With increasing age of the potato plants and the development of virus symptoms, the virus titre in the leaves declined and the initial weak correlation between the virus titre and field resistance ratings disappeared. Thus, screening secondarily-infected potato plants for field resistance to PLRV based on the concentration of viral antigen in leaves or in aphids fed on them should be avoided later in the growing season. The feeding rate ofM. persicae, measured by the number of honeydew droplets excreted, did not account for the reduced uptake of virus from older plants since it was not influenced by the age of the plant. Throughout the growing season, the feeding rate ofM. persicae nymphs on PLRV-infected plants was higher on genotypes with low levels of field resistance to PLRV than on genotypes with high ones.     

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     

New disease symptoms observed on field-grown potato plants with potato spindle tuber viroid and potato virus Y infections

Summary Potato spindle tuber viroid (PSTV) and potato virus Y (PVY) were isolated from plants of cultivar Kennebec with severe necrotic symptoms in the field. In the greenhouse, severe necrotic symptoms were reproduced only when potato plants were infected either simultaneously with PSTV+PVY, or with PSTV prior to PVY infection. Thirteen additional potato cultivars were tested in the greenhouse for this synergistic reaction and eight developed necrotic responses similar to cv. Kennebec. PVY concentration was significantly higher in doubly infected plants, compared with those infected with PVY alone.     

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.     

Retention of infectious potato virus X on common surfaces

Potato virus X in sap from potato leaves was applied by atomizer to surfaces commonly contacted during potato planting, growth, harvesting, grading and storage. The virus retained its infectivity toGomphrena globosa L. or Netted Gem potato for 10 seconds on leather, 3 hours on unpainted wood, iron, rubber and human skin, 6 hours on painted wood, jute and cotton, and 24 hours on soil.     

Some properties of potato virus S isolated from Peruvian potato varieties

Summary Potato virus S (PVS) was isolated from several Peruvian potato varieties showing different symptoms ranging from mild systemic mosaic to yellowing and bronzing of the lower leaves. The properties of PVS from the potato varietyMantaro in crude sap are as follows: thermal inactivation point between 56°C and 62°C, 6 days longevity in vitro at room temperature; and dilution end point between 10⁻⁴ and 10⁻⁵. This isolate had the unique property of giving systemic symptoms onChenopodium quinoa. Paper read at the Virology Section Meeting of the EAPR, June 1971.     

Enzyme-linked immunosorbent assay (ELISA) for the detection of potato leafroll virus and potato virus Y in potato tubers after artificial break of dormancy

Summary Conditions necessary for the detection of potato leafroll virus (PLRV) and potato virus Y (PVY) in tubers from primary and secondary infected plants were investigated. Tubers were analysed before and after breaking dormancy by rindite treatment. PLRV was reliably detected indormant tubers whereas PVY was readily detected only when tubers had been rindite-treated and held for two to three weeks at 22°C and high humidity in the dark. PLRV occurred in higher concentration at the heel end than at the rose end of infected tubers and the concentration remained nearly unchanged during the experimental period of 35 days, whereas PVY was found to be more concentrated at the rose end and was rapidly accumulating in the tubers after the break of dormancy. In dormant tubers PVY concentration dropped during storage at 22°C. The use of ELISA for tuber indexing is discussed.     

A rapid technique for assessing the resistance of families of potato seedlings to potato leaf roll virus

Summary A technique is described by which families of potato seedlings can be rapidly screened for resistance to potato leaf roll virus (PLRV) in the greenhouse with a minimum expenditure of labour and bench space. Seed of different families was sown in rows in four replicated blocks and the seedlings produced were exposed to viruliferousMyzus persicae when very young. Symptoms resembling those typical of secondary infection with PLRV developed within a few weks and families with few affected seedlings compared with the susceptible control were classed as resistant. Tests done at different times with similar sets of families gave similar results. Applying infectiveM. persicae by shaking heavily colonised potato shoots above the seedlings was more effective and far less time consuming than transferring them individually by hand using a fine paint brush.     

Incidence of potato virus Y and potato leaf roll virus in autumn potatoes in Israel

Summary Random sampling of autumn grown potatoes in Israel revealed potato virus Y incidences of 2.7, 2.5 and 3% during the years 1976, 1977 and 1978, respectively, figures only slightly higher than those found in spring-planted fields (2.3% in 1976), and which agree with visual estimates by the Inspection Service. Assaying random samples for potato leaf roll virus (PLRV) by aphid transmission to test plants, detected incidences as high as 38.8, 28.9, 36.7 and 33.3% during 1976, 1977, 1978 and 1979, respectively, although only minor levels were visable, indicating symptom masking in the autumn. Most of the infection was found to be secondary; ca. 25% of the local seeds for autumn planting, grown during spring, are PLRV-infected. The effect of the high incidences of PLRV on autumn yields is not known, but they are consistently lower than spring yields of the same varieties. Contribution No 315-E, 1980 Series, The Volcani Center, Bet Dagan, Israel.     

Biological factors influencing the transmission of potato leafroll virus by different aphid species

Summary Macrosiphum euphorbiae, collected in the field from potato plants infected with potato leafroll virus (PLRV), transmitted the virus to fewer potato plants in a field trial than did laboratory-rearedMyzus persicae. In the laboratory,M. persicae was the only efficient vector of PLRV fromPhysalis floridana seedlings, potato sprouts or excised leaves toP. floridana. Two clones ofM. euphorbiae and one clone ofAulacorthum solani transmitted PLRV from infected potato plants toNicotiana clevelandii as effeciently asM. persicae but a clone ofAphis gossypii was an inefficient PLRV vector. An isolate of PLRV, whichM. persicae transmitted inefficiently from potato toN. clevelandii, was also transmitted inefficiently byM. euphorbiae andA. solani.     

Effects of potato virus S and mild potato virus X on early and late harvest yields of the Kennebec cultivar in Maine

Yields of the Kennebec cultivar as affected by mild potato virus X (PVX) and/or potato virus S (PVS) were compared with those of a virus-free (VF) stock in both early and late harvest tests for the years 1972, 1973 and 1974. Results showed that VF stocks gave significantly higher yield weights than those with PVS plus mild PVX in late-harvested plots but in early-harvested plots results were reversed, though not significantly so at P=.05. Thus, the significant interaction of virus content with harvest date indicated that length of growing season can influence the outcome of tests designed to measure the effects of PVS and PVX on yield.     

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.