A comparison of some methods for detecting potato leaf roll virus in potato tubers in Israel

Summary The results obtained by four methods for detecting PLRV in potato tubers grown in Israel were compared with the results obtained by aphid transmission to test plants. The correlation coefficients for potatoes lifted in July (seed for autumn) and September (seed for spring) were: Igel-Lange test, 0.18 and 0.25; datura grafting with tuber sprouts, 0.71 and 0.63; aphid transmission from sprouts to test plants, 0.89 and 0.81; and visual assessment in the Florida test (for spring seed), Up-to-Date 0.58, Blanka 0.67 and Désirée 0.33. The Igel-Lange test proved unreliable; datura grafting is time consuming and PLRV symptoms were similar to reactions ofDatura stramonium to wounding and to stress. The results of the Florida test were somewhat variable and were obtained too late. The sprout test was deemed the most suitable as it is accurate and the results can be obtained within two months. Contribution No 156-E, 1979 series, Volcani Center.     

Potato leafroll virus and potato virus Y in seed potatoes used to plant the North Carolina crop

Incidence of potato leafroll virus (PLRV) and potato virus Y (PVY) was determined in seed potatoes (Solatium tuberosum) from Canada, Maine, Minnesota, New York, North Dakota, Nebraska, Pennsylvania, and Wisconsin used to plant the North Carolina crop in 1977, 1978 and 1979. Incidence of PLRV ranged from 0–5.2% (X = 0.57%) and for PVY from 0-5.6% (X = 0.62%) from all sources (112 seed lots). All PVY isolates (177) tested from potato caused a very mild veinbanding and mottling onNicotiana tabacum cultivars NC 95 and NC 2326. No serological difference was detected between these isolates and the common strain of PVY from tobacco in North Carolina. Essentially no spread of PVY occurred in three potato fields observed each year of the study.     

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.     

Relative resistance of potato cultivars to bacterial wilt

Fifty-one potato cultivars were evaluated for reaction to infection byPseudomonas solanacearum Sm. under high disease pressure in a field plot near Tifton, Georgia during the spring of 1978 and 1979. Plots were infested by clipping beds of thickly seeded tomato plants with a rotary mower contaminated with the wilt baterium in 1975, 1976, 1977 and 1978. The plants were incorporated into the soil following disease development. In addition to the use of infested soil, one stem on each potato plant was inoculated in late May by cutting with a knife blade dipped in a bacterial suspension. Most of the cultivars evaluated were highly susceptible to bacterial wilt. Ontario was the only cultivar highly resistant to the disease, with only 1% and 8% dead or dying plants in 1978 and 1979, respectively. Based on tuber infection, Ontario and Snowchip were the most resistant, with 9% and 14% infection, respectively, in 1978 and both 4% in 1979. Saco, Green Mountain and Sebago also showed some resistance.     

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.     

Growing seed potatoes in the Sinai Desert

Summary Seed potatoes grown during winter in the Sinai Desert (Sharm el Sheik) yielded in autumn significantly more (17%) than commercial seed grown in spring. Seeds grown in winter in an agricultural area at Yotvata yielded similarly to the commercial controls. Almost no spread of PVY or PLRV was found at Sharm probably because of the very low alate population there, whereas at Yotvata the incidence of PLRV was over 40% probably as a consequence of a high aphid populations during the growing season. Alate populations were also observed in the Sinai Desert at A-tur, a completely arid area, and at Firan, an isolated oasis where agriculture is practised. At A-tur only a few alatae were trapped, but large numbers were found at Firan, especially starting in March, but not during winter the proposed season for growing seed potatoes. The possibility of growing high quality seed potatoes at Sharm is discussed. Contribution No 107, 1980 series, Volcani Center.     

Production of seed potatoes in Cyprus: the effects of roguing and planting date on the spread of potato leaf roll virus,tuber yield,and infestation by potato tuber moth

Summary Field experiments during 1984–6 tested the effects of planting date on the development of aphid infestations and the spread of potato leaf roll virus (PLRV) in rogued or unrogued plots of potatoes, cv. Spunta. Plantings were made each month from December to April, the customary time for planting being February. Aphid infestation in early-planted plots was severe throughout the growing season; plots planted in February were also severely infested early in the growing season but the populations later gradually declined to undetectable levels. Nevertheless, the incidence of PLRV in the latter plots was as high as in those planted in December-January. Late-planted crops escaped aphid infestation and PLRV infection, either in part (March planting) or completely (April planting). Such crops, however, were uneconomical due to poor yields and heavy losses from potato tuber moth infestation. Roguing significantly reduced the spread of PLRV in all years but its interaction with planting date was insignificant.     

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.     

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.     

Environmental factors influencing aphid transmission of potato virus Y and potato leafroll virus

Summary The effect of temperature, relative humidity (RH) and light on aphid transmission of potato virus Y (PVY) and potato leafroll virus (PLRV) was studied using as vectorsMyzus persicae Sulz. andAphis gossypii Glov. Host susceptibility was enhanced by 48 h pre-inoculation exposure at 25°C and by 48 h post-inoculation exposure to 30°C. High RH (80%) in both pre- or postinoculation phases enhanced host susceptibility. Continuous fluorescent light (4000 lux) did not alter the rate of transmission of either virus. High RH (80–90%) and high temperature (25–30°C), when combined, increased virus transmission by 30–35%. Transmission rates were reduced by nearly 50% if RH was maintained at 50% in either of the two phases even if the temperature was 25 or 30°C. Both viruses were acquired by aphids earlier (13–20 days after inoculation) when the source plants were incubated at 25 or 30°C. Most virus was transmitted from plants inoculated with PVY 13 to 16 days and with PLRV 15 to 20 days previously. Transmission rates of PVY were enumerated from symptom expression on test plants and by Enzyme Linked Immunosorbent Assay (ELISA) whereas those of PLRV were enumerated from symptom expression alone.     

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.     

Comparison of three potato leafroll virus antisera and a single Beet western yellows virus antiserum for luteovirus detection in potato

Three potato leafroll virus (PLRV) antisera, representing European, British Columbian, and Californian isolates, performed similarly in detection of PLRV in ELISA tests of samples collected in three successive years at the Florida certification test plots and in tests of other samples collected in New York State. Although a range of absorbance values occurred, this was probably due to random variation in virus titers of samples rather than the occurrence of different virus strains or differential serological reactions by the antisera. Beet western yellows virus (BWYV) was detected in potato leafroll samples from nine states and provinces in North America. The BWYV-positive samples represented 40% in 1983 and 62.5% in 1984 of the total number of samples tested. These results confirm previous reports on the widespread occurrence of BWYV in potato with symptoms of leafroll.     

Transmission of potato leaf roll virus byMyzus persicae (Sulz.) from leaves and plants of different ages

Summary Young potato plants were a better source of potato leaf roll virus (PLRV) for aphids,Myzus persicae (Sulz.), than old ones. For plants 6, 7.5 and 9 weeks old, the best sources of PLRV were the lower, middle and upper leaves, respectively. The frequency of PLRV transmission from upper leaves did not change much with increasing age of plants nor did it change with different leaflets from the same leaf.     

Transmission of potato leafroll virus to or from tuber slices by aphids feeding through a membrane

Summary Eye-bearing slices, cut from healthy potato tubers and placed between Parafilm membranes, were inoculated with potato leafroll virus (PLRV) byMyzus persicae. PLRV was detected by ELISA and by transmission tests in tuber slices and in plants grown from the slices of the susceptible cv. Désirée, but not in those of the resistant cv. Arkula. These results suggest that PLRV replication and transport within tuber phloem is controlled by specific mechanisms of resistance.M. persicae was also able to acquire and transmit PLRV toPl floridana from slices cut from tubers of infected plants. The aphids effectively transmitted PLRV from slices cut from the sprouting rose end but they failed to transmit it from slices cut from the heel end of tubers.     

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.     

Effect of time of inoculation with potato leafroll virus on development of net necrosis and stem-end browning in potato tubers

The Green Mountain cultivar was used in field tests to determine the effects of inoculating potato plants at various times with the potato leafroll virus (PLRV) on development of internal necrosis of tuber tissue. Viruliferous apterae of the green peach aphid,Myzus persicae (Sulz.), were placed on each stem in all hills to be inoculated in each 3.0 m single-row plot. Planting and inoculation dates were varied in all field experiments and, in one, several vine-killing dates were also included. All harvested tubers were stored for approximately four months at 10°C to enhance development of internal necrosis prior to examination. Similar but smaller greenhouse studies involving both apterous and alate green peach aphids were also conducted using Green Mountain, Irish Cobbler, and Russet Burbank cultivars. All results showed that as inoculation was delayed relative to plant development, more net necrosis (NN) occurred. Conversely, when plants were inoculated early, stem-end browning (SEB) rather than NN predominated. A high percentage of naturally occurring SEB tubers (cv. Russet Burbank) were found by ELISA to contain PLRV. Plants produced by these tubers only rarely developed leafroll symptoms. These findings suggest a previously unsuspected causal relationship between SEB and PLRV. Implications of this apparent relationship on the epidemiology of potato leafroll in Maine are discussed.     

Monitoring current season potato leafroll virus movement with an immunosorbent direct tissue blotting assay

A direct tissue blotting assay (DTBA) was used to track the movement of potato leafroll virus (PLRV) from newly infected foliage to the tubers. Plant and tuber characteristics were recorded to assess plant growth stage at inoculation and PLRV effect on yield. Russet Burbank potatoes were planted at different times in 1991 and 1992 to provide plants of different maturities which were then inoculated using PLRV carrying aphids. Aphids were allowed to feed two to three days after which an insecticide was applied. Stems and tubers were tested periodically for PLRV with DTBA after inoculation. Indexed tubers were grown out and ELISA tested in the greenhouse the following winter to confirm results of summer serological tests. Plant age affected percentages more consistently than did inoculation date. When plants approximately 43 days from planting were inoculated at different dates, early inoculation produced a higher percentage of infected plants. Conversely, when plants approximately 62 days from planting were inoculated at different dates, late inoculation resulted in a higher percentage of infected plants. However, early inoculation of young plants resulted in the highest infection percentages. Tuber size and yield were negatively affected by higher percentages of leafroll regardless of the stage of growth at inoculation. DTBA is best used for detecting PLRV in foliage of plants grown from infected tubers (i.e. secondary PLRV). DTBA is less accurate for detecting primary PLRV.     

Potato leafroll virus concentration in the vascular region of potato tubers examined by enzyme-linked immunosorbent assay (ELISA)

Summary Enzyme-linked immunosorbent assay (ELISA), used in conjunction with a new rapid extraction method, showed that potato leafroll virus (PLRV) concentration in the vascular region of infected potato tubers decreases from the heel to the rose end. Lower virus concentration at the rose than at the heel end was found not only in dormant tubers but also in tubers three weeks after breaking dormancy although the difference was then less pronounced. These results were obtained from plants with both primary and secondary infection by one of two French virus isolates which behave differently in respect of either accumulation in the ants or in their serological properties or both.     

Selection of first-year potato seedlings for resistance to potato leaf roll virus

Summary In families obtained from crossing pairs of parents with high resistance to potato leaf roll virus (PLRV), individual clones differ greatly in their level of resistance. Inoculation of first-year seedlings was effective in selecting more resistant families, but it was not so effective in selecting more resistant individual clones.     

Host genes and transgenes that confer resistance to a Scottish isolate of potato leafroll virus are also effective against a Peruvian isolate

Summary Potato genotypes with host gene-mediated resistance (host-MR) and coat protein-mediated transgenic resistance (CP-MR) to potato leafroll virus (PLRV), were inoculated with a Scottish and a Peruvian isolate of PLRV. The coat protein transgene had been cloned from the Scottish PLRV isolate which had also been used during the screening and selection of genotypes with host resistance. Significantly less PLRV accumulated in plants with either host-MR or CP-MR than in plants of susceptible genotypes or in non-transformed control plants, but the two forms of resistance were equally effective against both PLRV isolates.