Spread of potato leafroll virus is decreased from plants of potato clones in which virus accumulation is restricted

Tubers of eight potato clones infected with potato leafroll luteovirus (PLRV) were planted as ‘infectors’ in a field crop grown, at Invergowrie, of virus-free potato cv. Maris Piper in 1989. The mean PLRV contents of the infector clones, determined by enzyme-linked immunosorbent assay (ELISA) of leaf tissue, ranged from c. 65 to 2400 ng/g leaf. Myzus persicae colonised the crop shortly after shoot emergence in late May and established large populations on all plants, exceeding 2000/plant by 27 June. Aphid infestations were controlled on 30 June by insecticide sprays. Aphid-borne spread of PLRV from plants of the infector clones was assessed in August by ELISA of foliage samples from the neighbouring Maris Piper ‘receptors’. Up to 89% infection occurred in receptor plots containing infector clones with high concentrations of PLRV. Spread was least (as little as 6%) in plots containing infectors in which PLRV concentrations were low. Primary PLRV infection in guard areas of the crop away from infectors was 4%. Some receptor plants became infected where no leaf contact was established with the infectors, suggesting that some virus spread may have been initiated by aphids walking across the soil.     

Evaluation of hairy nightshade as an inoculum source for aphid-mediated transmission of potato leafroll virus

Potato leafroll virus (PLRV) causes one of the most serious aphid-transmitted diseases affecting yield and quality of potatoes, Solanum tuberosum (L.), grown in the United States. The green peach aphid, Myzus persicae (Sulzer), is considered to be by far the most efficient vector of this virus. Even the most strict aphid control strategy may not prevent the spread of PLRV unless measures also are taken to keep virus source plants within and outside the crop at a minimum. Hairy nightshade, Solanum sarrachoides (Sendtner), is one of the preferred weed hosts for green peach aphid. The potential of this weed as an aphid reservoir and virus source and its spread or perpetuation were investigated. With the use of double antibody sandwich enzyme-linked immunosorbent assay, it was confirmed that green peach aphid can transmit PLRV to hairy nightshade and that aphids can become viruliferous after feeding on infected hairy nightshade plants. Transmission from hairy nightshade to potato is 4 times the rate of potato to potato or potato to hairy nightshade. The green peach aphid preferred hairy nightshade over potato plants and reproduced at a higher rate on hairy nightshade than on potato. Therefore, a low level of PLRV-hairy nightshade infection could enhance the disease spread in the field.     

New approaches for restricting spread of potato leafroll virus by different methods of eradicating infected plants from potato crops

Experiments were made at Invergowrie in 1984 and 1985 to compare the spread of potato leafroll virus (PLRV) after removing infected plants by three different methods; conventional roguing, desiccation with diquat, or incineration for 45–60s using a propane gas flame. Potato leaf roll 'infector' plants, grown in plots of virus-free Maris Piper seed potatoes, were artificially infested in June with aphids (Myzus persicae) from a laboratory culture, and removed from the plots after 2 or 3 wk. In both years, natural infestations of potato aphids were scarce during this period. There was no significant difference in the proportion of tubers infected with PLRV in adjacent plants after the neighbouring infector plants had been rogued by hand or desiccated with diquat, but the proportion was considerably reduced following incineration of the infector plants. In 1984, the spread of PLRV in conventionally rogued plots was also significantly reduced by a mixture of deltamethrin plus heptenophos, applied four times from 80% crop emergence, and was almost eliminated by a treatment with aldicarb granules, either at planting, or as a side-dressing 5 wk later. In 1985, delaying infector removal by 8 days in early July significantly increased the spread of PLRV to neighbouring plants from 2.3% (1 July) to 8.3% (9 July). A single application of deltamethrin plus heptenophos to infectors 1 wk before removal did not significantly decrease spread. Although incineration was quick and effective, the value of this method of eradicating infector plants in seed potato crops is limited because it failed to destroy infected tubers.     

Insecticidal control of aphids and the spread of potato leafroll virus in potato crops in eastern Scotland

Field experiments were carried out in eastern Scotland in 1976-78 to test the ability of granular insecticides, applied to soil at planting, and of insecticide sprays applied to the foliage, to control aphids and spread of potato leafroll virus (PLRV) in potatoes. The three years provided contrasting opportunities for virus spread. In 1976, the main vector of PLRV, Myzus persicae, arrived in early June and multiplied rapidly in untreated plots, and PLRV spread extensively. In 1977, M. persicae arrived 4–6 wk later than in 1976 and most spread of PLRV, which was less than in 1976, occurred after the end of July. In 1978, few M. persicae were recorded but the potato aphid, Macrosiphum euphorbiae, arrived early and very large populations developed in untreated plots. However, little spread of PLRV occurred in 1978, supporting other evidence that M. euphorbiae is an inefficient vector of PLRV in field conditions. In each year, granular insecticides decreased PLRV spread to a quarter or less of that in control plots. Thiofanox gave somewhat better and longer-lasting control of aphid populations than disulfoton, especially of M. persicae, but did not give greater control of PLRV spread. Application of three (1976) or five (1977) sprays of demeton-S-methyl to plots treated with granular insecticides further improved the control of M. euphorbiae but had less or no effect on M. persicae, especially where organophosphorus resistant aphids (R1 strain) were found. These supplementary sprays of insecticide did not further improve the control of PLRV but, in 1978, four sprays of demephion or pirimicarb to plots not treated with granular insecticide decreased PLRV spread. These data, together with previous findings, indicate that the amount of virus spread depends on the date of arrival and rate of multiplication of M. persicae in relation to the timing and effectiveness of removal of PLRV sources in crops. It is concluded that in Scotland insecticide granules should be used routinely only in crops of the highest grade of seed potato. Their use for other grades need be considered only in years following mild winters, when aphids can be expected to enter crops earlier and in larger numbers.     

蚜虫中携带马铃薯卷叶病毒检测方法的改进

马铃薯卷叶病毒( Potato leafroll virus,PLRV)对马铃薯生产的危害极大,是一种极为重要的马铃薯病毒病。 RT－PCR是马铃薯卷叶病毒检测较为常用的方法,该方法检测准确率高、成本低、适用范围广。但在实际生产中其检测对象多为染病植株,对PLRV传播的主要介体桃蚜( Myzus persicae)的检测,则由于蚜虫体积小、RNA提取难度大、成本高、且不能复检,因而在生产中不能被广泛使用。该研究以马铃薯感病植株和带毒蚜虫为材料,利用改进的RNA提取方法从它们中提取到PLRV的RNA,并以CP 基因设计特异性引物,进行PCR检测。结果表明：该方法提取的RNA完整性好,可用于蚜虫中PLRV检测,且同样适用于对马铃薯感病植株的检测。另外,通过对田间有翅蚜和无翅蚜携带 PLRV 情况进行检测发现,无翅蚜 PLRV 检出率为100％,有翅蚜PLRV检出率也高达60％,证明该体系在生产中的实用性。该研究使用改进的RNA提取方法,提取蚜虫中RNA,并利用RT－PCR进行了PLRV检测,与以前的方法相比简单实用,可被应用于生产检测中。该研究结果为马铃薯生产中PLRV的防控提供了一种新的手段。     

Quantitative studies on the uptake and retention of potato leafroll virus by aphids in laboratory and field conditions

Enzyme-linked immunosorbent assay (ELISA) was adapted for the efficient detection and assay of potato leafroll virus (PLRV) in aphids. Best results were obtained when aphids were extracted in 0.05 M phosphate buffer, pH 7.0, and the extracts incubated at 37 °C for 1 h before starting the assay. Using batches of 20 green peach aphids (Myzus persicae), about 0.01 ng PLRV/aphid could be detected. The virus could also be detected in single aphids allowed a 1-day acquisition access period on infected potato leaves. The PLRV content of aphids depended on the age of potato source-plants and the position of source leaves on them. It increased with increase in acquisition access period up to 7 days but differed considerably between individual aphids. A maximum of 7 ng PLRV/aphid was recorded but aphids more usually accumulated about 0.2 ng PLRV per day. When aphids were allowed acquisition access periods of 1–3 days, and then caged singly on Physalis floridana seedlings for 3 days, the PLRV content of each aphid, measured subsequently, was not strongly correlated with the infection of P. floridana. The concentration of PLRV in leaf extracts differed only slightly when potato plants were kept at 15, 20, 25 or 30 °C for 1 or 2 wk, but the virus content of aphids kept on leaves at the different temperatures decreased with increase of temperature. PLRV was transmitted readily to P. floridana at all temperatures, but by a slightly smaller proportion of aphids, and after a longer latent period, at 15 °C than at 30 °C. The PLRV content of M. persicae fed on infected potato leaves decreased with increasing time after transfer to turnip (immune to PLRV). The decrease occurred in two phases, the first rapid and the second very slow. In the first phase the decrease was faster, briefer and greater at 25 and 30 °C than at 15 and 20 °C. No evidence was obtained that PLRV multiplies in M. persicae. These results are compatible with a model in which much of the PLRV in aphids during the second phase is in the haemocoele, and transmission is mainly limited by the rate of passage of virus particles from haemolymph to saliva. The potato aphid, Macrosiphum euphorbiae, transmitted PLRV much less efficiently than M. persicae. Its inefficiency as a vector could not be ascribed to failure to acquire or retain PLRV, or to the degradation of virus particles in the aphid. Probably only few PLRV particles pass from the haemolymph to saliva in this species. The virus content of M. euphorbiae collected from PLRV-infected potato plants in the field increased from early June to early July, and then decreased. PLRV was detected both in spring migrants collected from the plants and in summer migrants caught in yellow water-traps. PLRV was also detected in M. persicae collected from infected plants in July and August, and in trapped summer migrants, but their PLRV content was less than that of M. euphorbiae, and in some instances was too small for unequivocal detection.     

Restricted distribution of potato leafroll virus antigen in resistant potato genotypes and its effect on transmission of the virus by aphids

Enzyme-linked immunosorbent assay was used to measure the concentration of potato leafroll virus (PLRV) antigen in different parts of field-grown secondarily infected plants of three potato genotypes known to differ in resistance to infection. The antigen concentration in leaves of cv. Maris Piper (susceptible) was 10–30 times greater than that in cv. Pentland Crown or G 7445(1), a breeder's line (both resistant). Differences between genotypes in antigen concentration were smaller in petioles and tubers (5–10-fold) and in above-ground stems (about 4-fold), and were least in below-ground stems, stolons and roots (about 2-fold). PLRV antigen, detected by fluorescent antibody staining of tissue sections, was confined to phloem companion cells. In Pentland Crown, the decrease in PLRV antigen concentration in leaf mid-veins and petioles, relative to that in Maris Piper, was proportional to the decrease in number of PLRV-containing companion cells; this decrease was greater in the external phloem than in the internal phloem. The spread of PLRV infection within the phloem system seems to be impaired in the resistant genotypes. Green peach aphids (Myzuspersicae) acquired < 2800 pg PLRV/aphid when fed for 4 days on infected field-grown Maris Piper plants and < 58% of such aphids transmitted the virus to Physalis floridana test plants. In contrast, aphids fed on infected Pentland Crown plants acquired <120 pg PLRV/aphid and <3% transmitted the virus to P. floridana. The ease with which M. persicae acquired and transmitted PLRV from field-grown Maris Piper plants decreased greatly after the end of June without a proportionate drop in PLRV concentration. Spread of PLRV in potato crops should be substantially decreased by growing cultivars in which the virus multiplies to only a limited extent.     

Restricted multiplication of potato leafroll virus in resistant potato genotypes

Plants of a range of potato genotypes differing in rating for field resistance to potato leafroll virus (PLRV) were inoculated with the virus by grafting or by aphids (Myzus persicae). Plants of all genotypes tested became infected by each inoculation method and PLRV was detected by ELISA in the upper leaves of all genotypes within 26 days after grafting. Most genotypes with high resistance ratings developed only mild primary and secondary symptoms whereas those with low resistance ratings developed more pronounced symptoms. However, one genotype (G7461(4)) with a high resistance rating was very severely affected. The concentrations attained by PLRV in genotypes with high resistance ratings were only 1–10% of those in genotypes with low resistance ratings. These differences in virus concentration were found in young leaves of plants with primary or secondary infection, whether inoculated by grafting or by aphids and whether grown in the glasshouse or the field. In older leaves, differences in virus concentration between genotypes were at least as pronounced as those in younger leaves. In contrast, PLRV concentration in vascular tissue at the heel end of tubers of plants with primary infection was similar for all the genotypes tested. Although low PLRV concentration was consistently associated with high resistance rating it is not the only form of resistance to PLRV occurring in potato.     

The effects of temperature on the availability and acquisition of potato leafroll luteovirus by Myzus persicae

The concentration of potato leafroll luteovirus (PLRV) did not differ in potato plants with secondary infections grown at 15°C or 27°C. Detached leaves of plants grown at 15°C or 27°C were used as sources of PLRV for peach-potato aphids (Myzus persicae Sulz.) both at 15°C and 27°C. At comparable temperature during virus acquisition, aphids which fed on leaves of plants kept previously at 15°C contained more viral antigen detected by ELISA than aphids which fed on leaves of plants grown at 27°C. The aphids which acquired PLRV at 27°C contained evidently more viral antigen than those which acquired PLRV at 15°C. The greatest amount of PLRV was found in the aphids which acquired the virus at 27°C from the leaves of plants kept at 15°C. The ability of M. persicae to transmit PLRV to Physalis ftoridana Rydb. generally decreased with decrease in the amount of PLRV in vectors.     

Multiple components of the resistance of potatoes to potato leafroll virus

In glasshouse experiments the ranking of potato genotypes for resistance to infection with potato leafroll virus (PLRV) using three concentrations of aphid-borne inoculum was the same as their field resistance ratings. In field-grown plants this resistance to infection increased in all genotypes as the plants aged but its rate of increase differed between genotypes. In tests on field-grown plants infected by aphid- or graft-inoculation, the proportion of virus-free progeny tubers increased the later the date of inoculation but was greater in resistant than in susceptible genotypes. This trend was most pronounced in the resistant clone G7445(1), in which the virus failed to move from the foliage to the tubers of some plants infected in glasshouse tests. The spread of PLRV will thus be minimised in crops of resistant compared with susceptible genotypes for three reasons: plants have greater resistance to infection, systemic spread of virus from their foliage to tubers is less likely and, as shown previously, the low concentration of virus particles in leaf tissue makes infected plants less potent sources of inoculum for aphids.     

Green peach aphid (Homoptera: Aphididae) action thresholds for controlling the spread of potato leafroll virus in Idaho.

Arbitrary green peach aphid, Myzus persicae (Sulzer), action thresholds (0, 5, 10, 20, and 40 aphids per 100 leaves) were tested in 3 yr of field experimentation to determine if they could be maintained and if they would significantly impact aphid densities and limit the incidence of potato leafroll virus (PLRV). In 1997 and 1998, significant linear relationships between thresholds and final percentage of PLRV (expressed as the percentage of tubers infected with PLRV) were observed: there was a trend toward lower PLRV incidence with decreasing action threshold in 1999. There were significant relationships between thresholds and mean number of apterous aphids in 1998 and 1999, indicating that reduction of PLRV resulted from reduced within-field spread by apterae. In almost all cases, aphid densities exceeded threshold levels from one week to the next, clearly showing that the thresholds could not be maintained. Over all experiments, four to nine seasonal applications of methamidophos were warranted by the magnitude of the threshold. Imidacloprid applied at planting to the zero aphid threshold reduced the number of methamidophos applications from nine in the insecticide-at-detection treatment to five. A revised within-field green peach aphid management plan is recommended that includes systemic insecticide applied at planting, aphid sampling every 3-4 d, and foliar insecticide application following aphid detection.     

Effect of an alternate weed host, hairy nightshade, Solanum sarrachoides, on the biology of the two most important potato leafroll virus (Luteoviridae: Polerovirus) vectors, Myzus persicae and Macrosiphum euphorbiae (Aphididae: Homoptera)

Hairy nightshade, Solanum sarrachoides (Sendtner), is a ubiquitous weed in potato agro-ecosystems and nonagricultural lands of southeastern Idaho and the Pacific Northwest. This weed increases the complexity of the Potato leafroll virus (PLRV) (Luteoviridae: Polervirus)-potato pathosystem by serving as aphid and virus reservoir. Previous field studies showed higher densities of green peach aphid, Myzus persicae (Sulzer), and potato aphid, Macrosiphum euphorbiae (Thomas), the two most important vectors of PLRV, on S. sarrachoides compared with potato plants in the same fields. Some of the S. sarrachoides plants sampled in these surveys tested positive for PLRV. Viral infections can alter the physiology of plant hosts and aphid performance on such plants. To understand better the potential effects of S. sarrachoides on the PLRV-potato-aphid pathosystem, the life histories of M. persicae and M. euphorbiae were compared on virus-free and PLRV-infected S. sarrachoides and potato. Individual nymphs of each aphid species were held in clip cages on plants from each treatment to monitor their development, survival, and reproductive output. Nymphal survival for both aphids across plant species was higher on S. sarrachoides than on potato, and, within plant species, it was higher on PLRV-infected plants than on noninfected plants. With a few exceptions, similar patterns occurred for fecundity, reproductive periods, adult longevity, and intrinsic rate of increase. The enhanced performance of aphids on S. sarrachoides and on PLRV-infected plants could alter the vector population dynamics and thus the PLRV-disease epidemiology in fields infested with this weed.     

Resistance to potato leafroll luteovirus can be greatly improved by combining two independent types of heritable resistance

Twelve potato clones were exposed to infection by aphids with potato leafroll luteovirus (PLRV) in three field trials in order to assess their resistance to infection. Up to 92% of the plants of some clones became infected, although other clones were relatively resistant to infection and one clone remained virus-free in all three trials. The resistance of the same 12 clones to PLRV multiplication was assessed in glasshouse-grown plant: lants were graft-inoculated and their daughter tubers were used to grow plants with secondary infection. High concentrations of PLRV were found in some clones (c. 1700 ng/g leaf) while in others much less virus accumulated (as little as 60 ng/g leaf). However, clones in which little virus accumulated were not necessarily those which were most resistant to infection in the field, and there was no association between the two types of resistance. Nevertheless, both types of resistance were found in some clones. The clone G8107(1), which remained virus-free in all the field exposure trials, was also the most resistant to PLRV multiplication. The combination of these two types of resistance in cultivars should help to eliminate the spread of PLRV in crops.     

THE CORRELATION OF APHID NUMBERS WITH THE SPREAD OF LEAF ROLL AND RUGOSE MOSAIC IN POTATO CROPS

An analysis of the results of experiments in different parts of England and Wales from 1941 to 1947 on the spread of potato leaf roll and rugose mosaic showed that leaf roll spread was correlated with the number of alate Myzus persicae (Sulzer) caught on sticky traps throughout the potato-growing season; there was some correlation with the maximum count of M. persicae per 100 leaves, but this possibly results from the correlation between trapped aphids and the number per 100 leaves. Spread of rugose mosaic (potato virus Y) was correlated to a lesser degree with number of M, persicae , perhaps because other aphid species are often vectors. With both diseases higher correlations were obtained when the infected plants were dispersed among the healthy crop than when they were placed together in a row. It is concluded that it is possible to predict the average health of potato stocks in the following year from average trap data; further work may enable the health of individual stocks to be predicted.     

Infection of potato plants with potato leafroll virus changes attraction and feeding behaviour of Myzus persicae

Potato leafroll virus (PLRV; genus Polerovirus, family Luteoviridae) is a persistently transmitted circulative virus that depends on aphids for spreading. The primary vector of PLRV is the aphid Myzus persicae (Sulzer) (Homoptera: Aphididae). Solanum tuberosum L. potato cv. Kardal (Solanaceae) has a certain degree of resistance to M. persicae: young leaves seem to be resistant, whereas senescent leaves are susceptible. In this study, we investigated whether PLRV‐infection of potato plants affected aphid behaviour. We found that M. persicae's ability to differentiate headspace volatiles emitted from PLRV‐infected and non‐infected potato plants depends on the age of the leaf. In young apical leaves, no difference in aphid attraction was found between PLRV‐infected and non‐infected leaves. In fact, hardly any aphids were attracted. On the contrary, in mature leaves, headspace volatiles from virus infected leaves attracted the aphids. We also studied the effect of PLRV‐infection on probing and feeding behaviour (plant penetration) of M. persicae using the electrical penetration graph technique (DC system). Several differences were observed between plant penetration in PLRV‐infected and non‐infected plants, but only after infected plants showed visual symptoms of PLRV infection. The effects of PLRV‐infection in plants on the behaviour of M. persicae, the vector of the virus, and the implications of these effects on the transmission of the virus are thoroughly discussed.     

Replicase mediated resistance against potato leafroll virus in potato Desirée plants

Potato leafroll virus (PLRV) is a major menace for the potato production all over the world. PLRV is transmitted by aphids, and until now, the only strategy available to control this pest has been to use large amounts of insecticides. Transgenic approaches involving the expression of viral replicases are being developed to provide protection for plants against viral diseases. The purpose of this study was to compare the protection afforded by the differential expression of PLRV replicate transgene in potato plants cv. Desirée, Plants were genetically modified to express the complete sense PLRV replicase gene. Two constructions were used, one containing the constitutive 35SCaMV promoter and the other the phloem-specific RolA promoter from Agrobacterium rhizogenes. Transgenic plants were infected with PLRV in vitro, using infested aphids. In plants in which 35SCaMV controlled the expression of the PLRV replicase gene, signs of infection were initially detected, although most plants later developed a recovery phenotype showing undetectable virus levels 40 days after infection. In turn, those plants with the RolA promoter displayed an initial resistance that was later overcome. Different molecular mechanisms are likely to participate in the response to PLRV infection of these two types of transgenic plants.     

Characterization of the expression and inheritance of potato leafroll virus (PLRV) and potato virus Y (PVY) resistance in three generations of germplasm derived from Solanum etuberosum

Potato virus Y (PVY) and potato leafroll virus (PLRV) are two of the most important viral pathogens of potato. Infection of potato by these viruses results in losses of yield and quality in commercial production and in the rejection of seed in certification programs. Host plant resistance to these two viruses was identified in the backcross progeny of a Solanum etuberosum Lindl. somatic hybrid. Multiple years of field evaluations with high-virus inoculum and aphid populations have shown the PVY and PLRV resistances of S. etuberosum to be stably expressed in two generations of progeny. However, while PLRV resistance was transmitted and expressed in the third generation of backcrossing to cultivated potato (Solanum tuberosum L. subsp. tuberosum), PVY resistance was lost. PLRV resistance appears to be monogenic based on the inheritance of resistance in a BC₃ population. Data from a previous evaluation of the BC₂ progeny used in this study provides evidence that PLRV resistance was partly conferred by reduced PLRV accumulation in foliage. The field and grafting data presented in this study suggests that resistance to the systemic spread of PLRV from infected foliage to tubers also contributes to the observed resistance from S. etuberosum. The PLRV resistance contributed by S. etuberosum is stably transmitted and expressed through sexual generations and therefore would be useful to potato breeders for the development of PLRV resistant potato cultivars.     

Detection of BMYV and BWYV isolates using monoclonal antibodies and radioactive RNA probes, and relationships among luteoviruses

In order to discriminate between sugar beet infecting beet mild yellowing virus (BMYV) and other isolates of beet western yellows virus (BWYV), monoclonal antibodies (MAbs) and radioactive riboprobes were used. With MAbs prepared against BMYV or potato leafroll virus (PLRV) no distinction could be established between BMYV and BWYV. Seven probes were synthesised from a lettuce infecting BWYV isolate; their localisation in the genome is known and they cover almost its entire length. Probes from the '3 part of the genome hybridised with all BMYV and BWYV isolates whereas those from the '5 part did not recognise BMYV isolates, showing that a divergent '5 region exists in the genomes of BMYV and BWYV. Probes also readily detected the virus in single aphids. The relevance of this finding for epidemiological studies is discussed.
MAbs and riboprobes were also tested against other luteoviruses (PLRV; barley yellow dwarf virus (BYDV) MAV, PAV and RPV strains). The serological relationship between BMYV and PLRV was confirmed and an epitope common to PLRV and BYDV-RPV was found. Using probes, PLRV and BYDV-RPV were found to share domains of homology with BWYV. BYDV-PAV showed weak homology with BWYV, while BYDV-MAV showed none.     

Increased sensitivity of detection of plant viruses obtained by using a fluorogenic substrate in enzyme-linked immunosorbent assay

The fluorogenic substrate 4-methylumbelliferyl phosphate (MUP) of alkaline phosphatase was compared with the chromogenic substrate p-nitrophenyl phosphate (NPP) in tests for plant viruses by enzyme-linked immunosorbent assay (ELISA). In tests on leaf extracts of squash infected with prune dwarf virus, Chenopodium quinoa and apple infected with apple mosaic virus (ApMV), and potato infected with potato leafroll virus (PLRV), MUP increased sensitivity 2–16 times, the smallest and greatest increases being obtained with ApMV (in apple) and PLRV respectively. In similar tests on 21 dormant PLRV-infected potato tubers, sensitivity was increased 2–4 times with 13 tubers, but the two substrates gave the same detection end-points with eight tubers. When individual seeds of potato plants infected with the Andean potato calico strain of tobacco ringspot virus were tested, the virus was detected in virtually all seeds by MUP-ELISA, but detection by NPP-ELISA was inefficient unless absorbance values were measured after overnight incubation at 4 °C, instead of after 2 h at room temperature. In tests on Myzus persicae carrying PLRV and Sitobion avenae carrying barley yellow dwarf virus (BYDV), both viruses were consistently detected in a greater proportion of individual aphids by MUP-ELISA than NPP-ELISA irrespective of whether incubation was for 2 h at room temperature or overnight at 4 °C. The effeciency of detection of virus in single viruliferous aphids by MUP-ELISA was not decreased by grouping with one or four non-viruliferous aphids but was decreased (PLRV) or greatly decreased (BYDV) by grouping with nine. MUP-ELISA and transmission tests to Physalis floridana seedlings (2–3 day inoculation access periods) both detected PLRV in most individual M. persicae, but the results obtained with the two methods did not correlate completely. In similar tests for BYDV in individual S. avenae, virtually all aphids transmitted BYDV to oat seedlings during a 3-day inoculation access period but it was subsequently detected by MUP-ELISA in less than half of them. By contrast, MUP-ELISA detected PLRV in most viruliferous M. persicae even after they had fed for 3 days on Chinese cabbage, a non-host for this virus.     

A major quantitative trait locus for resistance to Potato leafroll virus is located in a resistance hotspot on potato chromosome XI and is tightly linked to N-gene-like markers.

Potato leafroll virus (PLRV) causes one of the most widespread and important virus diseases in potato. Resistance to PLRV is controlled by genetic factors that limit plant infection by viruliferous aphids or virus multiplication and accumulation. Quantitative trait locus (QTL) analysis of resistance to virus accumulation revealed one major and two minor QTL. The major QTL, PLRV.1, mapped to potato chromosome XI in a resistance hotspot containing several genes for qualitative and quantitative resistance to viruses and other potato pathogens. This QTL explained between 50 and 60% of the phenotypic variance. The two minor QTL mapped to chromosomes V and VI. Genes with sequence similarity to the tobacco N gene for resistance to Tobacco mosaic virus were tightly linked to PLRV.1. The cDNA sequence of an N-like gene was used to develop the sequence characterized amplified region (SCAR) marker N127(1164) that can assist in the selection of potatoes with resistance to PLRV.