Spread Of Potato Leafroll Virus Research Articles

Transmission Efficiency of Potato Leafroll Virus by Four Potato Colonizing Aphid Species in Tunisian Potato Fields

This study investigated the transmission efficiency of Potato leafroll virus (PLRV) by four potato colonizing aphid species, Myzus persicae, Macrosiphum euphorbiae, Aphis gossypii and Aphis fabae, reported from leaves and yellow water trap. Physalis floridana was used as a test plant for virus transmission. DAS-ELISA was used for virus screening of samples as well as virus detection on the test plant after transmission experiment. A 2-h period was sufficient for the tested aphids to acquire PLRV virions. However, a difference in the transmission potential occurred according to the aphid species. The highest potential was recorded for M. persicae and M. euphorbiae, at 90 and 80%, respectively. For the first time, the study revealed the PLRV transmission efficiency of A. fabae, estimated at 50%. The lowest potential rate of 30% was recorded for A. gossypii. This study highlights the PLRV transmission capacities of four potato colonizing aphids suspected to play a key role in the spread of PLRV in potato seed production sites.

Dynamics of Myzus persicae arrestment by volatiles from Potato leafroll virus‐infected potato plants during disease progression

AbstractGreen peach aphid, Myzus persicae (Sulzer) (Hemiptera: Aphididae), an important pest of potato (Solanum tuberosum L.) (Solanaceae), preferentially settles on Potato leafroll virus (PLRV)‐infected potato plants as compared with non‐infected ones, primarily in response to volatile organic compounds (VOCs) released by the plants. In this study, we examined the dynamics of these effects, measuring arrestment of apterous M. persicae in response to VOC from upper, middle, and lower leaflets of PLRV‐infected potato plants at the same stage in disease progression (4 weeks after inoculation), but inoculated at 1, 3, or 5 weeks after transplanting (WAT). Sham‐inoculated plants were used as controls and VOC were collected and quantified. Aphid arrestment was greater on PLRV‐infected plants inoculated at 1 and 3 WAT as compared with sham‐inoculated plants, but this preference was reversed in plants inoculated at 5 WAT. Relative arrestment of M. persicae by infected plants and VOC release was greater for lower and middle leaflets than for upper leaflets at 1 and 3 WAT compared to sham‐inoculated plants. The reverse was observed in plants inoculated at 5 WAT. Findings indicate that aphid preference is influenced by VOC release from PLRV‐ or sham‐inoculated potato plants and that VOC emissions and aphid preference depend upon the age at inoculation and leaf position within the potato plants. The implications of these dynamics in vector behavior for spread of PLRV in the field in natural and managed systems are discussed.

The Importance of an Invasive Aphid Species in Vectoring a Persistently Transmitted Potato Virus: Aphis glycines Is a Vector of Potato leafroll virus

Experiments were undertaken to determine soybean aphid (i) landing rates in potato fields, (ii) population dynamics on potato, (iii) feeding behavior compared with green peach aphid on potato using the electrical penetration graph technique (EPG), (iv) acquisition, retention, and transmission of Potato leafroll virus (PLRV), and (v) if soybean aphid-infested crop borders could increase PLRV spread in seed potato. Soybean aphid (Aphis glycines) landed on potato but failed to establish colonies. EPG showed no significant differences between the aphid species in preprobe, xylem phase, sieve element salivation, and phloem sap ingestion durations on potato. Soybean aphid acquired PLRV 78% of the time, and 75 and 70% of individual aphids retained infectivity after 72 and 144 h, respectively. Soybean aphid transmitted PLRV to susceptible potato with 6 to 9% efficiency. Prior to the invasion of this exotic pest, soybean borders were commonly used in Minnesota and North Dakota to protect seed potato against spread of Potato virus Y. In 2002 and 2004, PLRV incidence was not different in potatoes with soybean borders whether treated with insecticide or not. In 2005, with extreme soybean aphid pressure, potatoes with untreated (no insecticide) borders had significantly greater PLRV spread. This is the first report of soybean aphid transmitting PLRV.

Hairy Nightshade as a Potential Potato leafroll virus (Luteoviridae: Polerovirus) Inoculum Source in Pacific Northwest Potato Ecosystems

Hairy nightshade, Solanum sarrachoides, is a solanaceous weed found abundantly in Pacific Northwest potato ecosystems. It serves as a reservoir for one of the important potato viruses, Potato leafroll virus (PLRV) (Luteoviridae: Polerovirus), and its most important vector, the green peach aphid, Myzus persicae (Homoptera: Aphididae). Laboratory research indicated an increased green peach aphid settling and performance on S. sarrachoides than on potato. It also revealed that green peach aphids transmitted PLRV more efficiently from S. sarrachoides to potato than from potato to potato. To test the efficiency of S. sarrachoides as an inoculum source in the field, a two season (2004 and 2005) trial was conducted at Kimberly, Idaho. Two inoculum sources, PLRV-infected potato and PLRV-infected S. sarrachoides, were compared in this trial. Green peach aphid density and temporal and spatial PLRV spread were monitored at weekly intervals. Higher densities of green peach aphids were observed on plots with S. sarrachoides and inoculum sources (PLRV-infected S. sarrachoides and potato) than on plots without S. sarrachoides and inoculum sources. PLRV infection in plots with PLRV-infected S. sarrachoides was similar to or slightly higher than in plots with PLRV-infected potato as an inoculum source. Temporal and spatial PLRV spread was similar in plots with either inoculum source. Thus, S. sarrachoides is as efficient as or a better PLRV inoculum source than potato.

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 BC(3) population. Data from a previous evaluation of the BC(2 )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.

Low-level jet streams associated with spring aphid migration and current season spread of potato viruses in the U.S. northern Great Plains

Green peach aphid, Myzus persicae (Sulzer), is the principal and possibly sole vector of Potato leafroll virus (PLRV) and a key vector of Potato virus Y (PVY) in the U.S. northern Great Plains. Current season spread of PLRV and PVY in seed potato depends primarily on flight activity of winged aphids and the availability of virus inoculum. M. persicae does not overwinter outdoors in the northern Great Plains, populations are reestablished each year by spring immigrants from the south. The objective of this research was to relate spring low-level jet (LLJ) streams to intensity of M. persicae flight activity and current season spread of PLRV and PVY. Synoptic weather maps were used to identify LLJ events that could have brought aphids into the northern Great Plains and to record the timing and duration of each event from 1 May to 30 June. HYSPLIT (Hybrid Single-particle Lagrangian Integrated Trajectory), a trajectory analysis model developed by the National Oceanic and Atmospheric Administration and Australian Bureau of Meteorology, was used to track pathways of air particles to their source. Captures of M. persicae and other potential vector species by a regional aphid trapping network ( Aphid Alert) from 1992 to 1994 and 1998 to 2003 were used as measures of aphid flight activity. Results from winter grow-outs of the Minnesota Seed Potato Certification Program were used as measures of available PLRV and PVY inoculum. As a possible surrogate for aphid flight activity we used data on spring wind events to predict current season seed lot rejections due to PLRV and PVY. Statistical models were developed relating frequency and duration of spring wind events to subsequent M. persicae abundance and severity of PLRV and PVY spread in the northern Great Plains. Results showed that the cumulative LLJ duration fit best with cumulative M. persicae capture through the first week of August ( R 2 ranging from 0.597 to 0.883), and the current season spread of PLRV fit best with inoculum and cumulative M. persicae capture through the first week of August ( R 2 = 0.75, P = 0.015). LLJ duration did not reliably predict seasonal abundance of PVY vectors other than M. persicae. The model for prediction of PVY using LLJ duration and PVY inoculum was considerably less reliable ( R 2 = 0.30, P = 0.34) although using both M. persicae capture through the first week of August and PVY inoculum fit better ( R 2 = 0.39, P = 0.231).

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.

Reduced Field Spread of Potato Leafroll Virus in Potatoes Transformed with the Potato Leafroll Virus Coat Protein Gene.

Russet Burbank potato was transformed with plant expression vectors containing the potato leafroll luteovirus (PLRV) coat protein (CP) gene. Transgenic potato lines contained a gene expression cassette with two copies of a PLRV CP gene in which the nucleotide sequence was modified to improve expression of the gene. In addition, the two copies of the PLRV CP gene were each driven by a different promoter. Field test screening for PLRV resistance identified 15 lines which showed moderate resistance to PLRV infection and virus titer build-up and a longer incubation period for systemic infection. By conducting field resistance assays during a period when the vector of PLRV was not present, it was possible to test whether the observed resistance was sufficient to restrict aphid transmission of PLRV in a field test. Two years of field testing demonstrated that PLRV-spread from an infected plant to adjacent healthy plants of the same line was severely restricted in nearly all the transgenic lines in the field. These lines have useful resistance to PLRV and could aid in managing PLRV disease in Russet Burbank potato.

Potato leafroll virus spread in differentially resistant potato cultivars under varying aphid densities

An action threshold of 3-10 green peach aphid,Myzus persicae (Sulzer), apterae per 100 lower leaves is recommended for use in Minnesota to prevent further spread of potato leafroll virus (PLRV) in potato,Solarium tuberosum L. This threshold was first developed and validated using the PLRV susceptible cultivar Russet Burbank. Here we report experiments to determine if higher aphid densities could be tolerated in PLRV resistant cultivars,i.e., Kennebec (moderately resistant) or Cascade (highly resistant), without an increase in PLRV infection. Insecticidal sprays were applied to plots when predetermined target aphid densities, based on number of apterae per 100 leaves, were reached: 3, 10, 30, and 100 (Russet Burbank); 10, 30, 100, and 300 (Kennebec); and 30, 100, 300, and 1000 (Cascade). The response variable was the average percentage of PLRV infected plants. Overall mean cumulative aphid-days and percent PLRV infection were 617 and 23.5% for Russet Burbank, 1,296 and 10.2% for Kennebec, and 4,816 and 9.5% for Cascade. For each cultivar, the highest target aphid density tolerated without an increase in PLRV spread was determined by comparing PLRV infection in plots sprayed on predetermined thresholds to PLRV infection in plots where aphids were rigorously controlled. This maximum density was 10 apterae per 100 leaves for Russet Burbank and 300 apterae per 100 leaves for Cascade. Results using Kennebec were ambiguous, but Kennebec was always more resistant to PLRV than Russet Burbank. Excised leaflet tests showed that the cultivars did not differ in resistance to green peach aphid. It appears that action thresholds based on green peach aphid apterae can be different depending upon the inherent PLRV-resistance of the cultivar.

Within-Plant Accumulation of Potato Leafroll Virus by Aggregated Green Peach Aphid Feeding

Aggregates of 20 green peach aphids placed on noninoculated leaves of plants previously inoculated with potato leafroll virus (PLRV) caused significantly more accumulation of PLRV at their feeding sites than in aphid-free leaves in both potato and Physalis floridana. Based on comparable periods of postinoculation, six times more viral antigen was detected by enzyme-linked immunosorbent assay in inoculated leaves of P floridana than of potato. The amount of PLRV detected in inoculated leaves was significantly correlated with the amount of PLRV that accumulated in aggregate feeding leaves, indicating that systemic spread of PLRV was titer-dependent. The movement of PLRV to aggregated aphid feeding sites probably followed the path of metabolites directed to the site of aphid damage and/or nutrient withdrawal. The accumulation of PLRV was not specific to the area of aggregated feeding but was evident throughout the leaf supporting the aphid colony. Aphids feeding in aggregates provided a mechanism that optimized virus acquisition from infected plants in which the pathogen was not uniformly distributed.

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

SummaryTubers 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.

Potato Leafroll Virus Spread in Relation to Densities of Green Peach Aphid (Homoptera: Aphididae): Implications for Management Thresholds for Minnesota Seed Potatoes

Tolerances for potato leafroll virus in potato, Solanum tuberosum L., seed are low in Minnesota, necessitating intensive insecticide use to control the vector, green peach aphid, Myzus persicae (Sulzer). We sought to determine the relation between low green peach aphid densities and potato leafroll spread, and to learn if insecticide use in seed potatoes could be reduced using green peach aphid apterae action thresholds. Plots containing leafroll infected plants were sprayed with methamidophos on thresholds of 0, 10, 30, and 100 apterae per 100 leaves in 1987 and 0, 3, 10, and 30 per 100 leaves in 1988. Percent potato leafroll infection was related to cumulative green peach aphid apterae-days (number of aphids per 100 leaves × number days present) and time of leafroll source introduction (R2 = 0.79). Leafroll infected sample units were more often adjacent than would be expected if occurrence were random in 17 of 18 apterae action threshold plots. The pattern of leafroll spread and the relation with green peach aphid apterae-days suggested that apterae were primarily responsible for spread of the virus. Percent potato leafroll infection in potato, cv. Russet Burbank, was related to the green peach aphid apterae thresholds at which insecticidal sprays were applied, although variation was large within a threshold. Data from these studies suggest that the threshold concept might be employed, but that the threshold for treatment would be very low, perhaps in the range of 3-10 green peach aphid apterae per 100 leaves. Experiments in 1988 showed little spread early in the season, even though aphids and leafroll infected plants were present.

The relationship between coccinellids and aphids in the spread of potato leafroll virus in a greenhouse

the relationship between coccinellids and aphids in the spread of potato leafroll virus in a greenhouse

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

SummaryExperiments 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.

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

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.

Spread and Control of Potato Leafroll Virus in Minnesota

We compared importance of green peach aphid, Myzus persicae (Sulzer), and potato aphid, Macrosiphum euphormae (Thomas), apterae and alatae in secondary spread of potato leafroll virus (PLRV) and the control achieved with foliar sprays alone to that of conventional practice using a soil systemic. Density gradients of apterae were produced by application of insecticidal sprays. Spread of PLRV from centrally located infector units was monitored in foliage serologically. PLRV infection ranged from 23.7 to 74% in 1985 and 12.3 to 77.2% in 1986. PLRV infection was significantly correlated with cumulative green peach aphid and with potato aphid apterae-days both years. Green peach aphid was more abundant than potato aphid and alatae were few both years. At all distances (1-5 m) from infector units, infection across rows was 38.7-60.6% of that within rows. Steep gradients of PLRV infection with distance from infector units indicated short-distance movement of vectors and suggested that apterae were of prime importance in PLRV spread. Check plots without a PLRV source and not augmented with green peach apllid (1986 only) had a final PLRV incidence of 1.5%, indicating only minor invasion of these plots by viruliferous alatae. Weekly application of methamidophos sprays reduced PLRV incidence to that in plotstreated with aldicarb at planting and weekly methamidophos sprays. It should be feasible to develop a seed potato pest management program for Minnesota based on foliar sprays alone.

Multiple components of the resistance of potatoes to potato leafroll virus

SUMMARYIn 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.

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

SUMMARYEnzyme‐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.

Potato Leafroll Virus: Evaluation of Resistance in Potato Cultivars

Potato Leafroll Virus: Evaluation of Resistance in Potato Cultivars

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

SUMMARYField 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.