Foliar symptoms of PLRV include leaf rolling, chlorosis (yellowing), reddening, 'leathering' of leaves, phloem necrosis, and stunting (Ragsdale et al., 1994; Rich, 1977; Rich, 1983). Plants infected early in the growing season may also be dwarfed, but if virus infection occurs late in the growing season foliar symptoms may not be exhibited. Potato plants develop resistance to foliar infection with plant age (Peters, 1981; Rich, 1977). Many times, infection can be seen in a circular pattern in the field, frequently surrounding what was most likely the original source of virus inoculum, an infected seed piece. Direct damage can also result from aphid feeding. Large numbers of aphids present can kill potato plants producing what is referred to as an 'aphid hole' in the field (Hines and Hutchinson, 1998).

PLRV travels through the phloem of the plant into tubers, reducing size and causing net necrosis. Net necrosis causes browning of the vascular system extending throughout the entire tuber as opposed to stem end browning which does not penetrate the tuber more than 1 inch from the stem end. Net necrosis is not normally seen in tubers produced from secondary, seed tuber-borne infected plants, with the virus concentration decreasing each year daughter tubers are propagated (Peters and Jones, 1981). Net necrosis is primarily seen when the virus is spread to healthy plants by aphids. Net necrosis may or may not be found prior to harvest, possible developing or becoming more severe in storage. Timing of the development of net necrosis is dependent upon the time in the growing season at which plants become infected. Tubers produced by plants infected with PLRV early in the growing season, June or July, may exhibit net necrosis at harvest. Plants infected in August or later will not likely exhibit symptoms of net necrosis at harvest, but develop symptoms later in storage. Also, plants infected with PLRV during the tuber bulking stage of growth in late July and early August are more likely to develop net necrosis. Differences in severity of net necrosis can be seen among virus strains (Bantarri, 1993; Peters and Jones, 1981). Net necrosis does not threaten all potato cultivars, but is a major problem in infected Russet Burbank tubers with a high percentage of crops intended for processing. Net necrosis has also been identified in Green Mountain and Russet Norkotah. Control of PLRV includes the isolation of fields targeted for certified seed production, the use of clean seed, control of aphids, removal of infected plants, and early harvest. Tubers with net necrosis are unacceptable for processing into chips and fries, causing serious losses to growers and potato processors.

The green peach aphid (Myzus persicae), the potato aphid (Macrosiphum euphonbiae), and the buckthorn aphid (Aphis nasturtii) are all colonizing aphids in a potato crop, and therefore may transmit PLRV (Radcliffe and Ragsdale, 1993; Ragsdale et al., 1994; Rich, 1983). Because of high efficiency for virus transmission and high populations in the crop, the green peach aphid is by far the most effective vector of PLRV (Radcliffe et al., 1993; Ragsdale et al., 1994; Simpson, 1977). In the Northeast and Midwest U.S., green peach aphids are responsible for 95% of PLRV transmission (Radcliffe et al. 1993).

Several species of aphids are known to transmit six viruses, persistently and non-persistently, which cause potato diseases in North America. The term persistent comes from the ability of a particular aphid species to carry this virus through molting, and possibly for life. Concentrated in phloem tissue, PLRV can be acquired only by an aphid that chooses an infected plant as a host and after extended feeding has taken place, not during sap sampling (Radcliffe et al., 1993; Ragsdale et al., 1994). For this reason, aphids that colonize a potato plant, as opposed to aphid species that are 'transient visitors' to the crop most effectively transmit PLRV. PLRV is the only known persistently transmitted virus of potatoes. Aphids acquire virus particles only while feeding on phloem cells (Radcliffe et al., 1993; Ragsdale et al., 1994). In order to reach the phloem tissue, the aphid must spend from 10 to 30 minutes on a host, with maximum transmission efficiency reached after about 12 hours after feeding (Ragsdale et al., 1994). PLRV infested plant sap is ingested by the aphid and passed through the gut wall into the bloodstream. The virus then enters insect tissues such as the salivary glands where it can be transmitted to the next plant with the aphid saliva during feeding.

In contrast, during non-persistent virus transmission such as with PVY, virus particles do not enter into the bloodstream of the aphid, remaining only in the mouthparts and front gut. PVY is concentrated in epidermal cells of the plant and any aphid can acquire the virus during sap sampling, even if the aphid does not colonize the host (Radcliffe et al., 1993; Ragsdale et al., 1994). The aphid retains the virus for only a few hours, and not after a molt, therefore, nonpersistent. Non-persistently vectored viruses can usually be mechanically transmitted also.

PLRV can be managed though the use of three fairly simple concepts: obtaining virus-free certified seed, killing volunteers and weed hosts, and early rouging of infected plants will all help to eliminate virus inoculum. Because PLRV is not mechanically transmitted, aphids are solely responsible for the in-season spread of the virus. Therefore, the management of aphids is an integral part of the management of PLRV. Virus spread can be minimized with insecticide applications for aphid control and the preservation of natural enemies (Mowry, 1994). Three circumstances exist in which the control of aphids may be necessary; the elimination of virus vectors during seed production, when cultivars susceptible to net necrosis such as Russet Burbank are grown, and during aphid population outbreaks. Aphid outbreaks frequently occur due to insecticide resistance and the elimination of natural enemies (Radcliffe et al., 1993).

Insecticide sprays for aphid control are recommended through the use of action thresholds. Economic action thresholds are set to predict the minimum number of aphids needed to cause economic loss and, therefore, the best timing for insecticide applications (Radcliffe et al., 1993). Action thresholds for insecticide application on fresh market potatoes varies among regions of the country, in the upper mid-west the threshold is set at 30 aphids per 100 leaves. Insecticide application timing is crucial for the effective control of aphids. Therefore, scouting fields and evaluating the population including both growth stage and number, allows for greater control with fewer applications. Insecticides aimed at Colorado Potato Beetles and leafhoppers may control aphids for the first application, but the second application often reduces natural enemies and promotes insecticide resistance. Many times, direct feeding damage done by aphids can be associated with aphid flare-ups due to insecticides applied to control other insects (Radcliffe and Ragsdale, 1993). Green peach aphid is one of the few aphid species resistant to every major insecticide class including organochlorines, organophosphates, carbamates, and phyrethroids. The only effective compounds with registration at this time are Monitor (methemidophos), and Admire (imidacloprid). Because of this, control measures directed toward aphids are not generally used in fresh market potato production, but low tolerances for PLRV in seed production require the use of some insecticides for aphid control.

Natural pests, in many instances, can effectively control aphid populations under cool and humid conditions, unless insecticides are used. Natural pests of the aphid include predators, parasites, and fungal pathogens. Predatory insects such as the ladybird beetles, lacewings, minute pirate bugs, hover flies, damsel bugs, and seed bugs are much more effective at controlling aphids than parasites (Hines and Hutchinson, 1998; Radcliffe et al., 1993). Both fungicides and insecticides are damaging to populations of aphid natural enemies. With the increasing importance of late blight and other fungal diseases on potato, there has been a dramatic increase in the amount of foliar fungicide applications. All major fungicides registered for the control of late blight kill beneficial fungi, increasing aphid populations dramatically (E.B. Radcliffe, personal communication). Mancozeb sprayed routinely can cause aphid populations to rise to 100 times those receiving no fungicide applications. Beneficial fungi offer possibilities for the biological control of aphids in the future, but high rates of fungicides used on potatoes limit the possibility of fungi as biological control agents (Radcliffe et al., 1993). Because management of potato leafroll and net necrosis is heavily dependent upon managing aphid populations, it is useful for potato producers to understand the biology of PLRV aphid vectors.

Aphids have both a winged (alatae), and wingless (apterae) forms. Wingless forms are the most common during the growing season, but both forms can transmit viruses. In Minnesota and North Dakota, wingless aphids dominate, but a few wingless aphids enter fields as virus carriers. Virus inoculum originates from the potato crop, either from infected seed or volunteers. While both the potato and buckthorn aphids over-winter in North Dakota and Minnesota, there is no evidence that the green peach aphid over-winters in severe climates such as the Red River Valley. Recent research has shown that this may not be the case, but the degree to with which the green peach aphid over-winters is not known (Hines and Hutchison, 1998; Radcliffe et al., 1993). Green peach aphids are known to originate in local greenhouses, or from long distance migration (Radcliffe and Ragsdale, 1993), migrating as far as 1,000 miles at altitudes of up to 10,000 feet (Radcliffe et al., 1993). Traditionally, aphids arrive in the northern Great Plains from southern areas via winds.

In regions too cold for continuous outdoor green peach aphid production, eggs serve as over-wintering structures on a restricted number of hosts, including woody plants and stone fruits. In the spring, eggs hatch and produce wingless aphids (stem mothers). These stem mothers reproduce without mating, giving birth to live young. Winged forms develop when populations become dense and migrate to secondary hosts while wingless offspring remain on the primary host and continue to reproduce. Before becoming adults, nymphs go through four instar stages (Simpson, 1977). During the growing season, these green peach aphids reproduce asexually with a new generation approximately every seven to ten days. Under warm, dry conditions aphid populations may double in less than two days (Radcliffe et al., 1993).

Winged green peach aphids have the ability to colonize over 100 species of secondary hosts. In order to find a suitable host, aphids must feed for a short time to determine sustainability; this process is called sap sampling. If the plant is unsuitable, the green peach aphid continues to move from host to host in search of a compatible feeding stimuli. When a suitable host is found the green peach aphid remains, reproducing almost entirely wingless offspring. As the quality of the secondary host diminishes, more winged offspring are produced to search for a more suitable host. As day length shortens in the fall of the year, both male and female fall migrants are produced. Upon return to the primary host, winged females give birth to wingless offspring, which in turn mate with male fall migrants. These females then lay fertilized over-wintering eggs deposited near buds on the primary host (Radcliffe et al., 1993).

PLRV damage done to potato crops in the upper Midwest has increased significantly over the past several years. Net necrosis, caused by PLRV infection, can cause rejection of a potato crop by french fry processing plants, causing significant economic impact to an individual grower. Seed certification is a major weapon in the fight against PLRV, limiting inoculum levels. Plants having secondary infection from seed tubers introduce the virus to aphids as they arrive in a field. Primary infection occurs when these aphids pass the virus to neighboring healthy plants in the field. Daughter tubers of some cultivars produced from these primary infections may, develop net necrosis. Therefore, along with clean seed, aphid control is of utmost importance. Aphid management starts with ongoing evaluations of the arrival and maturation of the population in a field. The limitation of insecticide use is important due to the ease of aphid resistance development. The increase in fungicide use on potato crops in this area has also caused aphids to be more difficult to manage due to elimination of natural enemies.

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Ragsdale, D.W., Radcliffe, E.B., DiFonzo, C.D. and Connelly, M.S. 1994. Action thresholds for an aphid vector of potato leafroll virus. Pages 99-111 in: Advances in Potato Pest Biology and Management. G.W. Zehnder, M.L. Powelson, R.K. Jansson, and K.V. Raman eds. American Phytopathological Society, St Paul, MN.

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