Net necrosis of the potato tuber is the result of infection by potato leaf roll virus (PLRV). Net necrosis causes unacceptable discoloration in processed potato products and is not easily or quickly removed during trimming in processing plants. Fresh pack potato grading standards also have strict tolerances (typically less than 5% internal defects) and this defect presently cannot be detected without cutting the potato. Consequently, it is difficult to use potatoes containing net necrosis for either processing or fresh packing.

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Seed-borne PLRV infection produces stunted plants with yellowed, rolled-up leaves that have a leathery texture. The rolled leaves will also produce a "rattling" sound when shaken. Symptoms are more subtle in plants with current-season infections. Early-season leaf roll infection typically stunts plant growth and causes the potato leaves to have a yellowed, leathery appearance with rolled leaves, much like the seed-borne infection, only milder. Late season infection of leaf roll virus may show little or no foliage symptoms, although the virus can still move down into the tubers.

Tuber

Tubers from infected plants are usually smaller than those from healthy plants and may develop a brown discoloration (net necrosis) in the flesh. Tuber symptoms are caused by the selective death and damage to cells in the vascular tissues of the tuber. The fact that only specific cells within the tuber, the phloem tissues, are affected by this problem while others remain unaffected causes the characteristic "net" symptom. Symptoms first appear as light to dark brown strands of discoloration in the flesh at the stem end of the tuber. These necrotic (dead tissue) strands can continue to progress the length of the tuber either in the field or after months in storage.

Plants produced from infected seed are not only stunted but also have reduced yield both in tuber numbers and in tuber size. Often the tubers produced by such plants have no net necrosis symptoms. Large tubers that show the severe net necrosis symptom are usually the result of current season infection.

Defects that resemble net necrosis

There is a strong resemblance between PLRV net necrosis and another tuber defect known as stem-end discoloration (SED) or stem-end browning. Unlike PLRV, SED is believed to be a physiological disorder. Net necrosis is difficult to distinguish, even by trained personnel, from stem-end discoloration particularly at harvest or early in storage. However, rapid and relatively inexpensive laboratory tests are available that can identify leaf roll virus in potato tubers.

Potato leaf roll virus can only be spread by aphids that colonize potatoes. Although many types of aphids may land on potato plants, there are only three species, green peach aphid (Myzus persicae), potato aphid (Macrosiphum euphorbiae) and, in southwestern Idaho, foxglove aphid (Aulacorthium solani) that normally colonize and produce nymphs on potatoes in Idaho. Green peach aphid is considered to be the most important of the colonizing aphids. This is because the green peach aphid is easily the most efficient vector of PLRV, even though the potato aphid, for instance, is often far more abundant. Green peach aphids have a distinct teardrop shape, and despite the "green" designation in the common name, are usually light yellow to pink or orangish in color and often reside on the underside of the lower leaves on potato plants. In that part of the canopy there is considerable protection against direct sunlight and wind. Potato aphid are green and football-shaped and will run rapidly to avoid the sun when leaves are turned over. Both the potato and foxglove aphid are capable of spreading PLRV but at such a low efficiency that these species are not considered important vectors of PLRV.

PLRV is an extremely small, nearly spherical virus particle (its diameter is only 0.000001 inch). The virus is referred to as "phloem restricted" because it only infects and multiplies in the phloem tissues of the host plant. This feature greatly influences the characteristic foliar and tuber symptoms associated with PLRV as well as the relationship of the virus with the aphid vector.

The insect vector is essential to disease spread because mechanical transmission, like that which occurs when the leaves of an infected plant rub on those of a healthy one, does not occur with PLRV. In fact, the transmission process is actually quite complicated with this virus. The virus does not overwinter in the egg of the aphid. This means that the newly hatched aphids are virus-free and must acquire the virus by feeding on a PLRV- infected plant. After feeding, the virus must circulate from the gut of the aphid, through the circulatory system until it finally gets into the salivary glands, from which it is secreted along with saliva when the aphid feeds on healthy plants. Only after this process has occurred can the aphid spread the virus and this sequence of events may require 24 hours or more to occur. Unfortunately, once an aphid becomes infected, it remains so for the rest of its life. New-born aphid nymphs are also virus-free and must acquire the virus, just as outlined above, before they are capable of transmitting. Spread of the virus between plants within a field and between fields can be done by the winged forms of the aphid, but much of the spread within a field is accomplished by the wingless forms.

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There are at least three potential sources of green peach aphids infesting potato fields:

1. overwintering aphids on primary hosts
2. overwintering aphids on secondary hosts
3. infested bedding plants in commercial nurseries

The primary hosts of the aphid are peach and apricot trees and they overwinter on these trees in the egg stage. In the spring, the eggs hatch and, after several generations on the primary host, winged forms are produced that migrate to secondary hosts including potato. The green peach aphid has over 100 secondary hosts, both cultivated and noncultivated plants, on which it can develop and maintain populations throughout the spring, summer, and fall. Because it has such a large secondary host range, viruliferous (infected aphids) green peach aphid can overwinter in the adult stage in sheltered areas, such as on weeds in drainage ditches when the winters are mild. In the spring, these aphids become active and immediately begin to reproduce, eventually infesting crops in the surrounding areas. The host plants must remain green all winter, so this type of overwintering may occur in some areas of Idaho. Green peach aphids are also introduced into Idaho potato production areas on infested bedding plants, such as cole crops, eggplant, forget-me-not, peppers, and petunia that are sold in local retail stores. Surveys conducted over various years showed that all areas in southern Idaho had sources of infested bedding plants. In general, secondary hosts play only a minor role in the overwintering of green peach aphid. Peach trees and bedding plants are the major sources.

Although there are reports that certain weeds can be infected with PLRV, potato is by far the most important source of the virus. This means that there are several potential sources of PLRV to be found in Idaho. First, there is always a small level of virus infection in certified seed. Second, backyard gardeners who plant year-out seed can inadvertently increase the local PLRV inoculum. Coupled with the fact that many bedding plants are infested with green peach aphids, which may be planted along with infected potatoes, these gardens can become sources of migrating aphids carrying PLRV. Third, volunteers, especially from fields having high levels of PLRV infection the previous year, can be a significant source of PLRV. Green peach aphids developing on volunteers that are surrounded by a crop that is not a suitable aphid host will tend to produce winged forms earlier in the season. Having acquired PLRV from the volunteer, such aphids may migrate to nearby potatoes already capable of transmitting the virus.

Seed certification programs allow only a very small level of PLRV in certified seed. In Idaho, for instance, during the second field inspection the allowable amount of PLRV is only 0.08% for G4 seed, 0.03% for G3, 0.02% for G2 and none at all allowed in nuclear and G1. Very small percentages of PLRV in seed potatoes do not normally pose any risk for the commercial producer. However, even very small percentages of virus can be a problem if green peach aphids appear very early and in abundance. For example, a 120 acre pivot with 0.05% PLRV will contain some 870 infected plants for aphids to encounter. In years that are very favorable, aphid populations can become so large that even a very low percentage of PLRV-infected seed could result in sufficient spread to cause a problem, because as the aphid population increases, so does the probability that some of them will encounter infected plants. Control of aphids with insecticide application is the most important means of managing PLRV spread in production years that are highly favorable for aphids. A fourth source is potato fields themselves. For long season varieties such as Russet Burbank, early harvest varieties and genetically modified potatoes can be sources of large numbers of aphids. In Idaho, 30% of the potato crop consists of varieties other than Russet Burbank and these are generally harvested early and not treated to control green peach aphid. Aphids leaving those crops will probably be highly viruliferous and could rapidly transmit leafroll. Consideration should be given to spraying these early varieties to reduce aphid numbers spreading to late season potatoes.

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Control of PLRV spread begins with the selection of certified seed. As stated earlier, potato seed certification standards allow only very low levels of leaf roll virus during field inspections and winter grow out tests. Year-out seed is not recommended because it has not been inspected or rogued for diseased plants nor has there been a winter test performed that would indicate how much new infection might have occurred during the growing season. Another important point is to insist that certified potato seed be tagged. This ensures all certification standards have been met, including the final shipping point inspection, and will help reduce inoculum sources of leaf roll as well as other seed-borne diseases in commercial fields.

Severe outbreaks of leaf roll or net necrosis typically occur during years when green peach aphid populations are high and/or persistent. Therefore, green peach aphid control is of paramount importance in limiting the spread of PLRV. Controlling the overwintering aphid eggs on peach and apricot trees can help to limit the size of the migrating spring population. One effective recommendation is to spray these trees every spring with crop oil to kill the aphids before they leave their winter host sites. Remove unmanaged, unsprayed or unneeded peach and apricot trees.

Scouting Potato fields should be monitored for green peach aphids throughout the growing season. When to start monitoring depends upon the seasonal and local conditions, as well as the need to detect aphid populations early. Normally, monitoring should begin in western and central Idaho early to mid-June. In eastern Idaho, late June to mid-July is early enough. Leaf samples should be taken on a diagonal transect across the field. Aphids are often, but not always, found on the edges of the field, typically the south or west windward borders or near buildings or trees. Look on the older, lower leaves that are beginning to turn yellow.

Currently, a new threshold of at detection of surviving wingless aphids should be used for decisions to apply insecticides. Be sure to monitor the field every 3 to 4 days. Monitoring and spraying should continue as long as the vines are green and growing. This is because mid-to-late-season infection may not result in identifiable foliar symptoms, but can still cause tuber net necrosis.

Potato fields adjacent to urban communities are particularly susceptible to green peach aphid infestations and should be carefully watched. Gardens probably contain bedding plants with these aphids and may not be managed to prevent their buildup and eventual migration. Green peach aphids may also be present on peach and apricot trees in homeowner yards.

It is important that aphid monitoring be continued even after the application of an insecticide to ensure that populations are being managed. Several factors may contribute to aphid control failure when using insecticides including, ineffective insecticides, inadequate coverage, and insecticide resistance. Moreover, aphid populations may rebound after insecticide residues fall below lethal levels. Some of the newer chemicals may require several days to completely kill aphids. You may need to wait a few days before judging effectiveness.

Several chemical control methods for green peach aphid are available. A few insecticides applied at planting provide good early season control in commercial production areas but most of these wear off by mid-July to early August. As a result, fields may need additional chemical control later in the season.

The choice of insecticide may depend on several factors in addition to cost. Some systemics provide control over pests other than green peach aphid, such as potato beetle, wireworm and nematode. Local resistance of green peach aphids to specific chemical families may also be an important consideration in some areas so fields should be monitored following chemical application to confirm effective control. There are also restrictions on the minimum days after application until harvest for many chemicals.

Storage Management

Net necrosis cannot be actively controlled during storage. This is due, in part, to the fact that not all leaf roll-infected tubers will show net necrosis and that the severity of the symptom is variable from tuber to tuber. Since it is simply a matter of time until some of the leaf roll infected tubers express net necrosis symptoms, each potato storage should be surveyed for net necrosis symptoms beginning shortly after harvest. Samples of potatoes with net necrosis symptoms can be sent to a diagnostic laboratory to confirm virus presence and distinguish net necrosis from stem-end discoloration. Marketing decisions can then be made before net necrosis becomes heavily expressed in the tubers.

Fortunately, net necrosis will not spread from tuber to tuber in storage and the tubers affected with net necrosis will not decay or break down. However, the net necrosis symptom can increase within infected tubers. The degree of increase within a tuber in storage is dependent upon how much net necrosis is present at the start of storage and the length of storage. In other words, tubers not showing net necrosis at harvest could develop net necrosis in storage and tubers that had net necrosis to begin with could show substantial increases as storage time lengthens.

Recent storage tests indicate that the increase in net necrosis, regardless of the initial levels, during the curing period (the first 2 to 3 weeks) and for the subsequent 4 to 5 weeks is minimal. Thereafter, the increase can become noticeable and measurable. Generally, potatoes stored at 50^o F have a higher rate of net necrosis development than those stored at 38 to 42^o F. There is no practical method to detect the presence of the virus in individual tubers loading into storage, and thus, no way to estimate the threat of net necrosis developing. Potatoes in storage should be sampled and examined weekly to measure any increase in net necrosis symptoms. This type of monitoring will help producers make marketing decisions before net necrosis exceeds defect tolerances in the tubers.

Some potato varieties are more resistant than others to developing net necrosis under similar PLRV infection rates in the field. Advanced breeding selections and standard varieties are routinely tested each year for net necrosis reaction at the University of Idaho Kimberly Research and Extension Center where insecticide efficacy tests for green peach aphid control are also conducted.

The varieties Gem Russet and Umatilla Russet showed almost no serious, and very little moderate net necrosis. CalWhite, Frontier Russet, and Shepody were as susceptible or more susceptible than Russet Burbank to net necrosis symptoms. Shepody showed more variability in net necrosis symptoms from year to year compared to the other varieties, but occasionally showed serious net necrosis. Russet Norkotah resistance to net necrosis was better than Russet Burbank, but not as good as Ranger Russet. Genetically modified potatoes are not susceptible to PLRV and do not show net necrosis symptoms but they are not resistant to green peach aphid. This information may be useful in selecting varieties for leaf roll integrated pest management programs.

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Chart of Chemical Control Methods for Green Peach Aphids

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