Potato virus

Quite possibly. What will happen? Nothing good. The point is, you could get in serious trouble or cause serious trouble for others years after you think that you have gotten away with it. You should probably contact your state plant health authorities, but I am trying to be realistic here. This is not going to change.

People have been breaking the rules for as long as they have existed and will continue to for as long as they do. Social media provides abundant evidence of this. Foreign varieties are going to make their way into private collections and then they are your problem. That has been a progression as I have moved from hobbyist to professional. I have written previously about how I got a surprise visit from the USDA after ordering true potato seeds from Europe.

So, this article is partly my penance. This document sets out to help you develop the knowledge necessary to test and respond to viruses if you find them and to hopefully avoid making costly and counter-productive mistakes.

If you better understand the scope of the problem, then you can make better decisions and you can help others to be better informed so that they can do the same. Some viruses are classified as quarantine pests in the USA, with mandatory reporting to the agricultural authorities.

This is noted in the descriptions below. If you live in another country, you will have a different list of quarantine viruses. Potato collectors should have increased awareness of potato viruses, beyond those that are common in the commercial crop. Collectors have unique exposure to viruses for a variety of reasons. Potatoes grown in gardens have much different virus exposure than potatoes grown in monocrop fields. For example, most gardens contain other Solanaceae like tomatoes, peppers, and eggplants, which can be sources of viruses that are not common in potato fields.

I have noticed that Tomato Spotted Wilt Virus is fairly common in potatoes from private collections, but I have never seen it from a commercial source. I would guess that it spreads from tomatoes, in which is is a very common virus. Many collectors grow potato varieties that have been maintained as clones for decades without any virus cleanup.

There are still plenty of heirloom varieties in the USA that predate plant health regulations. These varieties may carry viruses that have been extirpated in the commercial crop. Viruses like Potato Spindle Tuber Viroid, Potato Aucuba Virus, and Potato Latent Virus have been detected in private collections in recent years even though they have been out of the commercial crop for a long time.

Collections also often contain varieties that escaped quarantine. Anyone who has spent much time in social media knows that people routinely carry potatoes across borders. There must literally be hundreds of such varieties floating around out there. I suspect that there are a lot more exotic potato viruses floating around in collections than anyone realizes.

Viruses are usually determined to be extirpated from a region when they no longer show up in the testing results of certification programs. That is a sensible enough approach when you are looking to minimize economic damage, but it means that there could be many exotic and economically unimportant viruses present in the country without any recognition. Collections are rarely tested. Even when they are tested, they are mostly tested for things that are common, because testing is expensive.

So, when you are testing a collection of older varieties or varieties of unknown origin, you should probably cast a wider net. It is commonly believed that growing plants from true potato seed TPS is a guaranteed way to eliminate viruses. It is largely true if your exposure is limited to viruses that are common in North America. Of course, growing potatoes from true seed preserves the genetics, but not the phenotype, so this method is no help for preserving particular clones.

Exotic viruses may be. The most common example is Potato Spindle Tuber Viroid , which spreads easily through true seed. Some viruses are transmitted from pollen directly to seeds, without infecting the parent plant. This is a really difficult problem, since you could collect seeds from a plant that you have tested and found clean, only to later discover that those seeds were directly infected with a virus. The viruses in the USA that are capable of transmitting via this route are not common in potatoes and are inefficient, but there are viruses elsewhere that both infect potatoes easily and transmit efficiently by the pollen to seed route.

Some viruses are able to infect the seed coat and can be difficult to sterilize. This includes Tobacco Mosaic Virus and other Tombamoviruses. Even with standard surface sterilization treatments with bleach or trisodium phosphate, Tobamoviruses can sometimes still transmit at a low rate on seeds. Many people are under the impression that you can produce virus free true seeds with heat treatment.

This advice mostly comes from tomato growers. Heat treatment as it is usually described for tomatoes reduces the odds of transmission of bacterial and fungal pathogens on TPS, but does little to eliminate viruses if they are carried in the seed.

The incidence of some viruses can be reduced by dry baking seeds, which may be a reasonable precautionary measure if you think you have exposure to viruses that might infect true seed, as long as it is followed up by testing to confirm. Whenever you acquire new, non-certified varieties from other collectors, there is a pretty high risk that they will carry some kind of virus.

Because of this, I quarantine new varieties that I am considering using for breeding or adding to my collection. Viruses spread by direct contact between plants, by insect feeding or potentially even just by insect pollination in some cases, or by contact with soil organisms.

To quarantine, I grow plants in pots, on either a concrete or plastic surface with no soil contact. I cover each pot with a frame covered in insect netting and I use a wide spectrum pesticide on the plants. This is the only part of my operation where I use pesticides. The goal is to keep insects off of the aerial plant and to prevent exchange of soil organisms with the environment. I keep the plants like this for the first season, giving plenty of opportunity to observe them for symptoms and to collect foliage for testing.

If I encounter something suspicious or can confirm infection by testing, I destroy the plant and sterilize the soil. The soil goes into a plastic barrel with bleach and then sits sealed in the sun for several years. I put the actual plant matter in an incinerator and reduce it to ash. If you have a collection that is substantially free of the common potato diseases, it is worth considering quarantining even certified seed potatoes.

That translates to 1 in plants. Remember that viruses typically have multiple host species. If you are protecting a potato collection, you should also quarantine other host plants that are likely to carry potato viruses.

I recommend quarantining most clonally propagated plants, particularly other Solanaceae and Andean root and tuber crops. You might also consider any clonally propagated Asteraceae. There are several different kinds of tests available for plant viruses. These fall into two categories: serological immunological tests and genetic tests. Serological tests are usually virus specific, while genetic tests can be used to test for wider groups of related viruses or even for totally unknown viruses.

The widest selection of tests is available from commercial labs. I have purchased from both Agdia and Nano Diagnostics in the USA with no complaints about either one, but you may be able to get some testing done and probably at a lower cost through your state extension or local university. Serological tests are based on the use of animal antibodies that were produced by exposure to the plant virus.

The antibodies are then exposed to a sample to see if they react. With one notable exception, immunological tests are usually specific to a particular virus, occasionally cross reacting only with other closely related viruses. The exception is the Potyvirus group, which can probably detect every virus in the Potyvirus genus. Because there are a lot of potato Potyviruses, this is a very valuable test and I recommend that everyone screen for Potyviruses.

The simplest, consumer-oriented tests are field test kits. These are usually a dip-stick kind of test, where you grind up some plant material and insert a test strip, which develops a positive or negative indicator. These tests require no skill beyond being able to read and follow instructions.

I use a lot of these. They are handy, keep well in the refrigerator, and require no setup, so I am likely to test something suspicious immediately rather than put it off until I have a lab day. Unfortunately, they are only available for the most common viruses and not all of those. ELISA requires some tools and some skill and practice, but if you can follow moderately complex instructions, like making a pastry or wiring a three way switch, you can manage ELISA.

If you only want to do the occasional test or doing your own ELISA is too much work, you can have a lab perform the tests for you and the cost is usually pretty reasonable. The next step is PCR. You will probably not do your own PCR. To warrant this expense, you should have some suspicion that the virus you are interested in is present. Also in this general category is nucleic acid hybridization, which is typically used to test for specific viroids.

Potato Spindle Tuber Viroid is a serious concern in potatoes and this is the only reason you would generally be interested in a NAH test. You can bulk several samples from different plants together.

In that case, you will have saved a lot of money if the test comes back negative, but you will need to resubmit individual samples to determine the source if you get a positive. This has the benefit of being comprehensive, but it is currently outside of the budgets of most collectors. It will have to come down in price by another order of magnitude to be practical for most people. The price is likely to keep coming down though and this will become a very interesting and useful option when it does.

Once you know that you have an infected variety, you face the decision of what to do about it. If you have uninfected plants of the same variety or if it is possible to acquire disease free material elsewhere, then the decision is easy: destroy the infected plants.

If you are conserving a rare variety and have no source of clean material, then you may want to consider cleaning it. Cleaning potatoes of viruses is a complex procedure that requires some facilities, tools, and skills that you will have to obtain. I will not detail the procedure here, but I will provide an overview. Removing viruses usually requires a combination of meristem culture, chemotherapy, and thermotherapy.

Meristem culture is the process of removing very small sections, less than half a millimeter in size, from the growing tip of the plant and growing them on tissue culture media.

The reasons why meristem culture works are complex and not completely understood. In some cases, meristem growth can outpace viral replication. In others, the virus primarily infects tissues that have not yet formed in the meristem. RNA silencing may also defend against viruses entering the meristem.

A technician with excellent technique, who can obtain the smallest possible tips, can have a very high rate of disease free plants, but some viruses are able to thoroughly infect the meristem. In practice, is it usually necessary to follow meristem culture with chemo- and thermotherapy.

Chemotherapy involves exposing the plant to an antiviral usually Ribavirin that disrupts the viral replication process. These techniques are usually applied in tissue culture, because the smaller amount of plant material in culture is more sensitive to these therapies.

Every virus responds differently. Some are very sensitive to one therapy technique or the other. Some are resistant to both. Sensitive viruses may easily be cleared in one pass. Resistant viruses may only yield a small number of clean plantlets and may require more than one round of therapy.

This can be an expensive and time-consuming process if the viruses that you want to remove are recalcitrant. In that case, you need to culture and treat large numbers of meristems and then test all of them to find the ones that have come through the process clean.

If ELISA tests are available for the virus that you are removing, you can probably manage the cost, but if you need to resort to PCR testing, the cost of testing your cultured plantlets could easily run to thousands of dollars. This is usually the case only with rare viruses though.

Most common viruses can be tested with ELISA and you can run hundreds of tests for a few hundred dollars. In a best case scenario, the process can take as little as three months. In the worse case, it can take years and you might never be successful. One of the most common questions that I receive is whether an easier and cheaper way exists to de-virus potatoes. The answer is generally no, but there are exceptions.

For the most part, you will probably be wasting effort trying to clean varieties in non-standard ways, but if you read the available research about a given virus, paying close attention to its temperature sensitivity and mode of infection within the plant, you might be able to devise a low tech approach.

The following methods are known to work for some viruses:. There are some studies that show promising results with electrotherapy for some viruses, which involves passing a small current at high voltage through sprouts, cuttings, or even tubers. While the virus clearance rate is much lower than for meristem culture, this is technically less demanding, so the lower odds of success may be an acceptable trade.

Electrotherapy is generally done with an electrophoresis power supply, set to variable voltage and fixed current generally between 10 and 35 mA, with treatment periods of 5 to 20 minutes.

Tubers tend to die at the high end of current and duration, but that is also where the virus reduction is usually greatest. Tolerance appears to differ between varieties, so you will probably need to experiment to determine how much the plants that you are treating can withstand. Probes can be inserted at either end of a tuber and connected directly to the supply, or the tubers can be immersed in electrolyte solution like salt water with electrodes at either end of the bath.

At the end of treatment, some tubers may be virus free, but you will have to test to confirm. I have found electrotherapy at the mid to high range to be effective for clearing PVY. I have had no success clearing PVS with electrotherapy. Some viruses do not uniformly infect tubers. You could grow all the tubers from an infected plant and then test each plant to see if you get lucky. This approach may work with many viruses in the first year of infection. It generally becomes much less useful in systemically infected plants grown from infected tubers, but some viruses are particularly inefficient and may still turn up some clean tubers.

This approach only makes sense when there is a serological test for the virus, since it would get pretty expensive with genetic testing. Some viruses are so vulnerable to heat that you can apply heat directly to tubers in order to clean them.

This usually involves putting the tubers in an incubator for several weeks. Some of the tubers usually will die, but some of those that survive may be clean. Five weeks at about degrees F is probably a good place to start. I have noted in the virus descriptions below when research has determined a specific protocol. You may have to reduce the temperature or duration a bit if the tubers all die. You will have to test the survivors to determine if any of the tubers are clean.

You could potentially save yourself some time and money this way, although this approach has not been tested with many viruses and probably works only with those that are unusually vulnerable. Research has shown some success with the treatments listed in this table. The plant is able to survive infection with the virus and is either asymptomatic or has mild symptoms.

Virus replication may be limited compared to plants with no resistance or it may be normal. Because the virus survives, the plant is able to transmit the virus. The plant acts to limit the spread of the virus and generally remains asymptomatic with, at most, localized and short-lived areas of infection. Cells infected with the virus die, leading to obvious necrotic symptoms. The reaction may be localized, in which case the plant will survive, or it may be systemic, in which case the plant may not survive or may survive but fail to produce.

I have done a lot of testing of varieties that came from private collections though, so I probably have an uncommon perspective. The screening recommendations in these profiles are not intended for casual growers, but for people who are maintaining collections for the long term without access to replacement virus-free stock. Because of that, they are considerably more conservative than the standards used in most seed potato certification programs.

I have limited this list to viruses that have been observed to transmit naturally in potatoes. Plant viruses can often be inoculated into additional hosts that they do not have the opportunity to infect naturally. I assume that viruses that infect wild potatoes also can infect domesticated potatoes.

The number of known potato infecting viruses is likely to increase and probably by a lot. New viruses continue to be discovered, particularly with increasing use of high throughput sequencing. In fact, we are probably now entering the golden age of virus discovery.

A recent survey in Peru turned up a large number of new viruses in potato that have not yet been described. The more comprehensive and inexpensive the tools become and the more the search widens, the more viruses we are likely to uncover. If I have missed any viruses that infect potatoes naturally in the field, please let me know. Frequency in Potato USA is my assessment of the literature in combination with my experience.

Seed Transmitted is my assessment of whether the variety might be transmitted in true potato seeds. Color: Red indicates that a virus is serious, widespread, or both, and I recommend testing for it if you can. Yellow indicates that a virus is mostly a nuisance, although it may be widespread.

I recommend testing for these if you have seen them before, if you are in an area where they are more common, or if you are made of money. Green indicates that a virus is either not particularly damaging or too rare to test for unless you know that you have exposure. Gray indicates that the virus is not known to be present in the USA. Not a common virus in potatoes from what I can tell. It is one of those that turns up from time to time in well-traveled potatoes.

The primary symptom is known as calico and produces large blotches of yellow on the leaves, with clear boundaries between the yellowed and normal areas. In some potato varieties, there may also be tuber necrosis, beginning at the proximal end and spreading in storage.

Primarily spread by aphids and also by contact or mechanical means. It may also be asymptomatic. A small amount of transmission though seed has been observed in some potato relatives, so it seems likely that this virus infects TPS to some degree. Tubers are infected unreliably, so it may be possible to recover disease free tubers from infected plants.

Field tests are available. Not worth screening unless you know it is in your collection. You are at greater risk if you grow potatoes near alfalfa, since alfalfa tends to be heavily infected, even through seed. In one study Kaiser , tubers stored at This virus is found in wild and domesticated potatoes in the Andes and is present in some heirloom ulluco varieties, which is the main reason why I no longer grow them. In the Andes, it is primarily spread by contact and mechanical means, but is also reportedly spread by flea beetles.

Symptoms range from mild to severe mosaic. It can transmit through true seed, although reportedly at a low rate. It is also said to infect tubers unreliably, so it may be possible to recover clean plants from tubers of infected plants. This virus is a very good reason not to grow the old ullucos or to keep them absolutely isolated from potatoes if you insist on it.

No field tests. You should screen for this if you have potential contact between potatoes and heirloom ullucos. This virus was reclassified as a strain of Eggplant Mosaic Virus EMV and then returned to its own species, so you may need to search under both names for information.

A virus of limited distribution in the central Andes. This virus was once considered a strain of APLV , but has since been broken out into its own species. See APLV for more details. More info: EPPO. Probably not present in North America or at least very rare if so. Causes mild to severe mottle light and dark patches and deformation of the leaves. In some cases, this may lead to systemic necrosis. Spreads by plant to plant contact. Necrotic symptoms in tubers often increase after storage. Some varieties such as Russet Norkota and Shepody rarely show symptoms, but can carry the virus and serve as reservoirs for aphid transmission.

Yukon Gold is particularly susceptible to tuber necrosis. Since PVY is a nonpersistent virus and is transmitted quickly by aphids, the use of insecticides to control spread is generally not effective. The best strategy to control PVY is to use seed potatoes certified to have low virus content. Mineral oils can be used to reduce spread by aphids, but must be re-applied at frequent intervals to be effective.

Minimize mechanical spread by sanitizing equipment and reducing traffic through the field. Infected plants can serve as a source of inoculum for the rest of the field, so rogue pull out and dispose of symptomatic plants.

Solanaceous plants such as tomatoes, peppers, nightshade and ground cherry can harbor the virus and serve as a source of inoculum. Avoid planting potatoes next to weedy ditches and hedgerows, and practice good weed control within the field.

Potato leafroll virus PLRV is a phloem-limited Luteovirus which is transmitted by aphids in a persistent manner. In contrast to PVY and AMV, PLRV takes longer to be acquired minutes and transmitted 24 to 48 hours by aphids, since the virus needs to move into the gut, through the body and back out through the salivary system of the aphid.

Symptoms of PLRV include a characteristic upright character and rolling of the leaves, chlorosis yellowing or reddening, leaves with a leathery feel, phloem necrosis dead spots along the leaf veins , stunting reduced height of the plant, and net necrosis in tubers. The severity of net necrosis will vary depending on when the plant was infected, and may increase during storage. Some varieties are more susceptible than others, including Russet Burbank, one of the most commonly grown commercial potato varieties in the western US.

Symptoms of PLRV on foliage rolling of leaves, stunting of plants and tubers net necrosis. Colonizing aphids are the most important vectors for this virus because transmission requires an extended feeding period. The green peach aphid, Myzus persicae , is one of the most important vectors. The use of clean seed is critically important.

Roguing of infected plants helps prevent the spread of PLRV and early harvest can help prevent late-season infection. Handling plants will not spread the virus, since PLRV is not mechanically transmissible. It remained unknown until the 's because its symptoms are very inconspicuous.

Seed potatoes are not yet certified for PVS, which contributes to its widespread distribution. Most potato cultivars are symptomless. On some cultivars, if infected early in the season, will show a slight deepening of the veins, rough leaves, more open growth, mild mottling, bronzing, or tiny necrotic spots on the leaves.

PVS is a Carlavirus , and is nonpersistently transmitted by aphids, including Myzus persicae , the green peach aphid. It is also mechanically transmissible, and transmissible through tubers. Management PVS is very difficult to detect using visual cues. Insecticides are ineffective in controlling nonpersistently transmitted viruses.

Crop oils may be used early in the season. Plants tend to be resistant to infection by PVS later in the season. Prevent mechanical spread within the field by sanitizing tools and minimizing movement through the field. Rogue remove any symptomatic plants. Subscribe by Email Get the latest site news by email Please, insert a valid email.

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