What Is Potato Ringspot: Recognizing Corky Ringspot In Potatoes

By: Liz Baessler

Corky ringspot is a problem affecting potatoes that can lead to real trouble, especially if you’re growing them commercially. While it may not kill the plant, it gives the potatoes themselves an unpleasant look that’s hard to sell and less than ideal to eat. Keep reading to learn more about recognizing and managing corky ringspot in potatoes.

Symptoms of Corky Ringspot in Potatoes

What is potato ringspot? Corky ringspot of potatoes is caused by a disease called tobacco rattle virus. This virus is spread primarily by stubby root nematodes, microscopic worms that feed on plant roots. These nematodes will feed on infected roots, then move on to the roots of uninfected plants, spreading the virus underground without your knowledge.

Even once a potato is infected with corky ringspot, you may not realize it, as the symptoms are almost always underground. Occasionally, the leaves of the plant will appear smaller, puckered, and mottled. Usually, however, symptoms are only inside the potato, manifesting as darkly colored, cork-like textured rings, curves, and spots inside the flesh of the tuber.

In tubers with thin or light skin, these dark areas can be seen on the surface. In severe cases, the shape of the tuber can become deformed.

How to Manage Potatoes with Corky Ringspot Virus

Unfortunately, there is no way to treat corky ringspot of potatoes, not least of all because you often don’t know you have it until you harvest and cut into your tubers.

Prevention is key with corky ringspot. Only buy seed potatoes that are certified to be free of the virus, and don’t plant in soil that has already shown to contain the virus. When cutting potatoes for seed, sterilize your knife frequently, even if you don’t see any symptoms. Cutting into infected tubers is a common way for the virus to spread.

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Plant parasitic nematodes are microscopic roundworms that feed on plant roots. They live in soil and plant tissues, and more than one species may occur in a field. They have a wide host range, and vary in their environmental requirements, the symptoms they cause, and the risk to potatoes.

Overwintering root-knot nematode juveniles invade roots and later tubers, establish feeding sites, and develop into the adult stage. Adult females are swollen, sedentary, and lay eggs in a gelatinous matrix on or just below the root surface. Second-stage juveniles (J2) hatch from these eggs and move through the soil to invade other roots and tubers. When no crop is present, the nematodes survive as eggs or J2, which can be found in the soil. Root-knot nematode feeding reduces the vigor of plants and causes blemishes on tubers. The latter can lead to a severe reduction in tuber quality and, as a result, affected potatoes become unmarketable. Lesion nematodes damage roots by feeding and moving through cortical tissues. In addition, Pratylenchus penetrans increases the susceptibility of potato plants to Verticillium wilt and blemishes tubers. Pratylenchus neglectus is common in potato fields but has not been shown to damage potatoes in California. Stubby root nematodes feed on root surfaces, and can result in formation of numerous stubby roots. Yield loss has not been reported. The major problem caused by this nematode species is transmission of tobacco rattle virus, which causes corky ringspot disease on developing tubers.

Potato (Solanum tuberosum)-Corky Ringspot

Cause Tobacco rattle virus (TRV) is transmitted by the soilborne nematodes, Trichodorus spp. and Paratrichodorus spp. The presence of TRV, coincident with its nematode vector, is widely scattered in the Pacific Northwest and the resulting disease can cause extensive damage. Corky ringspot has been more severe when potatoes follow mint. All currently grown varieties can be infected by TRV but there is variation in the incidence and severity of tuber necrosis among varieties infected with TRV.

Symptoms There usually is no apparent foliage symptom under field conditions. Internal symptoms are generally necrotic arcs or brown spots but larger necrotic areas can sometimes be seen. Tuber symptoms caused by TRV can be confused with symptoms caused by Potato Mop-Top virus and Potato virus Y . Arcs on the surface of tubers can be sometimes seen in smooth skin varieties.

  • Do not plant potatoes in fields with a known corky ringspot history without proper chemical control of nematode vectors. Fields with unknown history should be sampled for the presence of the nematode (stubby root) and then assayed for the virus. Failure to adequately control this disease can result in complete loss.
  • Control nightshade, pigweed, and purslane while rotating out of potatoes to reduce the virus reservoir.

Chemical control Control of the nematode vector is mandatory if the field has a history of the disease.

  • Use Telone II as a preplant fumigant at recommended rates. Restricted-use pesticide.
  • Vydate C-LV at recommended rates at planting and on recommended intervals during the crop season will suppress nematode populations. Restricted-use pesticide.

Potatoes With Corky Ringspot - How To Manage Ringspot Of Potatoes - garden

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Temperate Tree Fruit and Vegetable Research: Wapato, WA


At this Location

Title: Stubby root nematodes and associated corky ringspot disease

FU, ZHEN - University Of Washington
SATHUVALLI, SAGAR - Oregon State University
Swisher Grimm, Kylie
CHENG, ZHIQIANG - University Of Hawaii

Interpretive Summary: Corky ringspot disease of potato is an economically devastating disease in the United States caused by the nematode-transmitted virus, tobacco rattle virus. In collaboration with researchers at Washington State University, Oregon State University, University of Hawaii and AgNema, scientists at the USDA-ARS Temperate Tree Fruit and Vegetable Research Unit in Prosser, WA, and the USDA-ARS Horticultural Crops Research Unit in Corvallis, OR, generated a review of the current knowledge of corky ringspot disease in the Pacific Northwestern United States. Here, our latest understanding of the stubby root nematode vector, the tobacco rattle virus itself, and chemical and cultural management practices are explored. This summation of corky ringspot disease provides an informative newsletter to be disseminated to growers in the Pacific Northwest.

Technical Abstract: Stubby root nematodes (SRN) damage plant roots, and more devastatingly, they vector tobacco rattle virus (TRV), the causal agent of corky ringspot disease on potatoes. Corky ringspot is a major factor in the devaluation and culling of potato tubers in the Northwest USA. A recent field survey showed that SRN were third in prevalence of plant-parasitic nematodes following root lesion and root-knot nematodes. There is little current data on the prevalence of TRV, as the last field survey was conducted over two decades ago. Past research has shed light on the distribution and ecology of the nematode, plant-nematode interactions, and chemical and cultural control methods of corky ringspot. However, more research is warranted to obtain an updated status of TRV in the Northwest and to determine whether populations of the virus and nematode are genetically different throughout the Northwest potato growing region. Here we briefly overview the basic biology of the nematode, associated TRV, and primary management strategies of corky ringspot in the Northwest.


Meloidogyne chitwoodi, Columbia root-knot nematode, has caused significant crop damage in high organic matter soils at low densities in Modoc and Siskiyou counties. In fields where this root-knot nematode is a problem, long-term integrated management tactics such as crop rotation, cultural controls, fumigation, and nematicides are necessary to prevent a substantial devaluation of the crop due to nematode-induced tuber blemishes.

Fumigants (metam sodium and 1,3-dichloropropene) and non-fumigant nematicides (ethoprop and oxamyl) are chemical control options for minimizing Columbia root-knot nematode crop damage in soils with low organic matter content. However, in silty clay loam soil with high organic matter when Columbia root-knot numbers exceed 500 second stage juveniles (J2) per 1000cc of soil, neither 1,3-dichloropropene nor metam sodium provide acceptable levels of control.

The following management tactics for Columbia root-knot nematode may increase efficacy in high organic matter soils):

  • A combination of fumigation (1,3-dichloropropene) and application of non-fumigant nematicide (oxamyl) can be effective. However, using this combination has not been tested in Californian in high organic matter soils.
  • Metam sodium broadcasted and immediately incorporated using a rotary tiller. (Based on one year of research.)
  • Multiple depth injection of 1, 3-dichloropropene or metam sodium

For other nematode species there are no precise guidelines for economic threshold levels under California cropping conditions. Fields infested with stubby root nematodes and tobacco rattle virus have been found in Monterey and Kern counties and in the Santa Maria area of Santa Barbara County. Fields with known virus infestations should not be planted to potatoes without an effective control strategy in place for stubby root nematode, which can transmit the virus that causes corky ringspot in potato.


The following measures will help prevent spread of nematodes to uninfested fields: (1) using certified planting material (2) cleaning soil from equipment before moving between fields (3) keeping irrigation water in a holding pond so that any nematodes present can settle out and pumping water from near the surface of the pond (4) preventing/ reducing animal movement from infested to uninfested fields and (5) composting manure to kill any nematodes that might be present before applying it to fields.

Crop Rotation

Crop rotation can be useful in reducing nematode numbers except of stubby root nematode and the tobacco rattle virus which have wide host range and so crop rotation is not practical. Alfalfa is not a host of M. chitwoodi, Race 1 cereals are nonhosts of M. hapla and there are several nematode resistant tomato varieties that can be used if M. incognita is a problem. Research in the Pacific Northwest has shown that cover crops of rapeseed, mustard, oilseed radish, or sudangrass reduce numbers of root-knot nematodes when incorporated as green manure. At present there are no nematode resistant potato varieties available. For managing Columbia root-knot nematode, consult the UC Nematology website.

Cultural Control

Fields that are left fallow but kept weed-free usually have an 80 to 90% per year reduction in root-knot nematode populations. Infested tubers left in the field after harvest can be a source of inoculum. Destroy potato plants that subsequently emerge from these tubers to restrict nematode reproduction. Avoid storage of tubers infected by M. chitwoodi as blemishes can increase during storage.

Common name Amount per acre REI‡ PHI‡
(Example trade name) (hours) (days)
Pesticide precautions Protect water Calculate VOCs Protect bees
Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide's properties and application timing, honey bees, and environmental impact. Always read the label of the product being used.
(Vapam HL) 37.5–75 gal See label NA
(Sectagon 42) 30–75 gal See label NA
COMMENTS: Contact your farm advisor for advice on the most effective application method for a particular situation. Fumigants such as metam sodium and 1, 3-dichloropropene are a prime source of volatile organic compounds (VOCs), which are a major air quality issue.
(Mocap EC) Maximum label rate See label NA
COMMENTS: Contact your farm advisor for information on maximizing the effectiveness of this material.
(Telone II) Label rates See label NA
COMMENTS: Helps to control lesion nematode, which is associated with Verticillium wilt and stubby root nematode, which is associated with corky ringspot. Fumigants such as metam sodium and 1, 3-dichloropropene are a prime source of volatile organic compounds (VOCs), which are a major air quality issue.
(Vydate C-LV) Label rates 48 7
Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.
* Permit required from county agricultural commissioner for purchase or use.
NA Not applicable
§ Do not exceed the maximum rates allowed under the California Code of Regulations Restricted Materials Use Requirements, which may be lower than maximum label rates.

UC IPM Pest Management Guidelines: Potato
UC ANR Publication 3463

Watch the video: How to Choose the Best Potatoes to Grow in Your Garden

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