Nematodes are microscopic soil-dwelling pests. There are several plant-parasitic nematodes of concern for potato growers as they can majorly impact yield. Nematodes attack the roots. Generally, symptoms of nematode infestation appear like water or nutrient stress with yellowing foliage, wilt and stunting.
Plants afflicted by nematode damage may be more susceptible to root rot pathogens. Growers will typically see symptom hot spots where the nematode infestation is concentrated. Nematodes move with the soil, which means they can easily be spread field to field on contaminated equipment.
Root-Knot Nematodes (Meloidogyne spp.). Root-knot nematodes are found in all potato growing regions of the world. In the U.S., the Northern root-knot nematode (Meloidogyne hapla) is widespread across cooler climates. The Columbia root-knot nematode (Meloidogyne chitwoodi) has more restricted distribution and is only found in the Northwest region of the U.S. Both species can infect potato tubers and causes a bumpy or rough appearance, and dark spots just under the surface of the skin. These internal and external blemishes diminish the potato market value (Fig. 1). The Columbia root-knot nematode is a quarantine nematode in certain export markets, for instance with Mexico.
Potato Cyst Nematodes (Globodera spp.). The potato cyst nematodes (PCN), Globodera pallida (Fig. 2) and Globodera rostochiensis (Fig. 3), found in Idaho and New York, respectively, are regulated quarantine pests in the United States. Potato cyst nematodes originated in the Andes with their primary host, potato, and are now found worldwide in most potato growing regions. PCN is difficult to control because eggs can remain viable in cysts for decades and only hatch in the presence of signals from a host or closely related plant. Each cyst contains hundreds of eggs, which can lead to exponential population increase when a susceptible potato variety is planted. PCN can cause up to 80 percent yield loss when infestation levels are high. The infestation of PCN in the U.S. has been contained by USDA-APHIS and respective state departments of agriculture. The New York infestation with the golden nematode (G. rostochiensis) is controlled through a strict rotation with resistant varieties. For the pale cyst nematode (G. pallida), lack of resistant varieties precludes management with rotation and an eradication effort is in place through the use of soil fumigants.
Root-Lesion Nematodes (Pratylenchus spp.). Many species of root-lesion nematodes are capable of infecting potato. Pratylenchus penetrans, which is associated with potato early dying, is one of the most important lesion nematodes. Root-lesion nematodes can greatly reduce yield and tuber size. Because of the wide host range of root-lesion nematodes, they are very difficult to control with crop rotation.
Stubby-Root Nematodes (Paratrichodorus & Trichodorus spp.). Stubby-root nematodes are not a threat to potato production by themselves; however, they can vector Tobacco rattle virus (TRV) which causes corky ringspot. Corky ringspot can cause up to 50 percent yield loss and cause necrotic rings deep within the tuber flesh that break through the surface creating a corky appearance. Stubby-root nematodes are found throughout U.S. potato growing regions, with P. allius being the most prevalent.
General Management Strategies
Plant-parasitic nematodes are difficult to control because of their diversity in biological traits (host range, feeding habit, interaction with other organisms) and because they are soil dwelling animals. A singular management strategy is unlikely to control all nematodes found in a potato field equally; therefore, an integrated nematode management strategy must be employed to keep production-limiting nematodes in check. Components of this strategy include fumigants/nematicides, resistance, crop rotation, and decision support systems.
The use of fumigants and nematicides are central to the management of plant-parasitic nematodes. This is especially true for M. chitwoodi and stubby-root nematode because of the low damage tolerance to tubers, and thus markets. Pre-plant fumigants such as 1,3-dichloropropene and metam sodium are broad spectrum and have the added benefit of eliminating many other soilborne pests. Nematicides are chemicals that directly kill the nematode and are most effective if used jointly with other control strategies.
Plant resistance should be the first line of defense to combat plant-parasitic nematodes. However, with the exception of the golden nematode, resistance to nematodes in commercially acceptable potato varieties for the U.S. market is not available. There are efforts underway to develop resistant potato varieties to M. chitwoodi, pale cyst nematode and stubby-root nematodes. Breeders have identified resistance in several wild Solanum species that are currently being introgressed into cultivated varieties.
Crop rotation should always be part of an integrated nematode management plan regardless of the other strategies available. Rotation does not only reduce nematode population levels in soil, but is also useful in reducing the impact of other soilborne pathogens. Because many plants are hosts for nematode, it is essential that the target nematode is known so as not to select a rotation crop that will make the problem worse. Weed control is also essential for rotation to be successful because many weeds are hosts for plant-parasitic nematodes.
Decisions on how to best manage plant-parasitic nematodes in potato are only as effective as the information available to growers. Many pieces of information are essential to management practices. Knowledge of the type of nematodes present in a field and the number in soil will help to establish the best strategy to use. While well-defined action thresholds for nematodes are not available, efforts are underway to assess the impact of nematodes on potato productivity through historical records and field research for the development of predictive tools to guide management decisions.
Nematode control begins with monitoring hot spots of symptomatic plants. If an infestation is suspected, such as patchy distributions of stunted and yellowing plants, soil samples should be sent to diagnostic labs for nematode identification. Growers should always avoid introducing pathogens by planting certified seed. Resistant varieties specific to the infesting species can be used if available. Soil movement from infested fields should be minimized by cleaning equipment.
Help protect against nematode damage by maintaining good crop health with best irrigation and fertilization practices. Controlling weeds which can serve as alternate hosts to the nematode is important. Crop rotation with a nonhost or trap crop may be helpful if the species infesting the field is known. Soil fumigation and nematicides are generally effective controls for all nematodes. Nematistatic chemicals temporarily paralyze the nematode which may allow sufficient time for the crop time to grow to become less susceptible. Incorporation of green manures, especially mustards that have biofumigant effects, is also helpful in controlling nematodes.
Recently, a group of researchers led by Dr. Louise-Marie Dandurand at the University of Idaho, received a $6.8 million supported by the USDA National Institute of Food and Agriculture award number 2022-51181-38450 to mitigate the impact of nematodes on potato production. The project, dubbed ‘PAPAS’ for Potatoes and Pests: Actionable Science Against Nematodes, focuses on development of molecular diagnostic methods, improving potato varieties to have resistance against potato cyst and root knot nematodes, and discovery of novel nematicidal compounds.
Learn more about this project at www.potatonematodes.com.