Plant-parasitic nematodes (PPNs) come in many different shapes; most are slender and threadlike, but some are lemon-shaped, round, or appear swollen. Less than 1 mm in length, PPNs are almost invisible to the naked eye. All share a common feature, a sharp needle-like structure in their mouth called a ‘stylet’. The stylet acts like a small hypodermic needle which the nematode uses to pierce plant cells and suck out cell contents. Most live below the soil surface and have evolved many strategies to survive fluctuating soil temperature and moisture conditions, or to escape predation by organisms such as fungi, bacteria, and even predatory nematodes. Some can survive the onset of extreme heat or cold or drying of soil by entering a state of suspended activity called cryptobiosis. This ability to survive in an almost inanimate state is one reason some nematode species can survive in soil for many years without their host plants. Some plant parasitic nematodes also evade hostile environments and predators by living most of their lives inside plant roots and have only short periods of time in the soil. By using multiple survival strategies, nematodes withstand seemingly extreme environments.
Because nematodes are so adept at surviving adverse conditions, they can be dispersed long distances on infested soil or plant material. Any process that moves soil or plants is a pathway for the introduction of PPNs; inadvertent movement in trade of infested agricultural products such as bulbs, nursery stock, tubers, or on contaminated equipment. Once introduced, damage from PPNs often goes unnoticed and is easily attributed to other factors such as nutritional deficiency. The nematode problem may only become evident once the numbers increase to such an extent to have an economic impact. Remaining undetected, the nematode has had the opportunity to be spread throughout a region.
As more is known about PPNs and their damaging impact on crops, governments have increasingly placed nematodes on plant health legislation to prevent their introduction and spread. In the U.S., phytosanitary action specifically addressing PPNs was first implemented in 1909 against root knot nematode infested cherry trees from Japan. Soon after, in 1912, the US Congress approved the first Plant Quarantine Act to minimize the threat posed from this and other pathogens. A few years later, the damaging impact of the potato cyst nematode, Globodera rostochiensis or the golden nematode (GN) was recognized. The Golden Nematode Act was passed in 1948 by the US Congress to prevent the spread and protect the US potato industry from this damaging nematode. Europe’s first phytosanitary legislation, the Beet Eelworm Order, was adopted in 1943 to manage spread and levels of infestations of the sugar beet cyst nematode in Britain.
In today’s globalized world, intensified international trade has increased the risk of an introduction of noxious pests, including nematodes. Phytosanitary measures for the exclusion, suppression, containment, or eradication of plant pests have been developed throughout the world. Many of these measures are designed to minimize the transport and worldwide spread of pests or pathogens.
Legislation in most countries is based on international treaties and conventions. The International Plant Protection Convention (IPPC) is an international treaty established in 1952 that aims to secure coordinated, effective action to prevent and to control the introduction and spread of pests of plants and plant products. The IPPC is administered by the Food and Agriculture Organization (FAO) of the United Nations and is recognized by the World Trade Organization under the Agreement on the Application of Sanitary and Phytosanitary Measures (WTO-SPS Agreement). Currently, 183 signatories, including the United States, adhere to the Convention, and contracting parties to the IPPC must adhere to the agreements of this treaty. The Commission on Phytosanitary Measures (CPM) is the governing body of the IPPC and has developed the International Standards for Phytosanitary Measures (ISPMs). The WTO-SPS agreement, an outcome of the General Agreement on Tariffs and Trade (GATT), recognizes the right of WTO members to adopt scientific and valid measures to protect food safety and animal and plant health while making sure these measures are not applied in an unnecessary manner. Phytosanitary measures for the purpose of preventing spread or introduction of plant pests can only be applied to regulated pests. A ‘Quarantine pest’ as defined by ISPM-5, Glossary of phytosanitary terms , is ‘a pest of potential economic importance to the area endangered thereby and not yet present there, or present but not widely distributed and being officially controlled’.
National legislation and regulations provide the framework for protecting agriculture from invasive pests. Some of the regulations or measures that reduce pest prevalence in the exporting country include treatment of consignments, importing dormant plants, restricting import to certain seasons, and prohibiting the import of an affected commodity are implemented to prevent introduction of pests through trade. IPPC recognizes the right of countries to regulate the import of certain plant species to avoid entry of a pest, to inspect or quarantine specific consignments, and to define which pest species are not allowed entry. According to this agreement, countries have the right to decide their own level of acceptable risk, and to apply phytosanitary measures as required to protect plant health and safeguard trade as long as these measures do not discriminate against certain countries or commodities. Phytosanitary measures need to be science based or based on international standards such as the ISPM set by the IPPC.
Regional Plant Protection Organizations (RPPOs) were created by the IPPC and allow for regional collaboration and coordination of phytosanitary issues and standards for respective regions. Ten RPPOs are currently recognized by the IPPC. The US is a member country of the North American Plant Protection Organization along with Canada and Mexico. Individual countries have their own National Plant Protection Organization (NPPO). In the US, the NPPO responsible for operating the regulatory framework for phytosanitary measures based on international standards is the United States Department of Agriculture’s Animal Plant Health Inspection Service (USDA APHIS).
Potential quarantine pests that meet the above criteria are evaluated through a pest risk analysis. Nematode pests of potato are among some of the most highly regulated pests in the world due to their impact on trade and production systems and because they are readily transported and spread in infested tubers or in soil clinging to potato tubers. For the US, examples of regulated nematode pests include the potato cyst nematodes, G. rostochiensis, which is regulated by 119 countries worldwide, and Globodera pallida, which is regulated by 80 countries worldwide. Both species originated in South America. Introduction of G. rostochiensis into the US is believed to be from Europe on military equipment contaminated with infested soil used during the Great War, World War I. While the 2006 G. pallida introduction in Idaho remains of uncertain origin.
Canada and the United States have developed a bilateral set of guidelines for phytosanitary actions for potato cyst nematodes. Potato cyst nematodes are not widely distributed in either Canada or the U.S. and are under official control in both countries. PCN species survive in soil for decades as eggs contained in protective cysts which are difficult to detect. Potato cyst nematodes are difficult to control, and require the use of phytosanitary measures including surveillance and sanitation to minimize the risk of spread. Although resistance to some races of G. rostochiensis is available in commercial potato varieties suitable for the US and Canada, full resistance to G. pallida in commercially relevant russet-type potatoes grown in the Northwest is not available.
In the US, stringent adherence to phytosanitary programs has contained G. rostochiensis to eight counties in New York, fewer than 6,000 acres, despite its documented presence since 1941. The infestation of Globodera pallida, first found only in Idaho in 2006, continues to be contained to approximately 3,000 acres. Efforts in Idaho by both USDA APHIS and the Idaho State Department of Agriculture have concentrated on containment and eradication of G. pallida. Trade of potato from the US and from Idaho, originally interrupted by the detection of G. pallida, has resumed with Canada, Mexico, South Korea, and Japan.
Nematodes are excellent at invading new territory and becoming established before they can be detected. Strategies aimed at preventing nematode establishment are early detection, exclusion, quarantines, containment, and eradication. These strategies are intended to decrease the probability of an introduction, prevent spread, and minimize the risk of damage to economically valuable crops. Phytosanitary programs are essential for maintaining plant health in any country or region, especially when applied to nematode infestations. The challenge continues to be understanding nematode biology so that they can be detected, identified, and intercepted to prevent their introduction and minimize damage.
