This article appears in the February 2017 issue of Potato Grower.
Nematodes are the most numerically abundant animals on earth. It is estimated that four out of every five animals is a nematode and that densities can exceed 1 million individuals per square meter. Nematodes, also called roundworms, can be free-living in soil or water, or parasites of animals or plants. Nematodes are found in an astonishing range of habitats, from the poles to the tropics, on mountains and in deserts, in lakes, rivers and in the ocean.
In 1914, N.A. Cobb, the father of nematology in the United States, said, “If all the matter in the universe except the nematodes were swept away, our world would still be dimly recognizable, and if, as disembodied spirits, we could then investigate it, we should find its mountains, hills, vales, rivers, lakes, and oceans represented by a film of nematodes.”
Some nematodes feed only on plants and are referred to as plant parasitic nematodes. They may not appear as impressive as human parasites that cause river blindness, or filariasis in humans, but plant parasitic nematodes are responsible for an approximate 15 percent of crop loss per year worldwide, equating to some $78 billion.
Plant-parasitic nematodes can come in many different shapes; most are slender and threadlike, but some are lemon-shaped, round or appear swollen. And, being less than 1 millimeter long, nematodes are almost invisible to the naked eye.
They all share some common features. Plant-feeding nematodes have a sharp, needle-like structure in their mouth called a stylet, which acts like a small hypodermic needle the nematode uses to invade plant cells and suck out the cell contents. Most nematodes are subterranean—living below the soil surface—and have adapted to survive in potentially hostile soil environments. Nematodes have evolved many strategies to survive fluctuating soil temperature and moisture conditions, and to escape predation by other soil organisms such as fungi, bacteria or even voracious predatory nematodes. Some are able to survive the onset of extreme heat or cold, or drying of soil by entering a state of suspended metabolic 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 and resume activity only once favorable conditions return. 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.
Nematodes are aquatic, but can swim only short distances to reach their hosts. They use the moisture in spaces between soil particles to move through the water. Nematodes can be dispersed over long distances on nematode-infested soil clinging to farm equipment, muddy shoes or in plant material such as tubers, bulbs or roots.
Nematodes can easily be transported across borders in infested plant material or in soil. Essentially, any process that moves soil or plants is a pathway for introduction of these potentially invasive species. Invasion of agricultural lands by nematodes has come about though inadvertent movement in trade of infested agricultural products such as bulbs, nursery stock, seeds, tubers, or on contaminated equipment. Once introduced, apparent damage from nematodes has often gone unnoticed and has been attributed to other factors such as nutritional deficiency. As more has become known about nematodes and their damaging impact on crops, governments have increasingly placed nematodes in plant health legislation to prevent their introduction or spread.
In the U.S., phytosanitary action specifically addressing plant parasitic nematodes was first implemented in 1909 against root knot nematode-infested cherry trees from Japan. Soon after, in 1912, U.S. Congress approved the first Plant Quarantine Act to minimize the threat posed from that nematode. A number of years later, the damaging impact of the potato cyst nematode—Globodera rostochiensis or the golden nematode—was recognized. The Golden Nematode Act was passed by Congress in 1948 to protect the U.S. potato industry from the spread of 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 sugarbeet 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 to prevent risk of an entry from potentially damaging pests. Many of these measures are designed to minimize the transport and worldwide spread of pests or pathogens that may need to be contained or excluded. Legislation in most countries is based on international treaties and conventions.
The International Plant Protection Convention (IPPC) is an international treaty 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). The Commission on Phytosanitary Measures is the governing body of the IPPC and has developed the International Standard for Phytosanitary Measures No. 5, called the Glossary, which was adopted in 1997 by the FAO. The WTO-SPS agreement, an outcome of the General Agreement on Tariffs and Trade, states that the purpose of an SPS is limited to the protection of human, animal or plant life or health; it applies to all sanitary and phytosanitary measures that can affect international trade. Phytosanitary measures for the purpose of preventing spread or introduction of plant pests can only be applied to regulated pests. “Quarantine pest” as defined by the Glossary 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 plant resources from invasive pests. Implementing regulations or measures that ensure low pest prevalence in the exporting country, treatment of consignments, importing dormant plants and restricting import to certain seasons, or prohibiting the import of affected commodities are some of the measures taken to prevent or mitigate introduction of pests through trade. The 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 the agreement, countries have the right to decide their own level of acceptable risk and to apply phytosanitary measures as required to protect plant health as long as such measures do not discriminate against certain countries or commodities and have as little impact on trade as possible.
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. Nine RPPOs are currently recognized by the IPPC. The U.S. is a member country of the North American Plant Protection Organization along with Canada and Mexico. Individual countries have their own National Plant Protection Organizations. In the U.S., the organization responsible for operating the regulatory framework for phytosanitary measures based on international standards is the USDA’s Animal Plant Health Inspection Service (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 tubers. Phytosanitary measures work best for nematode species that have a narrow host range and a slow rate of reproduction. In the U.S., 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 U.S. is believed to be from Europe on military equipment contaminated with infested soil used during World War I. The G. pallida infestation discovered in Idaho in 2006 remains of uncertain origin.
Canada and the U.S. have developed a bilateral set of guidelines for phytosanitary actions against 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. Scientifically recognized as long-lived and capable of being highly destructive, both species can survive in soil for decades as diffincult-to-detect eggs contained in a protective cyst. Potato cyst nematodes are difficult to control, necessitating the use of phytosanitary measures including surveillance and sanitation to minimize the risk of spread. Although host plant resistance to some races of G. rostochiensis is commercially available for the northeastern U.S. and Canada, full resistance to G. pallida in commercially relevant russet potatoes grown in the Northwest is not currently available.
Stringent adherence to phytosanitary programs have contained G. rostochiensis to less than 6,000 acres in eight counties in New York, despite its documented presence since 1941. The infestation of G. pallida, first found only in Idaho in 2006, continues to be contained to fewer than 3,000 acres—less than 1 percent of Idaho’s total potato acreage. Efforts in Idaho by both APHIS and the Idaho State Department of Agriculture have concentrated on containment, sanitation and eradication of G. pallida. Of the nine original fields infested with G. pallida, eight no longer have active infestations. Trade of potato from Idaho—originally interrupted by the detection of G. pallida—has resumed with Canada, Mexico and South Korea; negotiations to resume export of Idaho potatoes to Japan are ongoing.
Nematodes are excellent at invading new territory and becoming established before they can be detected. They almost always have help from humans in their invasion. Also, nematodes are impossible to detect without collecting soil. One cannot look at a field and say whether specific plant parasitic nematodes are present without taking soil samples and extracting the nematodes from the soil. An acre of soil contains about 267,000 gallons of soil down to 10 inches.
The sampling system currently in use by the USDA takes about 1.5 gallons per acre and is the most stringent potato cyst nematode sampling protocol in the world. When nematode infestations are very small, surveys (even high-intensity surveys) may not detect the infestation. Globodera populations can increase in number very quickly under the right conditions, usually increasing a minimum of 20 to 50 times over a single cropping season. With the current USDA survey method, very small infestations can be detected after only two or three host crops, well before populations are likely to spread to additional fields.
Intervention strategies aimed at preventing potato cyst nematodes’ establishment through early detection, exclusion, quarantines, containment and eradication 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 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 their damage.