When we hear the word ‘nematode,’ many of us conjure up a picture of a worm or some other snake-like organism. The term is derived from the Greek, nemato, which literally translates to ‘thread-like.’ These organisms are members of the animal kingdom, among other multicellular invertebrates (animals without a backbone). But nematodes—also known as roundworms—are quite unique, so much so that they have been assigned their own major grouping in the phylum Nematoda. Though Nematoda look similar to flatworms (e.g. tapeworms and flukes of the phylum Platyhelminthes) and earthworms (of the phylum Annelida), they are biologically distinct.
Nathan Cobb, widely recognized as the father of nematology in the United States, failed to conceal his wonder and respect for nematodes when writing about their near-universal occurrence:
Not the least interesting thing about nematodes is the astounding variety of their habitats. They occur in arid deserts and at the bottom of lakes and rivers, in the waters of hot springs and in polar seas where the temperature is constantly below the freezing point of pure water. They were thawed out alive from Antarctic ice by members of the Shackleton Expedition. They occur at enormous depths in alpine lakes and in the ocean. As parasites of fishes, they traverse the seas; as parasites of birds they float across continents and over high mountain ranges.
It’s clear that nematodes can be found just about anywhere. But, what are nematodes? This article is a brief introduction to nematodes, and touches upon their importance to man and to our world of agriculture.
The diversity of nematodes living on this planet is immense, and their population numbers even more so.
Nematodes are the most numerous metazoans (multicellular organisms) on the face of the earth. If all the soil and rock were removed from the planet Earth, with everything else left in place, there would still be a noticeable sphere created by the nematode populations left behind.
Cobb once estimated that the top 3 inches of an acre of soil could contain as many as 3 billion nematodes. The top three-quarters of an inch of an acre of beach sand could have as many as 1.5 billion of the same.
Populations of other soil-borne organisms pale in comparison to those of nematodes. Ecologist Peter Warshall observed that the top few centimeters of soil covering a square meter of land could harbor a total combined population of 200,000 ants, mites, pot worms, slugs, earthworms, springtails, and beetles. In that same soil sample, you would find 12 million nematodes.
Björn Sohlenius, a scientist who has been studying nematodes and soil ecology for the last 50 years, once estimated that temperate grassland could hold as many as 9 million nematodes per cubic meter of soil. And it is possible that of these 9 million, more than 25% are plant parasites. Which leads us to the immense impact that nematodes have on the ecology of our planet.
Parasitic species of nematodes afflict nearly every plant and animal species on Earth.
A single dried wheat gall can contain as many as 90,000 wheat gall nematodes (Anguina tritici). And such quantities can be found in animals, and even people. After being treated with an anthelmintic drug used to treat human nematode infestations, more than 5,500 pinworms were passed by 1 man. When heavily burdened, a small puppy can pass more than 1,000 hookworm (Toxacara canis). The Guinea worm, which can reach lengths of over 1 meter, has in rare cases been found in excess of 100 worms per human host. Man is subject to parasitism by more than 32 species of nematodes. Among these, the hookworm, giant intestinal worm, and the pinworm are all common to North America.
The diseases of domestic animals caused by nematodes have long presented significant challenges to ranchers and farmers. In 1950, the estimated annual costs associated with animal-parasitizing nematodes exceeded $500 million in the United States. Contemporary estimates are difficult to come by, but elsewhere it’s estimated that gastrointestinal nematodes cost Brazilian cattle operations $7 billion per year, with their Mexican counterparts experiencing half a billion dollars in losses annually. But these losses are dwarfed by what plant growers are now facing.
An estimate calculated in 1981, when many now-banned nematicides were still available, attributed more than $6 billion in crop damage to insects, and more than $4.5 billion in crop damage and loss to nematodes. Today, researchers at Bayer estimate that as many as 20% of some crops are lost due to nematodes. The cost of such losses in the U.S. alone reaches $100 billion.
Nematode infestations cause significant crop losses due to the vast number of diseases and disorders they inflict on plants.
A dramatic, historical case of nematode damage is illustrated by the loss of the black pepper industry in Banka, an island off the coast of Sumatra in Indonesia. In 1922, more than 22 million pepper trees were in production in Banka. In 1930, the burrowing nematode (Radopholus similis) was first detected on the island, when it was observed to cause yellowing of plants in isolated fields. By 1950, more than 90% of the island’s pepper trees were completely destroyed.
A century previous, in the early 19th century, many plantings of sugar beets in Europe were initiated for the commercial production of sugar. But in the absence of crop rotations, and with continuous monoculturing, by 1850 many fields were left unproductive, being referred to as “beet tired soil.” In 1859, the cause of the decline in beet production in Germany was shown to be associated with parasitization by the sugar beet cyst nematode (Heterodera schachtii). This was among the very first instances that the parasitic, crop-damaging nature of nematodes was identified.
Today, closer to home, the citrus nematode (Tylenchulus semipenetrans) can, perhaps, be found in more than 90% of California citrus groves. It is considered one of the most critical issues impacting the citrus industry, and the most important pest among those afflicting citrus groves. The citrus nematode causes a slow but chronic debilitation of the tree (“slow decline”), typified in later years with stunting, defoliation, loss of yields, degradation of the fruit, and eventually death of the entire tree.
Root-knot nematode (Meloidogyne spp.) damage to stone fruit trees can be partially overcome with the use of resistant rootstocks, such as Nemaguard and Mariana 2624, in peaches, almonds, plums and apricots. Both rootstocks have displayed immunity to two species of root-knot nematode, Meloidogyne incognita and M. javanica. But immunity or resistance to all nematodes is virtually impossible. Today, as for many years now, the ring nematode (Criconemella spp.), dagger nematode (Xiphinema spp.), lesion nematode (Pratylenchus spp.), pin nematode (Paratylenchus spp.), and the stubby root nematode (Trichodorus spp.) remain a major problem wherever stone fruit trees are planted on these rootstocks.
The association of nematode infestations with a major disease of stone fruit trees, bacterial canker, has been demonstrated and continues to be a major disease complex in orchards throughout the world. The sheath nematode (Hemicycliophora arenaria) went unnoticed as an endemic pest of native desert plants in the Coachella Valley for many years. However, the devastating potential of this pest came to surface when valley agriculturists began to convert this virgin ground to crop land.
The sheath nematode’s short life cycle, spanning about 16 days, was found to be partly responsible for the explosive infestation of the crops planted in the valley. On tomatoes, for example, 150 females produced a population of 1.5 million in a period of 3 months. This case illustrates the characteristic course of parasitic associations that has occurred with introduced crops. Native nematodes have developed alongside their hosts over millions of years in a balanced relationship, maintaining but rarely devastating the host plants they occupy.
When a commercially grown crop is placed into the environment, the parasitic nematodes, hardened over the ages through selective pressures, are capable of multiplying to epidemic proportions on the ‘soft’ new host. Nematodes have been adapting and developing their associations for over 600 million years, and commercial agriculture is, in comparison, but a few seconds old.
Growers have begun to respond, looking to native hosts resistant to or tolerant of nematode exposure for inspiration in developing nematode-resistant varieties of crops. These include resistant lines of soybeans resistant to soybean cyst nematode, and seemingly countless resistant lines of VFN tomatoes hardened to the ravages of root-knot nematode. But, we have seen the extreme adaptability of nematodes in action, with these resistant variety placing selective pressures on parasitic nematodes that induce them to find novel evolutionary strategies to overcome the resistant qualities of host plants.
For this and many other reasons, growers cannot simply rely on nematode-resistant crop varieties to remedy diseases caused by nematodes.
There are some salient points that should be covered with respect to genetic lines as the sole answer to nematode problems, and the misconception that nematodes can be dealt with easily.
- More than one species of plant-parasitic nematode is capable of parasitizing a given host.
- Most species of nematode feed on many host species, and are capable of sustaining or carrying over on a variety of weeds and other natural vegetation.
- Many countries are as yet unaware of nematode damage, or the extent thereof, with control methodology that is lacking or primitive in nature.
- The increased flow of commodities between countries places continued demands upon inspection, which is always limited in its detection of entering pathogens and pests.
- The economics of the ‘cost-price squeeze’ places demands on farmers for optimal use of farmland. This in turn places restrictions upon the use of cultural control measures, such as fallowing or alternating with less desirable non-host or tolerant crops.
- Environmental awareness has prompted extensive testing of the safety of agricultural chemicals. This has resulted in the banning of various fumigants and non-fumigants aimed at controlling nematodes.
- Within and even outside of the economic limits of use, it is virtually impossible to eradicate plant-parasitic nematodes.
- Use of nematode-resistant plants or root stocks can rarely encompass corresponding accommodations for other pathogens as well as for other, nontarget plant-parasitic nematodes.
- As noted above, the use of resistant hosts can apply enough selective pressure for development of virulent pathotypes and strains of nematodes which can overcome resistance, as seen with root-knot nematodes on VFN tomatoes.
- The use of non-fumigant nematicides such as the carbamates and organo-phosphates can lead to tolerant, resistant, and cross-resistant strains.
- Soil sampling methods which can optimize detection of a nematode problem are often beyond the limits of physical and economic feasibility. In many cases the best time for sampling has already passed, while another important factor is the detectable population level of nematodes. For instance, the presence of the golden nematode cannot be ascertained until the population density has exceeded 100 million nematodes per acre.
- Nematode-fungal and nematode-bacterial interactions can lead to pathological conditions of a magnitude greater than the additive effect of each organism alone (e.g. the root-knot nematode acting in concert with fusarium wilt complex).
- The adaptive survival features of some species allow them to survive desiccation. Nematodes such as the wheat gall nematode (Anguina tritici) or the stem and bulb nematode (Ditylenchus dipsaci) are capable of surviving for years in a dry or ‘anhydrobiotic’ state, and revive upon remoistening. The same is true for many of the cyst-fanning species. If the soil is allowed to dry slowly over a long period of time, various unsuspecting species have been observed to survive in a dry state (e.g. dagger nematode, ring nematode, pin nematode, and others).
- Major parasites of pine and palm trees, the pine tree nematode (Bursaphelenchus xylophilus) and the coconut palm nematode (Radinaphelenchus cocophilus), respectively, can be transported from tree to tree by beetle vectors. Many other species of plant-parasitic nematodes are disseminated across vast expanses by wind, moving water, birds, mammals, agricultural equipment, and various forms of transportation.
All the above is why Sunburst Plant Disease Clinic has long dedicated itself to developing nutrition-based remedial programs that return plants to a state of peak health. This allows them to dedicate resources to natural immune processes that can suppress nematode populations and tolerate the effects of residual parasite populations. If you believe that your crops have been impacted by disease resulting from nematodes or other maladies, contact us today to learn how Sunburst can help.