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Nematodes: Science Fiction (and Scientific Fact) Under Our Feet

Alaska. Space. The deep oceans. All of these have been subject to the cliche, "The Final Frontier". We'd like to nominate a more appropriate candidate: The soil beneath our feet. 

As the scientific community learns more about the vast microscopic life within our soil and how it affects our garden, it's discovering fungi, bacteria, and creatures whose behaviors and appearances rival those of aliens in science fiction movies. 

Nematodes are no exception. 

These non-segmented worms inhabit a multitude of environments from volcanic undersea vents to polar ice sheets, and range in size from 0.3mm to nearly nine yards, with the latter having been discovered inside the placenta of a sperm whale. 

Fortunately for gardeners everywhere, most soil nematodes are usually only one millimeter long, so there's no need to worry that the Kevin Bacon film Tremors is actually a documentary...or that you have to walk without rhythm so as not to attract the worms that instilled fear in generations of Dune fans. 

While there are about 25,000 identified nematodes on the third planet from the sun, we'll focus on those that inhabit the soil in our very own backyards. 

Nematodes Up Close

Nematodes are a type of roundworm classified as the phylum Nemata, and those that affect our gardens are grouped into five main categories. 

  • Bacterial-feeders
  • Fungal-feeders
  • Predatory nematodes
  • Omnivores
  • Herbivores
  • Parasitic nematodes

The Greek word for thread is nema, and indeed, nematodes are threadlike in appearance. They lack a visible head, though their mouths take on various shapes according to the groupings listed above. 

Nematodes are thought to have evolved into a simple body design, rather than maintain a primitive physiology, which includes an outer cuticle and a long gut. "A tube within a tube" is a common description used when microbiologists are explaining nematodes to students and cocktail party guests, so as not to cause glazed-over eyes and the sudden onset of narcolepsy. Nematodes have no circulatory system, absorbing food directly through the gut wall into fluids, nerves, and muscle between the simple digestive tract and the outer cuticle. This cuticle provides structural support and sheds as the worms grow.

Many nematodes are able to suspend their biological systems in a state called cryptobiosis in extreme environmental temperatures or drought, a much deeper type of stasis than that experienced by beat rs and other creatures that hibernate...including bored and befuddled laypersons.

Some nematode biologists assign the term "predatory" and "parasitic" to one or more of categories; for example, predatory nematodes feed on bacteria, and nematodes that feed on harmful insects, or those that eat living bacteria, are technically predatory. Confusing? Biologists would agree. While they've identified specialists within the phylum, many nematodes are difficult to "put in a box", since nematode types are often difficult to differentiate, and more and more of these tiny worms are discovered each year. 

Some biologists just throw up their hands and place many nematodes in the "Other" category. According to a University of Florida information page, "It can be very difficult to distinguish whether a nematode is feeding on dead cells from a plant root or on fungi growing on the cell surface. Sometimes a nematode showing this feeding behavior may be classified simply as a root or plant associate." The microscopic size of nematodes doesn't help. 

But we'll give it a shot, based on currently available information. 

Bacterial-feeders have proven to add plant biomass and, through metabolism of bacteria, increases nitrogen in the soil (https://www.sciencedirect.com/science/article/pii/S003807170300333X). Of all the types of nematodes, some bacterial-feeders have the most unusual mouth structure, somewhat similar to that of the star-nosed mole. 

Fungal-feeding nematodes have short, tusk-like mouth appendages, called stylets, designed to pierce the fungal cell's outer membrane. The nematodes then feed upon the fungal cell fluid, and excess nitrogen is released in the nematode's waste. This process makes nitrogen and other nutrients more accessible to plants. 

Predatory nematodes eat other nematodes and protozoa. According to Oregon State University's Elaine Ingham, "They eat smaller organisms whole, or attach themselves to the cuticle of larger nematodes, scraping away until the prey's internal body parts can be extracted." 

Once again, gardeners should count their blessings that most garden nematodes require a microscope to properly view them, with no chance that they'll be discovered gnawing through their daisy-patterned rubber boots.

Some predatory nematodes are thought to eat insects in their egg or larval stage, while others live above ground to prey on adult insects known to harm crops. 

Omnivore nematodes eat both bacteria and fungi. 

Herbivore nematodes attack plant roots with their needle-like stylets. Some (ectoparasites) pierce the root wall to feed off the root's interior, while others (endoparasites), including the Root-knot nematode, actually enter the roots and cause tiny burls in the root structure. 

All herbivore nematodes can transmit disease to plants and prevent plants from absorbing water and nutrients. 

Nematodes in the Soil

Go out to your garden, and fill a spade with soil. That dirt likely contains a million nematodes, more or less. 

Different soil types host different ratios of nematodes by groupings, and this can vary by season, type of vegetation in the area, and general soil health. According to our friends in Florida, "Because they are responsive to so many different factors, it is believed that nematodes may be useful bioindicators of the condition of the soil environment."

Aside from general descriptions of nematode mouths, they're still very difficult to identify. Most agriculture extension offices or universities can accept soil samples to provide a wild guess, at best, at what types of nematodes live around your tomatoes and carrots. 

Which Nematodes Are Harmful? 

Soil biologists believe that the vast majority of nematodes, including those that are "free-living" in the soil, are important to nutrient cycling and maintaining a proper balance of living microorganisms and arthropods in the soil environment. Those that feed on plant roots, of course, are harmful, but nematodes that feed on fungi don't discriminate between the "bad" and the "good" fungi, and we're learning the importance of rhizomes in the process of aiding root systems' ability to absorb nutrients. 

Generally speaking, root-eating nematodes are, hands-down, the worst offenders. 

Some harmful nematodes are relatively easy to identify. For example, Root-knot nematodes are among the type that creates root burls, and other root-eaters cause dark spots and forking in root vegetables. Many commercial nematicides often boast their effectiveness against these particular worms. 

The best indicators that nematodes are attacking root systems in your garden plants? Yellowing, wilting and stunted plants, those that appear to be suffering drought in spite of your diligent watering schedule, and those that don't respond to fertilization and nutrient additives. 

Reducing Harmful Nematodes and Attracting the "Good Guys"

How do you get rid of the nematodes that spread disease, eat plant roots and predate on the good guys? 

  • Avoid chemical nematicides, which kill both bad and good nematodes.
  • Make sure your plants are healthy enough to withstand imbalances in the sub-soil environment. 
  • Remove plants that you suspect may harbor harmful nematodes. Be sure to dig out the roots, and be careful not to shake off the soil in the root ball. 
  • Dispose of symptomatic plants by burning, rather than composting. Never compost diseased plants. 
  • Innoculate your soil with beneficial rhizobia (bacterium) to boost plant health and increase food for bacteria-feeding nematode populations, which will help reduce diseases spread by root-feeders. This may also raise the percentage of nematodes which prey on ectoparasitic nematodes. 
  • Incorporate aged compost into your soil. In addition to feeding your plants, compost stabilizes soil moisture and contains copious amounts of fungus. 

Beware of Bad Advice

You'll read a lot of well-intended but inaccurate information about nematode control on the internet. Let's take a closer look at some common myths:

Till up soil to expose nematodes to sunlight, thus killing it. When you disrupt the soil in this manner, you're also killing the good nematodes and microbes your garden depends upon, as well as the fungal web that aids roots in the nutrient cycle. 

Plant marigolds around your garden. While it's true that marigold roots may secrete a compound deadly to nematodes, scientists haven't figured out how or why. Marigold roots could affect only root eaters, or it could work as a broad-spectrum nematicide, killing all nearby nematodes. The key phrase here is "nearby". On the website Garden Myths, Robert Pavlis' own research concludes that one would have to plant marigolds in the same space planned for future vegetables for 2-4 months and that a 1-2 foot variance of the target planting spot would make a huge difference. 

According to Pavlis and other sources, marigolds aren't the end-all, be-all companion plant. Different marigolds affect different nematodes, and marigolds are known to attract some of the very pests (thrips, spider mites) gardeners want to eliminate. 

Other plants promoted as nematode bane include painted daisy, castor beans, and dahlias, but one should remember that to nematodes, the distance from one square foot of garden space to the next is probably the same distance between Earth and Jupiter. 

Nematodes are bad. If you didn't know this before, you certainly know it after reading this post! Many garden products and websites seem to lump all nematodes into the "bad" category. (Not all Klingons are bad, either!) Be leery of any company that doesn't differentiate when they advertise; it means either they don't know what they're talking about, or they assume you're ignorant...and a good mark. 

Introducing Beneficial Nematodes

Maintaining a healthy soil environment and eliminating diseased plants is the best approach to preventing the effects of harmful nematodes and encouraging a balanced microcosm in your garden, but you can give your beneficial nematode population a boost. 

Some nematodes prey on harmful insects, and commercially-available products applied to crops aid in organic pest control. Steinernema feltiae, the most commonly-marketed beneficial nematode, eats gnat larvae and thrips pupae living in the soil, as well as wood-boring insects, without harming beneficial insects. Packaged nematodes in cryptobiotic state, when combined with water and applied through a hose sprayer, revitalize and get to work in the soil. (Kind of like Sea Monkeys, if Sea Monkeys lived in the dirt...and were worms.)

While you can give your beneficial nematode population a boost if it's clear your plants are suffering from soil-borne pest larvae and even root-feeding nematodes, nature tends to balance itself out. Predatory nematode populations grow when lots of baby or "prey" nematodes are around. This might detract from predatory nematodes chowing down on the root feeders. 

Healthy Plants Start With Quality Seeds

Your best defense against harmful nematodes is a healthy garden and vibrant plant food web. Young plants are most susceptible to pests and root damage, and fresh, quality seeds provide them with the nutrition they need to properly germinate and grow. Be sure you source fresh seeds from the best parent stock. Buying seeds from Seed Needs gives your garden a great start to a healthy season. 

(We apologize, though; we don't carry parasitic space pods.)

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