As a gardener, you’ve probably learned that it’s the little things that can sometimes have the greatest impact. Well, this episode focuses on some very little things – nematodes. These microscopic creatures are a part of the soil food web. They play a key role in nutrient cycling, but some of them can also devastate your plants. There’s a lot we still don’t know about these creatures, but I asked expert, Dr. William Crowe, to join me to share the facts on what we do know – the good and the bad.
Dr. Crowe is a professor of Entomology and Nematology at the University of Florida. When it comes to nematodes, Florida is where a lot of the action happens. Conditions in that state are particularly ideal for nematodes. As with all insects – when it comes to nematodes, there are good guys and bad guys. As Director of the University of Florida’s Nematode Assay Lab, Dr. Crowe studies both categories and oversees thousands of tests submitted to the lab each year by home gardeners and agricultural organizations alike.
Let’s start with the basics: What is a nematode? These creatures are unsegmented roundworms, and there are over 15,000 different species of them. In fact, they are the most abundant life form on our planet. Seven out of every ten creatures alive on Earth are nematodes.
Most species are microscopic in size, but some grow very large. In fact, one species of parasitic nematode can grow to be around 30’ long. You don’t need to worry about coming across that species in the garden. They only attack and parasitize sperm whales.
Don’t confuse earthworms as a nematode species. Unlike nematodes, earthworms are segmented. You can cut them in two, and both ends (or segments) will remain active for a time. Dr. Crowe says nematodes, which are unsegmented, are more like a water balloon. A water balloon can’t be pulled into two new balloons.
Although nematodes are typically found in soil, they are actually aquatic creatures. They move through the thin layer of water that exists around soil particles. They are swimming through soil. Their movement is similar to that of a porpoise at it skims the surface of the ocean – moving up and down, rather than wriggling through soil.
Most of the larger nematode species are parasitic to non-plant organisms for at least one portion of their life cycle. If you’ve ever had your dog vaccinated against heartworm, you’ve been protecting your canine friend against a parasitic nematode species.
Roundworms sometimes found in humans are also a parasitic nematode, and those can also grow quite large. However, the species which impact our garden or the other creatures living in the garden tend to be very small or microscopic in size.
Unfortunately for us, there are many species of bad guy – or parasitic – nematodes. In our gardens, it’s the plant parasitic species that cause problems. Different species have adapted to different environmental conditions (warm climate vs. cool climate, sandy soil vs. clay soil) but the majority of nematodes prefer warm temperatures and sandy soil conditions. That’s what makes Florida the Grand Central Station of North American nematode problems.
Since they are microscopic, it stands to reason that plant-parasitic nematodes can’t travel great distances. So, how do they travel from area to area? Unfortunately, that’s usually thanks to human activity.
Nematodes can be transmitted in soil or on plant leaves – for example, in or on a plant you purchase at a nursery. They can also be transmitted through seeds. The transmission method depends on species as well as plant variety.
Since they are so small, there’s no way to proactively avoid bringing nematodes into your garden through another plant or in seed, but the odds are pretty good there are some parasitic species already present in your garden. If their population isn’t strong, you might not notice their impact. In addition, most parasitic nematodes are very host-specific. They will only attack one plant species or family.
For example, just like a tomato hornworm will attack tomato plants and other members of the Nightshade family – like eggplants & peppers – they will leave your squash alone. Most nematodes are similarly host-specific.
One of the most common categories of the plant-parasitic nematodes is the root-knot group. Each species of root-knot nematode can cause the same symptomatic damage – galls or lumps on plant roots.
Once a root-knot nematode hatches from an egg within the soil, it locates a host plant by sensing exudates emitted from the host plant’s roots. The juvenile nematode injects hormones into an area of the roots to establish a feeding site. This activity creates the gall and damages the plant’s ability to transmit water and nutrients.
If you’ve ever seen a plant or a seed packet marked as VFN, that indicates the plant is resistant to Verticillium wilt, Fusarium wilt & nematodes (specifically, root-knot nematodes). Unfortunately, that doesn’t guarantee the plant won’t be affected by these creatures.
Nematode-resistant plants can detect the hormones injected by nematode juveniles. In response, the plant kills its own cells surrounding the area under attack from the nematode. That disrupts the nematode life cycle, killing the nematode.
Unfortunately, there are root-knot species which are overcoming this plant resistance. These species are becoming more common too. Nematode-resistant plants are not able to detect the hormone injected by these increasingly-problematic nematode species, so the plant isn’t able to defend itself.
Just about every variety of edible crop is on the menu of a root-knot nematode species. There are many varieties of ornamental plants that aren’t susceptible to nematodes, but some varieties can be hard hit.
Symptoms You Might Have a Problem
Damage from all species of plant-parasitic nematode can be difficult to diagnose in the garden. Common symptoms include:
- Galls or lumps on the roots
- Degradation of the root structure
- Stunted plant growth – in spite of nutrient availability in the soil or through fertilizer application
- Wilting – in spite of moisture in the soil
- Leaf drop
Each of these symptoms can also be an indication of another issue, which is one of the reasons nematode diagnosis is, essentially, impossible for the home gardener. Adding to the complications of diagnosis is that parasitic nematode activity leaves plants more susceptible to other plant diseases. So, your Tomato Spotted Wilt virus problem might actually be an indication of nematodes which are causing initial damage.
One way to know that root galls are, in fact, a result of nematodes is to try to pull the galls away from the roots. If the lump is caused by nematode damage, it will be part of the root itself. If something else is the cause, the lumps will be an appendage to the roots, so they will pull off fairly easily.
The only true way to determine whether or not parasitic nematodes are an issue for you is to send a sample to a plant disease clinic or a private testing lab. There are private labs in virtually every state, and your best bet is to stick with a local testing facility. That said, Dr. Crowe’s lab receives tests from across the country. Processing out-of-state samples requires more precautions, and that means those tests are more expensive.
Those extra precautions are necessary, because nematodes can be so easily transmitted. In fact, Dr. Crowe’s lab is unable to accept samples from Idaho, Nevada, Oregon, Utah, or Washington; because there are nematode species present in those states which haven’t yet invaded Florida.
This is also a good example of why there are quarantines and regulations in place prohibiting the carrying produce or plants between states.
There are generally two types of tests – predictive and diagnostic. A predictive test can determine what is present in your soil, so you can avoid planting susceptible varieties. If you’re experiencing damage, you’ll request a diagnostic test to determine the cause. In either case, the best first step is to contact the lab. There are specific requirements for preparing and shipping the sample that you need to be aware of before you get started.
Some species of nematode feed on leaf or bud tissue of a plant. They can move up through the plant to reach the foliage, or they can climb up through the microscopic film of water that covers the plant. Similarly, these pests will feed on the surface of the plant tissue or consume it from the inside out.
Although soil-dwelling nematodes are more commonly known, Dr. Crowe says that the foliar species are becoming a bigger problem.
These species are also more commonly transmitted in large commercial nursery environments where a sea of plants is often grouped closely together. Overlapping foliage provides plenty of opportunity for foliar nematodes to spread and increase in numbers.
If you determine that plant-parasitic nematodes are causing damage in your landscape, what can you do to control them? Chemical control options are generally available to large, commercial growers only – not home gardeners. If you’re like me and strive to keep your landscape chemical-free, that probably doesn’t break your heart. Fortunately, we can turn to some fairly effective organic options.
The first step is to gravitate toward plants bred to be nematode resistant (look for the N indicator on a plant tag or plant description). Those will be least likely to suffer nematode attack.
If you’re interested in trying the biopesticides approach, there are some products available. For example, you can purchase a fungus that parasitizes the eggs of root-knot nematodes. Dr. Crowe says biopesticides won’t eliminate a nematode problem, but it can keep their population down. Your best bet is to purchase from a biocontrol lab online.
Solarization won’t eliminate nematodes, but it can drive them deeper into the soil than the roots of many plants will reach. For this option to be effective, you need to cover the area with clear plastic and seal the edges to seal in the heat from the sun. Direct sun will be necessary to create temperatures high enough to impact nematode populations. Timing is also key. The hotter and drier the weather, the better.
The plastic must remain in place for about 6 weeks to complete the solarization process. The heat created will disinfect the soil to a depth of 6-8”. That gives plants time to mature and develop a stronger resistance to attack from the nematodes that gain access to roots reaching that deep into the soil.
If you apply organic amendments to your soil – like compost or shredded leaves – you’re already a step ahead of the game. The organic materials improve the soil’s nutrient level and its ability to hold water. So even if parasitic nematodes are present, the plants will have optimal access to the moisture and nutrients necessary to withstand some nematode damage.
In addition, there are fungi and bacteria naturally present in organic materials which will parasitize some nematodes.
Crop rotation is one of the most effective weapons for battling parasitic nematode issues. Since these creatures are host-specific and can only feed on live plant material, crop rotation can starve them out. There’s no magic bullet when it comes to duration, though. The amount of time nematodes can live without live host-plant tissue is diverse.
That said, most nematodes live a fairly short time. After it hatches from the egg, the juvenile is full of stored lipids which serve as its energy source. Unless the juvenile can find a food source, it will die.
Let’s say you continue to experience root-knot nematodes on your tomato plants. That means there are nematodes in your soil that feed specifically on the Nightshade family of plants. As you plan your garden, relocate Nightshade plants to a different area each year. In doing so, you deprive the Nightshade-loving nematodes in that space of a constant food source. Odds are good this won’t eliminate the population, but it will help to keep it in check.
When it comes to ornamentals, you can starve out a nematode problem by simply not repeating the same mistakes. If a specific type of plant in your landscape dies and you suspect nematodes, don’t replace it with the same variety of plant. Starve out the nematodes by planting a different type of plant.
Dr. Crowe likes to give the example of the azalea. Although root-knot nematodes are a big problem in Florida, there are no species which use azalea as a host. So, that’s a safe plant choice for avoiding root-knot nematode damage.
Some cover crops promote organisms which feed on nematodes. Dr. Crowe gave Sunn Hemp as an example. That crop promotes the population of nematode-trapping fungi. The fungus creates a sort of lasso that the nematode wanders into. Then, the fungus restricts on and kills the nematode. So, it’s not an exaggeration to say that it’s like the wild, wild West under the soil surface. Nature just never ceases to amaze.
Dr. Crowe cautions that, if you do utilize a cover crop for nematode control, it’s important to keep out weeds. Many nematode species use weeds as a host plant, so keep out the weeds to keep down the nematode numbers.
If you don’t have a lot of space to try crop rotation or cover crops, you might have more luck gardening in containers. Just be sure to keep containers off of the soil surface. Nematodes can make their way from the soil into containers.
As with all garden pests, the amount of damage you’ll experience is dependent on the number of pests you’re dealing with. It’s possible your soil has always contained parasitic nematodes, but the conditions keep the population low. If that’s the case, you may never notice damage at all. Pest control is always about tolerance level. You might say it’s a little bit of a “numbers game.”
Bad guy nematodes can be challenging to identify and eradicate, but there are also good guy nematodes in the neighborhood. The good guys can be effective allies in battling other garden pests.
Beneficial nematode species work as a natural biocontrol. They don’t parasitize plants. They parasitize soil-dwelling grubs and other insects. These species are naturally present in soil, but some can also be purchased as a treatment application.
These species are generally host-specific. They kill insects in one of two ways:
- Entering the insect to feed on it
- Injecting a bacteria into the insect – The bacteria kills and reproduces inside the insect, and the nematode feeds on the bacteria.
Varieties that enter the insect can’t be produced commercially in a biocontrol lab. It’s too difficult to properly provide a food source. Those species attack insects such as grasshoppers, cockroaches and mosquito larvae.
Nematodes that feed on bacteria can be grown commercially, and those are the species that can be helpful in killing Japanese beetle larvae and other grub pests in soil.
If you opt to purchase nematodes, avoid off-the-shelf products. After all, these are live organisms, or at least, they were when they shipped from the manufacturer. So depending on shipping conditions and how long they have been sitting on the shelf, the viability of what’s inside the package can vary pretty dramatically.
The best option is to purchase directly from a manufacturer or lab online.
Dr. Crowe’s nematode species recommendations for some common garden pests include:
- Heterorhabditis bacteriophora – effective against grubs
- Steinernema feltiae – effective against fungus gnats
- Steinernema carpocapsae – broad spectrum
Once you receive them, it’s important to apply the nematodes right away, although they can be stored in a refrigerator (not the freezer) for a few days. Avoid letting them sit in direct sun or in a warm area.
Inside the package will be a clay-like or granular material or a sponge. This is added to water and watered into the area you want to treat. The best time to apply is in the evening, because the nematodes need time to work their way into the soil to avoid the UV rays of the sun that can kill them.
You might wonder how beneficial nematodes find their prey. Some species can detect the CO2 emitted by the soil-dwelling grub or insect. Other species follow chemical signals put out by insects, other nematodes, or even plants.
In fact, it has been discovered that corn under attack from corn rootworms, put out an SOS to nematodes. The corn emits volatiles in the soil. The volatiles call in the nematode calvary to prey on the rootworms, and the corn lives to produce another ear.
So Much More to Learn
Scientists have only barely scratched the surface when it comes to soil food web research, and nematodes are no exception.
Recently, it’s been discovered that nematodes can be trained. These microscopic creatures can be taught how to follow a certain chemical signal in order to find a host. Nematodes live in herd-like groups, so when one nematode is trained to follow a signal, the rest of the herd follows to check out the action.
I don’t know about you, but I can’t wait to learn more about those types of developments.
Nematodes do much more than just parasitize plants and insects. They play an important role in nutrient cycling. If you know much on how the soil food web works, you know that the microorganisms in the soil consume materials, minerals and other microorganisms. Nutrients are released as microorganism waste or poop. The nitrogen released by nematodes is in a much more plant-available form than nitrogen released by soil bacteria.
So although some species might be causing damage, the population as a whole is doing great work to provide nutrients that healthy plants require.
We’re still in the early stages of learning how we might be able to take advantage of what nematodes have to offer. The bad guys get the most attention from the research community, but the scientists are making strides with both the beneficial and the plant-parasitic groups. You can bet that I’ll be keeping my eye on those developments and look forward to more podcasts discussions as this field opens up.
In the meantime, I encourage you to scroll to the top of the page to listen in to my conversation with Dr. Crowe. He’s passionate about this work and offered some other fascinating examples of the good guy and the bad guy nematodes. You might not be able to understand the species names he throws at you, but the principles will certainly resonate.
Links & Resources
joegardener Online Academy Three popular online courses, including gardening fundamentals and my newest course on starting your own plants from seed!
Master Pests, Diseases & Weeds – My online course teaching you how to master pests, diseases and weed control – Just $47 for lifetime access!