Much is asked about nematodes and how to avoid them entering your garden…. but what are they, how do they strike fear in our hearts (or at least our green-ish thumbs), do they have a “bright side” and is there any place for them within our soil ecosystem?
Like anything in our garden, when we intervene to get rid of a perceived pest, rather than letting nature’s pest control step in, you should always consider what you’re adding and what you’ll end up taking away….. (and also who will take its place – for better or for worse).
Let’s kick off with our Nematode Taxonomy. Starting with the 5 Kingdoms again…
- KINGDOM – The nematode taxonomy journey starts in the Kingdom called Animalia, like us.
- PHYLUM – The Kingdom Animalia is split into 35 Phylums, one of which is Nematoda (AKA Nematodes). Nematodes are described as “non-segmented roundworms that… mineralise nutrients contained in bacteria and fungi.” (Teaming with Microbes, Lowenfels, J. & Lewis, W., 2011) Within the many levels between Kingdom and Phylum Nematoda follow the path of symmetrical at the tissue level, three cell types and shedding of skin (Source). Its “skin” is the tough outer “cuticle” – is non cellular and is secreted by the outer layer of cells (the epidermis). (Source) Although this layer is both tough and flexible (Source), it will not grow with the nematode and hence is shed – debate-ably the first of many irrelevant facts for today. Nematodes are also referred to as ‘Eel Worms’. (Source)
So it was a very short trip down the Taxonomic ranks this time (as compared to the aphids), but perhaps this indicates that it might take more than one nematode to make a phylum…. so we’ll dig a little further down and figure out where the famous (or infamous) nematodes reside…..
(But hang on a minute… did someone say something about mineralising nutrients?… surely that’s a good thing? Let’s come back to this after we’ve learnt a little more.)
Though only about 20 000 species have been described some scientists estimate there may be as many as a million species all up.(Source) (others believe it could be up to 100 million – Source) It is interesting to note that these little guys are thought to be the second most dominant life form after arthropods and, in a rather strange analogy (that crops up in several articles, with exactly the same wording and no references….. so it must be true!) “because nematodes are so numerous and occur in so many habitats, it has been suggested that if we removed everything from our planet but nematodes, much of the topography of the Earth would still be recognisable as a film of nematodes!” (Source and many others..) Nematodes can be found everywhere – in the deepest ocean, to the tallest mountain; causing grief within animals (humans included) around the globe; grown in labs and killed in fields; etc, etc, etc. “A spadeful of soil can contain more than a million nematodes!“(Source) But once again…. or it can contain none of them…. its our choice.
The groupings of these currently known species, speaking purely taxonomically, completely lost me and I have had to admit defeat based on time (of course, it’s not intellect based!!!!). I’d hoped to do phylum, class, subclass and order, then slot in some famous / infamous examples for you. The best spot I found, if you wish to get technical, is this link which does the top down; and then you can get lost in wiki as it has good bottom up information on specific nematodes (but these do not match its top down pages or anyone elses! Best of luck, very keen to hear if anyone has success.) …. but let’s not get hung up on that. As a nice change, I reckon I’ll step away from the pure science and borrow the “Teaming with Microbes” (Lowenfels, J. & Lewis, W., 2011) to keep it simple and sticking to topic…soil…..(as you might have notices I have a problem sticking to topic!!) …. please feel free to follow any reference back and see what the scientists really reckon!
“Teaming With Microbes” Classification 1: PLANT EATERS – Herbivores
“Most plant feeding nematodes live in the soil and feed on plant roots, thereby reducing the plant’s uptake of water and nutrients, and reducing tolerance to other stresses such as drought.”(Source)
Root eaters can be split into two sub – categories: (a) ectoparasitic – feeding on the root surface, or (b) endoparasitic – feeding on the root from within. Both cause lesions which creates bulges often referred to as root knots. (Teaming with Microbes, Lowenfels, J. & Lewis, W., 2011)
In addition to harming the plant directly, some herbivorous nematodes also “transmit other disease-causing agents (e.g. viruses) to plants as they feed.”(Source)
“Teaming With Microbes” Classification 2: BACTERIA EATERS – Bacterivores
These nematodes, as the name states, eat bacteria and hence mineralise the nutrients the poor little guys contain, making them available to the plant once again in the form of nematode poop. So they are team players in our food web.
“Teaming With Microbes” Classification 3: FUNGI EATERS – Fungivores
These nematodes, also as the name states, eat fungi and again mineralise the nutrients the fungi have stored up within the fungal hyphae, making it available to the plant in the form of nematode poop. And again these ones are also team players in our food web.
“Teaming With Microbes” Classification 4: (HIGHER) PREDATOR NEMATODES
These nematodes feed on “protozoa, algae … and other small members of the soil food web – grubs, weevils, wasps and even small invertebrates such as slugs.” These ones also eat other nematodes and hence serve to moderate the population of the bacterivores and fungivores so that the colonies of bacteria and fungi are not decimated, but more importantly….THEY KEEP THE POPULATION OF THE HERBIVOROUS NEMATODES DOWN TOO! It is noted that various species of these type of nematodes have been used in pest eradication strategies (see below discussion). I reckon this makes it pretty clear that we need to take care of how we treat the bad guys as we have the potential for issues with friendly fire…… Once again Ms Nature has her balancing mechanisms.
Considering non-soil examples – most “worms” described as causing issues with humans or animals – hook, heart, tape …. – are indeed nematodes wreaking havoc. And there are lots and lots of other examples… (have a hunt, the internet is full of articles – they’re fascinating, if a little foul sometimes).
General Nematode features:
- Shape/anatomy: “The head of a nematode has a few tiny sense organs, and a mouth opening into the …throat… where food is pulled in and crushed. This leads into a long simple gut cavity lacking any muscles, and then to an anus near the tip of the body.” (Source)
- Mouths – “Plant feeding nematodes have a sharp needle-like structure in their mouth called a ‘stylet’. The stylet acts like a very small hypodermic needle and is used to pierce through cell walls and suck out the cell contents. An example of the losses caused by …. nematodes, is…. the pine wilt nematode (Bursaphelenchus xylophilus) which has been associated with the death of tens of millions of pine trees in Japan.” (Source) The Fungivores also have a stylet for piercing the walls of the fungal hyphae. Those nematodes that feed on bacteria, protozoans and even other nematodes lack this stylet. Other nematodes are famous for their hook mouths, but these are animal based parasitic nematodes and hence I’ll speak no more of them…. See the diagram below which shows you (a) the stylet in the plant parasites (which we called herbivores), (b) the microbivore (we split these into Bacterivores and Fungivores, plus a little bit of the predator group) and (c) the predators which I would not like to come face to face with in a dark alley (although it does remind me a little of the dragon from Never Ending Story….).
- Respiration – No respiratory system – gas diffusion occurs through its exposed surfaces.
- Vascular – Similarly there is no vascular system as nutrients and waste are diffused throughout the body cavity along the length of the gut. This process is “regulated by an excretory canal/duct along each side of the body.” (Source)
- Size – Unlike protozoa, the larger nematodes are typically 2 millimetres long and 50 micrometre diameter. Those how have been able to see them with the naked eye (a) describe them as wriggling human hairs (Teaming with Microbes, Lowenfels, J. & Lewis, W., 2011) and (b) must have better eyes and patience than me! Most plant parasitic nematodes are <1mm long and almost invisible to our eyes, but yet are thought to be responsible for “15% loss of crops per annum world-wide.” (Source) Having said that we are only talking about the typical nematode species when we mention the tiny ones….. there is a nematode living in the placenta of sperm whales (I hate to think how they found it!) which grows up to 9 meters long! (some sources have 13 meters.) And the name I hear you ask…. Placentonema Gigantissima, of course! (Teaming with Microbes, Lowenfels, J. & Lewis, W., 2011, among many others sources – everyone quotes this example) There’s also a nasty one in the tropics that is transferred by mosquito and damages the lymphatic system in humans… this bugger has been found to grow as long as 10cm. (Source)
- Locomotion – typically wriggling or thrashing by using the individual muscles that run the length of the worm on alternate sides. (Note that there are not any radial muscles as the cuticle prevents radial expansion. This is a key distinction when identifying nematodes.) (Source)
Interestingly, some nematodes can elect to suspend life (stop all metabolic processes) during unfavourable conditions and restart when conditions become more conducive to survival – a mechanism known as cryptobiosis. (Source) Sneaky little blighters – as if they did not have enough of their side!
So what do we care? (Nematodes in Western Australia):
Let’s visit/revisit a few of the infamous groups of nematodes (from the herbivore TWM class) local to WA and see why we should care about them being in our garden: (Most info courtesy of our local friendly Ag. Dept.)
Root Knot Nematodes (RKN, Meloidogyne) – The obvious beastie. There are more than 50 species of root-knot nematodes (Source) and they attack more than 2000 species of plant worldwide. The “RKN enter the roots as larvae, causing the plant roots to form galls or knots, and there may be excessive root branching…..The nematode larvae mature in the roots, where they mate. The female adults enlarge, remain in the roots, and lay eggs into an egg sac that exudes into the soil. The eggs hatch and the young larvae go on to infect more roots. Plants are damaged because the galls or root knots block the transport of water and nutrients through the plant. Nematode feeding sites in the roots can also provide entrance for other disease-causing organisms (such as fungi or bacteria), leading to increased plant damage.” (Source) Factors favouring them are – long growing season plants, sandy soils and plants already under stress.
Root Lesion Nematodes (RLN, Pratylenchus) – typically affect fruit trees, roses and turf in the urban setting, but cereal crops are damaged with a higher financial impact.”RLN use the stylet to puncture roots and enter the cells. They move through the root, piercing cells, extracting cell contents, and leaving behind a trail of both cell-killing metabolites and eggs. Root cell death results in browning and lesioning of the roots…The overall effect is a weak, shallow root system with many dead or dying areas. When the soil dries out, root lesion nematodes become inactive and survive in a dry form in the soil or in root tissue of old crops. As the soil moistens, the nematodes become active again and re-infect the fresh roots of the new crop.” (Source)
Two key threats to WA are the Potato Cyst Nematode (PCN, Globodera rostochiensis) and Stem and Bulb Nematode (Ditylenchus, attacks oat, faba bean, field pea, lentil, or canola.) (Source)
Deladenus Siridicola – On the goodies side – there are not many specific goodies which stand out from the crowd, but don’t think they are not out there – Deladenus Siridicola is just one example of nematodes which attack insect pests and is one which has been used as a biological insecticide. This nematode was “released to control the Sirex wood wasp (and) has been credited with saving the forestry industry up to 80 million US dollars per year in Australia.”(Source)
Where do they come from?
Most plant-parasitic nematodes enter the garden through infested soil or infested transplants. The same is thought to be the case with the good nematodes, but with less money involved in this aspect of the phylum there is very little documentation on it.
As they work underground you will not be able to identify a problem until you see the effect on your plants or unless you pull out a plant and discover the damage. A plant suffering badly from RKN may wilt, turn yellow, and/or eventually die, even if provided with moisture. (Source) But how many other issues cause similar problems and you wont have a definite answer until you pull it out. Most websites say to “If you think your plants are infected, dig up a plant, rinse the soil off the roots and inspect them…. (e.g. source) – which with some plants may have a worse effect than the nematodes themselves….
The following picture is what you are likely to see – the left has the classic cysts/bulges indicating the presence of RKN.
So what do we do?
It is understandable that when your livelihood depends on productivity and your close to the bone on revenue, it is critical to avoid production limiting soil biology. But when it’s just your garden, what lengths do you go to and what can you hope to achieve?
“Once nematodes are present, they are almost impossible to eliminate, but their damage to plants can be reduced.”(Source)
On a farming scale, brassicas can be used as green manure or alternate crops. Some “Brassica species (e.g. mustard, canola) have nematicidal properties which potentially can reduce nematode numbers below levels at which significant economic damage is caused.”(Source) The specified brassicas “contain compounds called glucosinolates, which provide the spiciness associated with their culinary cousins. These break down to become isothiocyanates, which are the bioactive compounds that act upon soil-borne creatures such as nematodes in a process called biofumigation (McLeod & Da Silva 1994)” (Source) For farms where the plants are picked rather than completely harvested (e.g. vineyards), and hence crop rotation is not possible, it is stated that the nematode repelling “plants need to be ploughed in so that the chemicals are released into the soil from the plant material.” (Source) This contradicts our previous studies which found that tilling is to be avoided.
Within a garden setting, it is suggested that crop rotation is best – i.e. move your planting of brassicas into the effected area or, for prevention, just move them around over time to minimise future potential invasions.
Even our own Department of Agriculture has stated: “It is virtually impossible to eradicate nematodes. There is no economically viable chemical control for nematodes in broad-acre crops, consequently they must be managed. The key to management is reducing their numbers in the soil through appropriate crop rotations. The ability of nematodes to multiply on a host plant is the over-riding influence on nematode population density, and thus the potential for crop damage, in both current and subsequent crops. Resistant or moderately resistant crop species or cultivars limit the capacity of nematodes to reproduce. Frequent sowing of resistant crops will therefore lead to a reduction in nematode levels, and a corresponding yield increase.” (Source)
Note that the eggs can survive for at least a year, so it is suggested that if you have had issues in one particular area, then the rotation of crops should avoid the nematode susceptible plants for closer to 18 months.
Many articles discuss the use of chemicals in the form of “fumigants or nematicides” in order to control the nematodes. It is however acknowledged that these “chemicals are generally very toxic and hazardous to the health of both humans and the environment. Some nematicides have been banned because they were found to easily leach through the soil and contaminate drinking water in aquifers. ” (Source)
There are nematode resistant rootstock products for various grafted species – e.g. grape vines – which could be looked into for planting in effected areas.
Apart from not recommending the nematicide chemicals especially in the home garden (Just think of what will end up in your produce produced from soil near by, when it this poisons drinking water in aquifers – eeek), specifying nematode resistant green manure and suggesting crop rotation, what else has the Department of Agriculture got to say? (Source)
- Relocate your garden;
- Don’t grow nematode susceptible plants in that location (See above chart);
- Plant nematode resistant plant varieties (“Plants that contain high levels of bio-fumigant compounds include: rapeseed (canola) Brassica napus (more info above), BQ Mulch (i.e. Brassica Mulch), marigolds (more info in link!) and Indian mustard (Brassica juncea). BQ Mulch, canola and mustard are cool season crops. Marigold is a warm season crop that when mixed with cowpea makes an effective, warm season, nematode-controlling green manure.” (Source) (Use caution if electing to plant Canola as it is one of the few approved GMO crops in WA. If you are concerned about introducing genetically modified organisms to your garden then check your seed source or avoid altogether. As an aside – canola is a selectively bred cultivar of rapeseed with reduced glucosinolates and low erucic acid …. “Roundup Ready Canola” has been genetically modified to tolerate glyphosate (I’d thought it was glyphosphate….there’s a learning for some of us!) the base chemical of Monsanto’s “Roundup”. “With its heavy use in agriculture, weed resistance to glyphosate is a growing problem. While glyphosate and formulations such as Roundup have been approved by regulatory bodies worldwide and are widely used, concerns about their effects on humans and the environment persist.” (Source) “Roundup Ready Wild Radish” was found in WA earlier this year. Whoops, sorry, digressing terribly now…. back to the topic….)
- Plant early season cropping plants (lettuce, onion, radish, leafy-greens, green peas or brassicas) to escape the serious damage due to the cooler weather meaning that the nematodes are not yet in full swing;
- Root Destruction – destroy and remove the crop roots during harvest.
- Organic Matter – I’d like to focus on a little more as it is in line with the “add life” philosophy – increased water and nutrients help plants tolerate the attack. The organic matter retains moisture and adds nutrients for the plants. It also increases the microbial action through decomposition activities. More microbes favours the build-up of organisms that feed on these and other microbes, including nematodes.
- Soil Solarisation – “High temperatures can help control nematodes and some other plant diseases which carry over in the soil – see the reference for the technique to use. This is also described in this link and states that “This is high enough (temperature) to kill disease pathogens but most beneficial soil organisms will survive.”
Using our add life philosophy perhaps the options to select are 2,3,4,6 and crop rotation (including a fair duration with specific resistant brassicas). 7 although taking life, provides a limited damage option.
This link has a lovely simple explanation of the nematodes themselves as well as the means of limiting their impact. Some options also talk about exposure of lower soils to air, but as we have learnt previously this has an adverse effect on our other soil life.
Home remedies…. Many remedies (both aggressive chemical and nature based) that I have read about involve treating the soil with specific fluids to kill off the RKN, but take care with the life removal approach, there are good guys that will be caught in the crossfire. Perhaps there are some ideas above that you might employ first, if indeed you have found that the RKN have actually succeeded in killing your plant, or you found them by accident……
Yes, like the embarrassment of admitting that at one stage (well okay two….three at most….) our children have had worms (or as I like to call them now – nematodes – ha ha, does it sound any better?), our garden too has suffered what we think are nematodes…. There has been no official diagnosis, but these photos are the submitted evidence, your Honour, judge for yourself.
A key point to note – the plant was very healthy as you can see from the first picture. It was removed to make way for a lovely mango (we had rescued from a friend’s home renovation site), also this Lovage needed splitting into multiple plants.
We have noticed this syndrome several times – i.e. healthy plants (visually) removed for other reasons, but then found to be displaying significant RKN damage. Which lead us to the question of what is the problem? (Speaking from an urban stand point where we are not dependent on our produce for our livelihood! Farming, I expect, would be a very different matter.)
Now, I would like to pretend that we were doing an experiment – to see how what nematodes do when soaked in water; whether a struck plant can grow new healthy roots; whether Lovage (botanical) is as happy in straight water as Water Lovage (??poisonous….) -, but really I split the gorgeous gal up, gave her a pretty rough hair cut and then…..we just got busy and ignored the poor thing. Two weeks later (and a week of windy storms followed by several days of “spring” (as only Perth can turn it on!)) please see below…. how sad our Lovage is looking in the photos I took this morning….
Perhaps it will all come back to haunt us by degrading our garden slowly and we’ll see the real damage in future years…. But we’ve been edible gardening in this yard for 6+ years, owned the property for 10+ years and there have been “lumpy roots” spotted from day we arrived, so (as a complete amateur…) I’m guessing we may have reached our balance already. The panic which screams at us on sighting such root damage…. “never plant in that bed again”; “killing off all life to start again” etc…., has perhaps been overcome by our (initial naivety, followed by reticence to take life, partially disguising perhaps a laziness and partially a curiosity to see what effect it might have…. but let’s give the Guru credit where it is due and as his knowledge evolved from childhood farming and then more urban observation/study over the past 6 years…unbeknown to me he has been using….) diverse planting strategies, relatively sparse planting of mustard (planted to attract wasps to help with the cabbage moths) and Stinking Roger (planted specifically for root exudates adversely effecting the herbivore nematodes – and there I was just loving the name!), facilitating healthy soil life (food web), avoidance of chemicals (I’d argue most of the earth is made up of “chemicals”, but here I mean chemicals as in the generically accepted term referring to the purchased, mass-produced, short-term-life-manipulating-variety…) and general promotion of garden health. As I say – only time will tell.
As an aside, in discussions held with the Guru on this topic, we’ve invented a new catch phrase – “Have your problems and eat them too!” – What else would you expect from edible gardeners?
RANDOM AND USELESS FACT OF THE DAY – There is a species of Nematode (Panagrellus redivivus) that has only ever been found in German beer mats. (Source) But wait…there are articles stating that “despite the precise rules that have evolved for documenting changes in a binomial, and our ability to track those changes through the literature, there must remain lingering uncertainty that Chaos redivivum and Panagrellus redivivus are the same nematode.” Amazing! Over the years, has Panagrellus ventured off “the mat” (and why would he do such a thing?…. does he prefer cider???)….plus, can it be said that he has become less racially exclusive….. controversy still surrounds this scavenger of the of yeast based beverage industry!
Information Added Post-Publishing –
Firstly, a huge THANK-YOU to those who have provided extra information for us all to learn from (and for pleasantly pointing out an omission in my summary). Very happy to hear from you through whatever media suits. I’ll include a section like this in the appropriate blogs to accommodate additional information coming in from readers. I’ll keep it separate so that those who have already scrolled through the above info can find it easily.
The addition of clay has been linked with a reduction in nematodes. (Perhaps, for those who know the Guru, this is another reason we are doing okay!) In fact, it is stated that the “soil texture has the greatest influence on, or is the factor most correlated with the pathogenicity of, the nematode community” in tropical and subtropical agriculture. (Source) Its slightly different for the more Mediterranean/Temperate climates and for different types of nematode as a review of many Australian articles and Ag Dept recommendations seem to suggest. From a species perspective, the Stem Nematode (that we have not yet fallen prey to in WA, but over East has) is better in wet, clay soils….. (Source, Source).
However speaking generally – the soil texture effect of adding clay (smaller diameter than sand) increases the moisture retention ability of the soil within the root zone – nematodes in WA typically like dry sandy soil, so you have a win. By increasing the moisture content and the surface area of particles (as we learnt previously) allows soil life to more colonies and they thrive. By adding organic matter to feed those colonies (and the pore space allow the freed nutrients to be retained in the pores), you also boost the food web population. Some of the food web life forms like to eat nematodes as well as the good nematode population increasing to mineralise nutrients. Another win.
Many studies have looked at keeping all other things (moisture, organic matter, etc) equal and just effecting the pore size – i.e. if the pores are smaller than the nematode, it cannot pass and hence cannot travel to the target. (Source) Thus sensibly stating that particle size, and hence pore size constraints, plays a direct role too. Having said that, these articles are typically based on determining the best conditions for pest-controlling nematodes and hence small particles are seen as a negative. (As an interesting aside, nematodes used for biological control are sometimes transported “sandwiched” in between two clay layers. (Source))
However in sandy soils (but striving for loam!) the chances of reaching the pore size throughout the root zone associated with this pore size is unlikely (unless we completely/expensively import our soil), so I’m guessing its the moisture and soil web we might be gaining the biggest benefits from.
Another interesting comment made was that clay soils make it difficult to remove the plant’s roots completely, so the RLN may be harder to identify and remove (Source))
An interesting video has also been suggested which has some amazing microscope work…. worth watching just for that – I’m hoping the nematode was a nasty one!
Conclusion – yes, congrats you’ve made it!
So for anyone thinking, I’ve never had a problem, but I want to do something to prevent them…. I sincerely hope this has opened your eyes to the phylum that is the nematodes; the options available to you for prevention; the lack of methods for eradication; the potential management techniques (and the impact of some of those on the good, the bad and the ugly); a bit of solidarity, brother; and hopefully a little less fear in the face of spotting those lumpy roots.
This topic came about as a result of a query which came through and, as I am new to all this, could not answer, so it seemed as good a reason as any to bring us all up to speed. Please feel free to fire topics through (publicly or privately) and I’ll put them on the list….
To be honest I’ve not yet chosen the topic for the next blog….but the front-runners look to be:
- a query from the peanut gallery – Habitat – the washing line of a friend. Suspicious Item – some very cute, neatly laid eggs. In the course of identifying them (as with most queries I get, I had no idea!), I have identified a whole new avenue of investigation to put on my list …. Grasshoppers, Crickets, Katydids and Locusts in WA.
- I’d like to start on some Permaculture Principle investigation….. as my course is fast approaching and I’d like to have my lingo sorted…. never bodes well to start a course already behind.
- I’d also like to do a garden update as the predators are out in force – I might try to do some short blogs (what??? Less than 5000 words???? Surely not!) that feature spring developments / activities.
- And of course, one of these days, I’ll need to get you addicted to Soil Carbon Sequestration and, at the same time, get myself started on studying this topic more diligently.
Too much to investigate and yet so many distractions along the way…. til next time.