Nematodes - Underground Root Destroyers
by Carolyn Elgar, Orange Country Rose Society
This article is a 2017 Award of Merit winner
Rose “poop out” or declined vigor is a frustration that rose lovers sometimes have to deal with. The plant’s foliage has decreased, bloom count is low, and the plant looks weak. Lots of things can cause this, but one is not visible above the ground. Nematodes are microscopic, unsegmented worms that feed on your plant’s roots, robbing it of water and nutrients. The damage they create shows as galls on on the roots, small hard lumps in the shape of a pea. Roots are strangled by the masses of these galls. Unfortunately once nematodes have invaded your soil, you are stuck with them for life unless you dig out the entire bed to a great depth and replace it with new soil. The challenge is to keep their numbers low and help plants become more tolerant of their damage. ABOVE: These roots show the galls of nematode infection.
There are many, many types of nematodes in the soil; some of them are actually beneficial. The one that feeds on rose roots is the root knot nematode, a variety that is not host specific which means it can come into your garden on the roots of other transplants. These nematodes survive over the winter as eggs in the soil. After the eggs hatch they enter a juvenile stage during which they seek out root tips to invade. Once they have pierced a root they become sedentary, feeding on the root tissue near their heads. After around twenty days, the attached nematodes begin to lay eggs. The most active stage of the nematode’s life is the juvenile stage when the worm is free in the soil. They are not fast movers and their reach rarely exceeds three to four feet from where they have hatched.
If they are limited in their movement, why are they such widespread problem? Nematodes can be spread to other areas in the garden on shoes, spades, trowels, and even by running water. Sanitation of tools after working in a nematode infested area is essential. Inspect the roots of any new transplants carefully for the galls. Cut that part of the root system off and throw it in the garbage, not the compost pile.
Nematodes may be a nuisance for the rose gardener, but they can be a disaster for farmers and commercial growers. As a result, a lot of research has been completed on different ways of controlling them. The studies encompass two categories: killing the juvenile nematodes and helping the plant gain the health needed to overcome the damage to the roots.
Nematicides for home use
In the past, before planting, farmers would fumigate their soil with nematicides to control the pests. The chemicals used are extremely toxic and many of them have been banned because of water contamination. They are only available to licensed users and very unsafe for the home gardener.
Gardening product companies have stepped in and offer a number of ways to decrease the numbers of nematodes in the soil. Neem, an extract from the seeds of the Neem tree is available in the form of Azadirachtin, which can be used as a soil drench. Residue from the crushed need seeds is available as a soil ammendment sold as Neem cake. In addition to decreasing nematodes, Neem cake acts as a slow release fertilizer that improves the soil.
Another plant extract, saponins of Quillaja saponaria, from the South American soaptree is the active ingredient in one commercially available nematicide. Geraniol, an alcohol extracted from plants such as lemongrass, geraniums, and blueberries, appears in several nematicides. Research shows that essential oil from grasses that include geraniol as a component decreased the number of junvenile nematodes in the soil.
In the category of improving the plants’ resistance to nematode damage, several organic amendments have been shown to impact nematode populations while offering other benefits to the soil. Worm castings, crab/shrimp shell, and mycorrihizae have research supporting their efficacy in controlling nematodes. All take some time to establish in the ground before becoming most effective.
Earth worm castings and vermicompost (worm) tea have been shown to inhibit egg hatching and decrease the number of juvenile nematodes in the soil. The tea seems to be most effective, but worm castings contain trace elements that improve the soil, and they improve the soil’s texture and water retention. (It is beyond the scope of this article, but worm castings have been proven to improve plant growth.) Earth worm castings are available at specialty nurseries and online.
Crab or shrimp shell contains chitin that attracts beneficial microbes that feed on it. Once these organisms have consumed the chitin in the shell, they feed on the eggs and nematodes. The addition of chitin to the soil creates communities of beneficial fungi and bacteria. Shrimp shell has a higher content of chitin and is available as an organic amendment at specialty nurseries or online.
Finally, there have been some interesting studies on how nematodes and mycorrihizae coexist. Mycorrihizae is a fungus that has a symbiotic relationship with rose roots. In an effort to reach more phosphorus, an element that is immobile in the soil, a rose’s roots exude a sugar that feeds the fungi which then become attached to the roots and grow long, thin strands that can penetrate the soil farther away from the plant where the phosphorus has not been used. Along with the uptake of phosphorus, mycorrihizae absorb other nutrients which benefits the plant’s health.
Because both mycorrihizae and nematodes depend on plant roots for nutrition, it is possible that there could be food competition between the two. The existence of mycorrihizae in the soil causes the roots’ chemical composition to change, possibly having nematicidal effects. Some studies have found that nematode eggs are decreased in soil that has been treated with mycorrihizae.
Perhaps the largest effect of mycorrihizae is its ability to increase tolerance to nematode damage in plants. Nematodes don’t travel far and may be contained to a part of the plant root system. The development of more healthy roots, the extension of these roots by the mycorrihizae, and the food competition from the increasing amounts of the beneficial fungi combine to make the plant more vigorous despite its nematode damage.
Mycorrihizae is available as an innoculent and in soil amendments available for order online. Be sure to read all the instructions as each product has different ways of applying the product. Mycorrihizae are fragile; systemic fungicides will kill them. Too much phosphorus will inhibit the development of your application because the plant will not be trying to attract the fungi if there is lots of phosphorus in the ground. Unfortunately, some rosarians use fertilizers high in phosphorus because they think it increases rose blooms. Organic fertilizers are better tolerated. Research shows mycorrihizae do better in loose, well drained soils that are undisturbed.
Resistant root stock
What many farmers have done is plant nematode resistant varieties. Similarly rose growers are increasing the availability of roses on Fortuniana root stock, a strong nematode resister. Fortuniana has been used for roses grown in the southern United States for several years; the soil there tends to be sandy and fast draining which is what nematodes prefer. As an added benefit, roses on Fortuniana, once established, grow tall and have large blooms. The root system is shallow and wide spread which also improves water uptake and increases plant vigor. It’s not easy to find roses on Fortuniana root stock in our area; the best resources are online nurseries or home propagators who have acquired and are growing this root stock.
Nematodes can surprise the gardener. You may not know you have them until you see the galls on the roots. Unfortunately, by that time the plant may have weakened considerably. Most people choose to dispose of the plant because it is in such sorry shape. A good approach might be to pull out the infected plants and treat the soil before replanting. Use one or more of the above approaches and make sure to add amendments to improve soil health. You could try to replace most of the soil but nematodes can survive deep in the soil and may reappear. One thing to be very careful about is the possibility of spreading them through your garden on tools that have been in nematode infected soil. The good thing is that nematodes don’t go far on their own; the bad thing is that we may give them a ride to a new home.