Nematodes Chilling Out Now, But Ready for Action

Nearly dead liriope plants with crown rot and root-knot nematodes.

Last week we received a sample of liriope from NCSU’s Centennial Campus. The main problem was Fusarium crown rot, but it also suffered from root-knot nematodes (Meloidogyne). Based on a microphoto I took, Dr. Weimin Ye of the NCDA&CS Nematode Assay Laboratory gave a tentative identification as the southern root-knot nematode, Meloidogyne incognita.

Root-knot nematodes are microscopic roundworms that are parasitic on cells found in plant roots. The nematodes have a spear-like stylet that allows them to pierce the root cells and feed on the nutrients found there. Root-knot nematodes also use the stylet to inject the cells with nematode "saliva" which contains substances that, among other things, cause the cells near the nematode's head to swell. This swelling eventually leads to the galls that are a symptom of root-knot nematode infection.
  
You may be surprised to know that we’ve found root knot nematode damage on samples from every month of the year, although about 80% of them arrive between mid-May and the end of October. In a blog post last month, Dr. Barbara Shew made mention of this particular pest in the context of the relatively unknown “Nematode Song”.

Meloidogyne species have an extremely wide host range. The PDIC has made a confirmed or suspected diagnosis of root-knot nematodes on many North Carolina host plants since January 2008. This list is by no means exhaustive. For example, figs are a known host that did not show up during this period.

Ajuga, Angelonia, Aucuba, Azalea, Beans, Beets (garden & sugar), Begonia, Bermudagrass, Boxwood, Butterfly-bush, Cockscomb, Coleus, Gardenia, Corn, Cotton, Creeping bentgrass, Cucurbits (Cantaloupe, Cucumber, Watermelon), Euphorbia, Foxglove, Hydrangea, Impatiens, Itea, Japanese holly, Lantana, Liriope, Okra, Pansy, Peach, Peanut, Pentas, Potato, Sasanqua camellia, Soybean, Spinach, Strawberry, Sweetpotato, Tobacco, Tomato.

Plants listed in bold are those with three or more diagnoses during the last six years. The plants most often diagnosed were boxwood (6 samples), cucumber (7), tobacco (5) and tomato (12). Of course this is due in part to the popularity of these plants in North Carolina.

Not all species of Meloidogyne are created equal. For example, all four of the most common species can reproduce on tomato, but only two of them are problems on peanut. Even within species there are races that differ in their host ranges.

Rather than repeat the very useful information on root-knot nematode damage and control from Charlotte Glenn’s previously cited article, I’ll just add a few comments. One is that if your interest is bedding plants, the University of Florida has a publication showing the relative susceptibility of various cultivars.

Charlotte mentioned how to tell a root knot from a legume nodule. Another challenge can be distinguishing between root knots and the normal storage swellings on roots of plants such as daylily, liriope and mondograss. In my experience, the average tuberoid swelling is larger than a root knot, but gall size can vary with the host plant. When in doubt, you can do a dissection under a magnifying glass or a stereomicroscope.

Root-knot galls (left) and swollen storage area (middle) on liriope roots.
The tip of a standard pencil (right) is for size comparison.

Choose swellings that are solid, then shave or pick off layers with a scalpel or razor blade. Look for the pearly white swollen posterior of the adult female nematodes. They are easily “popped”, so work carefully. With a little more effort you can lift out the entire nematode and see the narrowed head and esophageal region.

Two root-knot nematodes dissected out of their gall (arrows).
One is in hind view, the other in side view. Scale bar=0.4mm.

An interesting point to ponder at this time of year is how nematodes survive the “big chill” of January, especially in the absence of a host plant. The short answer is: eggs. Adult females are pretty much egg-producing machines, extruding them in a gelatinous matrix throughout the growing season. Egg masses can be seen on the gall surface if you look under magnification before washing the roots. They may be brown or white.

Brown masses on the surface of these galls are nematode egg masses.

An important part of the nematode’s reproductive “strategy” is that not all the eggs hatch as soon as conditions become favorable; some remain inactive. This inactive period is known as diapause and confers a survival advantage: if conditions improve only temporarily, the unhatched eggs will survive. Fortunately for our plants, root knot eggs will not survive more than about one year if they can’t hatch and infect a susceptible plant root. In this week’s sample, eggs at different stages of development could be seen in the same mass. Some eggs showed no signs of further development, while others already had active juvenile worms inside. In the photo below, you can even see the stylet of the unhatched juvenile. If you have doubts as to whether this nematode was alive, click here.

Meloidogyne eggs within an egg mass. Those at top and left are
undifferentiated. In the egg at right there's a well-developed juvenile.

There are limits, of course. Many root-knot nematode species will not survive the extreme winters of our northern states. That doesn’t mean that they don’t have nematodes up there. My first plant pathology professor, Dr. Dave MacDonald at the University of Minnesota, told us about a state trooper who found live nematodes after melting dirty snow from a blizzard that had picked up soil from points farther west.

There is so much more that can and should be said about these fascinating creatures. Here are two excellent print resources, used in the preparation of this blog. The third is an online introduction to the topic, from the American Phytopathological Society.

• Shurtleff, M.C., and Averre, C.W. 2000. Diagnosing Plant Diseases Caused by Nematodes. APS Press.
• Sasser, J.N., and Carter, C.C., eds. 1985. Meloidogyne: An Advanced Treatise. NC State University Graphics.
•  Mitkowski, N.A. and G.S. Abawi. 2003. Root-knot nematodes. The Plant Health Instructor. DOI:10.1094/PHI-I-2003-0917-01 Revised 2011  http://www.apsnet.org/edcenter/intropp/lessons/Nematodes/Pages/RootknotNematode.aspx

Note: Many other genera of nematodes are important pests of plant roots, although most don’t cause galls. These tend to live in the shadow of their highly destructive root-knotting cousins, and we’ll give them some attention in a future blog.

Mike Munster and Barbara Shew