Golf course notes
Why Are
Nematodes
— Aquatic Animals — a Problem in Turf? By Alan Henn, Ph.D., Professor and Extension Plant Pathologist, Mississippi State University
E
ven when soil has gotten so dry that plants are wilting and the spaces around the soil particles are filled with air, a film of water still surrounds each soil particle. Nematodes swim in that water, moving somewhat like snakes. While nematodes do not move very far in the soil horizontally (maybe an inch), they can move surprising distances vertically. In Mississippi, they seem to move toward the lowest levels of the putting green during cold weather and move higher in the soil when it is warmer. This means that soil samples taken during colder weather could underestimate the nematode numbers if only the upper four inches are sampled. That nematodes are found in the water means that a product targeting nematodes (a nematicide of some type, chemical or biological) must (1) move through the turf, (2) move through the thatch or stolon and root mass in ultra-dwarfs, (3) move into the water surrounding the soil particles and (4) enter the nematode through the cuticle. The need for water solubility is one of the major reasons we no longer have most of our “good” nematicides. Most 16 • Mississippi Turfgrass • Summer 2014
of them have been found contaminating ground and surface water around points of use. This should also caution us to use the products we do have wisely, by following label directions.
How do nematodes hurt turf?
Although really small (about 0.02 inches long), nematodes damage turf by feeding on the plants’ roots. Two factors about this feeding help us understand the damage we see: method of feeding and location of feeding.
Method of nematode feeding
Nematodes with the simplest feeding style simply poke a hole in a root cell using a “stylet.” Imagine a stylet to be something like a roofing nail with a tunnel (like a straw) along the axis. Once the stylet has penetrated the cell wall, the nematode injects digestive enzymes to liquefy the cell contents, which are sucked out through the “tunnel” or lumen in the stylet. The feeding of the root-knot nematode is much more complicated. The newly hatched juvenile pokes a hole in a cell and then burrows inside it to a favored feeding spot. Once there,
it secretes substances that modify the cells to make them larger and pull in more nutrients from the rest of the root. The female nematode stops moving and enlarges to look something like a balloon, the head being the tie area of the balloon. Her head moves from side to side, feeding on the swollen cells. The combination of the swollen cells and enlarged female make the small gall that can be seen on the turf roots. Dagger and sting nematode feeding methods fall between these two extremes — some cellular modification and some “eat and run.”
Feeding location
Root-knot nematodes and sting nematodes like to feed near the root tips. In some greens infested with sting nematodes, the end of the “stung” root can look similar to DNA herbicide injury. That is, the root tip may be slightly swollen or truncated. Rootknot-infected roots will be more swollen, ending in a “knot,” or may have a few knots along it. Since the nematodes are feeding near the root tip, the plant tries to put out side roots, which in turn are fed on by the