Bluegrass is a soft, dark green perennial grass of particular importance in the northern United States for use in lawns and for forage. Because bluegrass is a cool-season grass, it is less susceptible to nematode problems than grasses grown in the warmer areas of the South, according to the Cornell University Plant Disease Diagnostic Clinic. However, nematode infections remain possible and, once underway, can cause significant damage to bluegrass.
Nematodes are tiny parasitic worms that occur in the soil. A handful of soil can contain thousands of them, according to the University of Nebraska-Lincoln, and most are harmless to plants---some are even beneficial. However, a few species parasitize the roots of plants, either feeding on the outside of the roots or entering the root tissue. Parasitic nematodes can affect turfgrass, including bluegrass.
Several types of nematode afflict bluegrass. The University of California Integrated Pest Management identifies the root knot, ring and spiral nematodes as particular pests of bluegrass. University of Florida nematologist Dr. William T. Crow also notes that the root gall and stem gall nematodes can cause problems for bluegrass.
When nematodes attack the roots of bluegrass, they affect the plant's ability to take up water and nutrients from the soil. This increases the plant's susceptibility to other stressors in its environment. It cannot extract the water and nutrients they need, making it more susceptible to secondary infections. On hot days, the sun evaporates water from the grass faster than the roots can replenish it, causing the grass to grow thin and yellow. In order to maintain attractive turf grass, the stresses nematodes put on plants often encourage homeowners and groundskeepers to add more fertilizer and increase irrigation to compensate, both of which can harm the environment.
If your bluegrass appears yellow, wilted or thin, nematodes might be the problem. Digging up some of the grass may reveal galls---small swellings on the roots---and poorly developed, discolored roots. Because parasitic nematodes are too tiny to distinguish with the naked eye, you will need to test your soil with a nematode assay to confirm the diagnosis.
Before planting sod, make sure that you have it tested for nematodes, advises Dr. Crow. He also suggests raising the cut height when mowing, which reduces stress on the grass and helps it combat nematodes. If your grass is already affected, chemical, plant-based and biological nematicides can help bring the problem under control. The best choice of control measures depends on the precise diagnosis.