DISEASES OF ALFALFA - ROOT KNOT NEMATODE

Nematodes are microscopic worms that attack the root systems of plants, causing death and/or distortion of root tissue. The most common type of nematode found on alfalfa (and clover) is the root knot nematode, Meloidogyne spp. This nematode burrows into the interior of the root. Once inside, females set up housekeeping at a single location, their bodies swelling as they prepare for reproduction and egg-laying. As they feed, their saliva stimulates root cells to grow very rapidly and to an abnormally large size. This overgrowth of root tissue leads to the formation of a gall. If galling becomes severe, it may cause problems with water and nutrient movement in the vascular tissue, and lead to stunting or yellowing of plants. To compensate for loss of root function, infected plants (on the left in this photo) produce additional root tissue leading to root systems that are bushier than those of non-infected plants (on the right in this photo).

Root knot nematode galls on alfalfa roots (and on clover) are typically irregularly shaped, relatively small (~1/8 – 1/4 inch in diameter), and located in the middle of a root (see arrows). People often confuse galls with the nodules formed on roots by the nitrogen-fixing bacterium Rhizobium. Nodules, while similar in size, tend to be elongate or fan-shaped, and are attached to the side of the root. If cut open, nodules are pink inside, while galls are white.