Galls on cucumber roots caused by root-knot nematode. Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org
Wilting symptoms on cantaloupe caused by Root-knot nematodes. Affected plants often show symptoms of stunting, wilting or yellowing due to nematode feeding damage to the roots. Edward Sikora, Auburn University, Bugwood.org
Yellowing and death of plants infected with root-knot nematode. Edward Sikora, Auburn University, Bugwood.org
Do not confuse root knots caused by nematodes with nodules of nitrogen-fixing bacteria on legumes, such as soybeans (Glycine) shown here
Root-knot nematodes are microscopic, plant-parasitic roundworms in the genus, Meloidogyne. Normally they exist in sandy soil in hot climates or short winters, but they have spread to our region as well. In Missouri alone, 40 species of parasitic nematodes exist. About 2000 plants are susceptible to infection by these nematodes, and they can cause global crop loss. Root-knot nematode larvae infect plant roots that drain the plant’s ability to take up water and sufficient nutrients. Because of their microscopic size, the nematodes go unnoticed until serious symptoms appear. The pathogenic nematodes can allow entrances for root rots and wilts. They can also transmit viral diseases.
Symptoms and Diagnosis
A plant infected with root-knot nematodes often exhibits the same signs of nutrient deficiency. It may be stunted, yellowed, or wilted, and can lead to premature death of the plant. Below the ground, the roots of the plant are swollen or knotted with root galls, and unless a plant is removed from the soil to examine the roots, the cause of the poor plant growth remains undetected. The appearance of the galls on the roots can be confused with nitrogen-fixing nodules common in the legume family. However, it is easy to distinguish between them, as the nitrogen nodules can be easily rubbed off while the nematode galls cannot be removed.
The root-knot nematode is parthenogenic, that is a single female can reproduce without males and a new generation can occur every 28 days if conditions are ideal. Inside the gall, the enlarged female appears as a shiny white body, the size of a pinhead. She deposits 300 to 500 eggs in a protective jelly-like material. These glistening white to-yellow egg masses are present on the root surfaces. Juveniles emerge from the eggs in the soil and penetrate between and through cells at the center of the root, usually near the growing tip. These larvae actively feed and remain at this same site. The juvenile stage can over-winter even under very unfavorable conditions. New adult nematodes develop from the larvae and start the cycle again.
Integrated Pest Management Strategies
1. Never buy or plant any stunted-yellowed plants. When a new plant’s roots are examined and its roots contain root knots that will not easily rub off, do not plant it. If someone gives you a division of their plant whose roots contain these root-knots, never add it to your garden because it will contaminate all of your healthy plants with nematodes.
2. Keep plants healthy and avoid stressing plants and water plants during periods of high temperatures and drought. Also, keep other insect pests and fungal diseases under control. Control weeds as they can act as hosts to these nematodes. Keep tools clean.
3. Rotate crops, planting nematode-resistant plants when possible. It may take 3-5 years to kill off these nematodes before replanting the same plant.
4. Destroy any plants you discover that are infested with root-knot nematodes and don’t compost them. Replace all the soil before adding a new plant to this area.
5. Soil solarization has demonstrated some success in eliminating root-knot nematodes. Thoroughly moisten the soil and cover with clear plastic in full sun during the hottest months of the year. The soil must maintain these high temperatures for at least 4-6 weeks to be effective. Re-moisten the soil under the plastic if it dries out.
6. No chemical controls are advised for home gardeners. Some restricted-use chemicals are available for commercial, licensed applicators.
Strategies 1, 3, 4, and 5 are strictly organic approaches. Using appropriate organic insecticides and herbicides (or removal by hand) would be viable organic approaches to Strategy 2.