Symptoms of Dahlia Root Knot Nematode Damage
The primary symptom of root knot nematodes in dahlias is swelling or galls on the roots. The swellings make tiny, pimple-like bumps as large as an inch (2.5 cm.) across. If you aren’t sure, carefully dig the plant and shake off the loose soil to see what you’re dealing with. Dahlia root knot damage may also include yellowing of the leaves and wilting, especially during hot weather when the plant is water stressed. Galls on the roots make it difficult for the plant to absorb moisture.
Preventing and Treating Dahlia Root Knot Nematodes
Dahlia root knot nematodes are difficult to control and there isn’t much you can do. Professional growers use nematicides, but the chemicals haven’t been approved for home gardens. You may need to start over with new dahlias in an unaffected area of your garden. Be sure to look for nematode-resistant varieties. You can also take these preventative measures in the garden when planting dahlias:
Add a generous amount of manure, compost, or other organic material to the soil, especially if your soil is sandy. This won’t get rid of dahlia nematodes, but it will give the plants a fighting chance by getting more moisture to the roots.
Grow marigolds as a group throughout the summer. Most marigold varieties are known for controlling dahlia nematodes. However, avoid signet marigolds, as these may actually attract the nematodes you are trying to control.
You can try solarizing the soil as well. This is often helpful on a temporary basis. Water the infected area, cover it with clear plastic, and secure the edges. Leave the plastic in place for at least four to six weeks. Solarizing is effective only in hot weather.
