Nematodes

Root-knot nematodes spend part of their life inside plant roots, and part in soil. Second-stage juvenile nematodes hatch from eggs that remain in the soil after crop harvest. They then move through the soil and penetrate and enter the roots of host plants. Once inside the roots they become immobile, and develop into females. Mature female root knot nematodes are pear-shaped and about 0.01 inch long. The roots are modified by the nematodes and start to form the typical root galls. Mature females resemble tiny, white pearls; they sometimes can be seen with the use of a hand lens when root galls are cut open. New eggs are produced by the females that either hatch into second-stage juveniles during the same growing season, or remain in the soil after harvest as inoculum for the next host crop. The time it takes the nematodes to go through their life cycle depends largely on the nematode species, soil temperature, and type of crop, but can be as short as 3-4 weeks.

Root knot nematodes cause characteristic galls on roots; galls may be up to 1 inch in diameter, but are usually smaller. These galls interfere with the flow of water and nutrients to the plant; infected plants appear less vigorous than healthy plants, may be yellowed, are prone to wilt in hot weather, and respond poorly to fertilizer. Damage areas usually appear as irregular patches and are frequently associated with lighter-textured soils.

Assess the population level and damage potential based on soil sampling or the history of injury in previous crops. Because root knot nematodes feed and multiply on many weed species, weed control is an important aspect of their management.

Planting a field to nonhost crops can help to reduce nematode populations. Long-term, bare soil fallowing will also reduce, but likely not eliminate, nematode populations. Again, weed control is essential for effectiveness of fallowing. There are no known resistant varieties.

Soil solarization can provide control of many soilborne diseases, nematodes, and weed pests. By itself, it may control root knot nematode in the upper soil levels but reinfestation can occur over the course of the season because eggplant is a long season crop. Thus, soil solarization is effective only for early season control. For further information, contact your local farm advisor or see Soil Solarization: A Nonpesticidal Method for Controlling Diseases, Nematodes, and Weeds, (PDF) UC ANR Publication 21377.

Grafting of eggplant onto root knot nematode resistant rootstock, usually Solanum torvum, is used in high value production systems in some European and Asian countries. This method is an effective way to prevent nematode damage and build-up, but costs are prohibitive for most California production systems.

Cultural control is acceptable for use on an organically certified crop.

Although soil sampling provides more detailed information, checking for root galls is a simple way to confirm that aboveground symptoms are caused by root knot nematode injury. Check the roots of a few plants in midseason or later, even if the crop appears healthy. Check earlier if you see wilting, poor growth, or other symptoms; galls may appear as soon as a month after planting. Carefully brush or wash soil from roots to look for galls. Be sure to check some plants in the sandiest part of the field, where damage is most likely.

Check the roots of rotation crops as well, but remember that galls will not be present on nonhost crops and may not be as obvious on other susceptible crops as on eggplants. If the field has been fallowed or planted to a nonhost crop, look for root galls on nightshades and ground cherries. Other weeds may also have galls but are less reliable indicators of root knot activity. However, absence of root galls on other plants does not necessarily mean the soil is free of nematodes that could injure eggplants.

Soil Sampling

Soil sampling provides the best basis for management decisions, especially in coarse textured soils. You can sample whenever the soil is in good working condition, but the best times are in spring before planting and in fall after harvest. Because of the gradual decline in populations over the winter and the gradual increase during summer, samples from fall and spring represent the high and low extremes of the population. Sampling at other times yields intermediate results that are more difficult to interpret.