Nematodes

Plant parasitic nematodes are microscopic nonsegmented roundworms. The beet cyst nematode parasitizes more than 200 plant species in many different families. The most important agronomical hosts belong to the Chenopodiaceae (e.g. beets, sugarbeets, spinach) and Cruciferae (e.g. broccoli, Brussels sprouts, cabbage, cauliflower, canola). The beet cyst nematodes over-winter in soil as eggs or as juveniles in their eggshell within cysts. These dark brown, lemon-shaped encasements are the remainder of the female bodies. They contain 100 to 300 eggs that can remain dormant in soil for many years. Juveniles start hatching from the eggs and moving out of the cyst when soil temperature (greater than 50°F) and moisture are suitable. Hatching is promoted by the presence of root exudates from host plants. After entering the spinach root, a juvenile establishes feeding sites near the central cylinder by injecting compounds into surrounding root cells. These cells are transformed for the purpose of supporting the nematode for the rest of its life with nutrients.

In the following weeks, the nematode undergoes three molts before it matures as either male or female. The developing females swell until their back ends break through the root epidermis while their front part remains in place and continues to feed. Careful external examination of the roots reveals the pin-head sized, white bodies of the females. The adult males remain wormlike and move out of the root. The females excrete compounds (pheromones) that attract the males. Soon after mating, the males die. The females start producing eggs that remain mostly within their bodies. The body wall of the mature female thickens and tans. The females die during this transformation into tough, brown cysts. The cysts fall off the roots and protect the contained eggs against desiccation, predators, and microbial degradation. Under normal plant-growing conditions, the development rate of nematodes is strictly temperature dependent. To complete the life cycle, the beet cyst nematode requires about 4 to 7 weeks at soil temperatures between 18°C/65°F and 24°C/75°F (400 degree days; baseline 10°C).

Spinach is susceptible to other plant parasitic nematodes (e.g. root-knot nematodes, lesion nematodes, needle nematode) but economically significant damage has rarely been observed.

Infestations with the beet cyst nematode may be fairly regularly distributed throughout a field or, more commonly, may occur in several randomly distributed, localized areas. It typically takes several years before the population densities increase to levels that affect plant growth and cause obvious aboveground symptoms. Fields will have oval-shaped spots with spinach plants in the center most severely affected. The spinach appears small, chlorotic, and stunted. The symptomatic areas may be elongated in the direction of tillage operation (soil movement) and furrow irrigation. In heavily infested soils, spinach seedling emergence may be delayed or seedlings may be killed before emergence, resulting in a reduced stand. Root-knot nematodes are known to increase seedling susceptibility to damping-off and other soilborne fungal diseases.

Aboveground damage symptoms are unspecific and may be caused by other biotic or abiotic stress. If nematode species have not been identified previously, take soil samples to a diagnostic laboratory to be analyzed.

Once a field is infested with plant parasitic nematodes, it is almost impossible to eradicate the population. Therefore, sanitation should always be the first defense to avoid nematode problems. Although an infective juvenile can potentially migrate on its own a short distance through soil, dissemination of plant parasitic nematodes occurs primarily through passive movement of infested soil by machinery, tools, wind, water, crop residues, or animals. Thorough cleaning of farming equipment with high-pressure water between fields reduces the chance of dissemination of nematodes and other soilborne diseases.

Rotations with crops that are nonhosts of the cyst nematodes will decrease the pathogen population. As a general guideline, the nematode population is expected to drop by 30 to 50% after each nonhost crop. Weed management during rotations is extremely important as many weeds can act as hosts as well. This includes mustards (Brassica spp.), shepherd's-purse (Capsella bursa-pastoris), charlock (Sinapis arvensis), lambsquarters (Chenopodium album), purslane (Portulaca oleracea), black nightshade (Solanum nigrum L.), common chickweed (Stellaria media L.), redroot pigweed (Amaranthus retroflexus) and buckwheats (Polygonum spp.).

Consider a 3 to 4 yr rotation with nonhost crops or preplant treatment with soil fumigants if beet cyst nematodes have caused widespread problems in the recent past. An economic threshold for the beet cyst nematode has not been established in spinach. Population densities exceeding 300 eggs/100 cm³ soil may cause damage depending on soil temperature and plant susceptibility.

Delay fall seeding of spinach in the inland valleys after a root-knot nematode susceptible crop until soil temperatures decline below the nematodes activity threshold (less than 65°F).

In recent years the cultivation of certain Brassica trap crops have gained some interest for reducing beet cyst nematode populations, especially when used in combination with biofumigation or solarization. These trap crops attract the beet cyst nematodes to enter their roots but do not support their reproduction. Incorporation of the Brassica tissues into the soil and their pathogen-suppressive effect during decomposition are termed biofumigation.

Soil solarization can reduce many soilborne diseases, nematodes, and weeds. It requires a 4- to 6-week treatment during the hottest time of the year. The efficacy against plant parasitic nematodes is at best limited to about 12 inches in soil depth and is much reduced in cooler coastal areas. For more information, see Soil Solarization: A Nonpesticidal Method for Controlling Diseases, Nematodes, and Weeds, UC ANR Publication 21377.