Nematodes

Plant-parasitic nematodes are microscopic roundworms that can impair a crop’s functioning. Most nematodes spend their entire life in soil surrounding roots, while others reside primarily inside plant tissues. They feed by piercing living cells and sucking up the contents with a retractable, hollow, needle- mouth stylet. A typical life cycle includes several distinguishable developmental phases: egg, four juvenile stages, and the adult stage. The life cycle duration is species-specific and primarily influenced by the environmental temperature. Most species are active between 60° and 86°F. They survive during cooler seasons as eggs or juveniles.

Root-knot nematodes are by far the most important plant-parasitic nematodes in celery. They have a lifestyle classified as sedentary endoparasitic. Each female produces an egg mass that contains several hundred eggs embedded in a protective gelatinous matrix. A first-stage juvenile develops in each egg and molts into a second-stage juvenile. It hatches and invades its host close to the root tip. It establishes a feeding site near the central root cylinder. There it remains for the rest of its life through three additional molts to the adult stage. At soil temperatures of 77° to 82°F, the optimal range for the development of root-knot nematode species, they complete their life cycle within 3 to 4 weeks.

While feeding, root-knot nematodes inject compounds into the root cells that induce abnormal changes in the plant’s morphology and physiology. The cells enlarge, causing the root to form characteristic galls or knots. Galls interfere with the plant's normal water and nutrient transport, which causes below- and aboveground symptoms such as stunted growth, chlorosis (yellow leaves), and reduced yields. Severely galled crops often wilt earlier or more  than healthy plants, even when soil moisture is adequate. Affected plants may occur in oval patches stretched in the direction of pre-season soil movement by plowing or bed-shaping. Crop damage is most significant in warm regions or seasons with coarser-textured soils such as sand or sandy loam. Apart from the root galls, most of the described symptoms are not diagnostic and may be related to other abiotic and biotic problems, such as fungal diseases, irrigation problems, or nutrient deficiencies.

The severity of crop damage is related to the nematode species, their population density at planting, and soil temperature. It is critical to know these factors to make rational management decisions. If a previous crop had problems caused by root-knot nematodes, population density may be high enough to damage celery seedlings.

The microscopic, transparent nematodes are difficult to detect and to identify. Specialized commercial labs offer diagnosis and population enumeration crucial for risk analysis and devising potential integrated pest management strategies.

Soil Samples

• Divide the field into sampling blocks of under 5 acres, each block representing representing cropping history, crop injury, or soil texture.
• Randomly take several subsamples from within the root zone (4 to 18 inches deep) throughout each block.
• Mix all the subsamples from a single block thoroughly to make a composite sample of about one pint for each block.
• Place the composite samples in separate plastic bags, seal them, and place a label on the outside with the following information: your name, address, field location, and the current/previous crop, and the crop you intend to grow
• Place plants suspected of having root-knot infestation in a separate plastic bag.
• Ideally, keep samples at 54° to 59°F in a cooler and transport them as soon as possible to a diagnostic laboratory. Do not freeze the samples!

Contact your farm advisor for more details about sampling, finding a laboratory for extracting and identifying nematodes, and interpreting sample results.

Currently, no nematode-resistant cultivars are available. Implement the following cultural practices for nematode management:

• Rotate celery with non-host or poor host crops to reduce population levels of various root-knot nematode species. For example, strawberries are a poor host for most California root-knot species except for the Northern species (M. hapla). Proper nematode management in a rotation crop can considerably benefit the following celery crop.
• Manage weeds in crop rotations; many common weeds are good hosts to root-knot nematodes. 
• If possible, transplant early.
• Plant in cooler weather if possible. When soil temperatures are below the nematode's activity threshold (62° for M. hapla and 65°F for M. incognita), planting celery transplants reduces early root damage and slows nematode population increase.
• Thoroughly clean machinery and other equipment with water between fields to avoid transferring root-knot nematode to uninfested fields.

Currently, there are no damage thresholds for root-knot nematode on celery. If root-knot nematodes are present, consider applying soil fumigants or contact nematicides to mitigate crop damage.