Nematodes

Soil-dwelling nematodes are typically elongated, unsegmented, microscopic roundworms that occur in virtually every terrestrial environment. The vast majority of these species feed on microorganisms and microfauna without harming plants. They are an important part of the soil food web and nutrient cycling processes. In contrast, plant-parasitic nematodes feed on plants by puncturing and sucking the cell contents, and can considerably reduce crop yield and quality. Most plant-parasitic nematodes have hollow, needlelike mouthparts called stylets. The stylet enables the nematode to puncture plant cells and inject various compounds, which support the feeding process and may influence host physiology and morphology. A nematode life cycle typically includes an egg, four juvenile stages, and the adult stages. The length of the life cycle depends on the nematode species and the temperature of its environment.

Stem and Bulb Nematode

Stem and bulb nematode live mostly within the plant, feeding on cells in stems, leaves, and bulbs. The species has many biological races with different host ranges. The race that infects onions and garlic requires mating to reproduce; a female lays 200 to 500 eggs within the host tissue. Optimal egg development occurs between 59° and 70°F. The life cycle takes approximately three weeks at 59°F. Both juveniles and adults are able to infect plants. Under moist, mild conditions (59° to 68°F), stem and bulb nematodes can move in a thin water film on the plant surface to enter their host through stomates. Multiple generations can occur during one season, potentially resulting in a huge increase in nematode numbers. They are capable of surviving drought for many years in or on dry plant material, only to become active again with rehydration.

Root-Knot Nematode

Root-knot nematodes live primarily within the roots of their hosts. Only the second-stage juveniles are motile, while the other stages are sedentary. At least four root-knot nematode species may cause damage in onions and garlic.

Root-knot nematodes have a large host range and complete their life cycle in 3 to 4 weeks under optimal conditions. Juveniles hatch from eggs in their second stage. They are able to infect host roots at soil temperatures exceeding 41°F (M. chitwoodi), between 55 to 57°F (M. hapla), or between 59 to 64°F (M. incognita, and M. javanica). After initial penetration, the nematodes move to a site near the vascular cylinder, where they establish a permanent feeding site. Once they have established this site, they become sedentary. After three further molts, the nematodes develop into slender, wormlike males that can leave the roots, or into pear-shaped females that stay at the feeding site.

Most root-knot nematode species can reproduce asexually, with the exception of M. hapla. The females lay several hundreds of eggs in a gel sack that provides protection against desiccation and predation.

Stubby Root Nematode

These nematodes live exclusively in the soil and feed on host roots. In contrast to the previously mentioned nematodes with hollow stylets, stubby root nematodes have a curved, solid spear that is used to puncture cells. The nematode secretes saliva from its mouth that hardens into a hollow tube, allowing the nematode to feed on cell contents. A new tube is formed each time the nematode moves to a new cell. Males are rare, as the stubby root nematode reproduces asexually.

Lesion Nematode

Lesion nematodes spend large proportions of their life history inside of hosts‘ roots. These nematodes move through root tissues destructively while feeding on plant cells. When conditions become less suitable, they may leave the host to search for more suitable roots. While lesion nematodes have been shown to reduce growth and yield of onion and garlic crops in other states, they have not been reported to cause problems on onions or garlic in California.

Field-level, aboveground symptoms of nematode problems often show as oval patches of stunted, thin plant stands. However, such symptoms may also be the result of other biotic and abiotic causes.

Stem and bulb nematodes can cause substantial crop damage and are of major concern in garlic and onion production worldwide. Migratory feeding by these nematodes creates cavities, leading to shortened, distorted or collapsing leaves and bloated tissues. The bulbs often soften at the neck. Cutting the bulb in half may reveal brown rings. Secondary invaders such as bacteria, fungi, and onion maggots may then cause soft rot and greatly accelerate the decay of the bulbs. Even during bulb storage at low temperatures, the nematodes may continue to develop, reproduce, and typically in combination with microbial contamination, damage the bulbs during storage.

Root-knot nematodes can also cause stunting and uneven stands of plants. The feeding process induces the characteristic knots or galls on the host's roots. Their size depends on the nematode species and the response of the host. Gall formation by root-knot nematodes impedes the uptake of nutrients and water that may lead to stunted growth. Nutrients produced by photosynthesis are diverted to sustain the nematodes instead of supporting plant growth. The feeding process also lowers the host's resistance to biotic and abiotic stress, such as the ability to deal with drought or microbial infections.

Stubby root nematodes feed primarily on the root tips of their host, which damages the meristematic tissues (growing root tissues). It causes the roots to remain very short. This impedes the ability of the host to take up nutrients and water, which reduces growth and bulb size.

Plants with lesion nematodes typically have fewer roots than noninfested plants, and may exhibit elongated, brown-to-black lesions along the roots.

Because nematodes are tiny, colorless, and transparent, they are not easy to detect or identify. To make appropriate management decisions, it is critical to accurately diagnose the nematode species present and know how numerous they are. If a previous crop in the particular field had problems from nematodes that are also listed as pests of onions or garlic, their numbers may be high enough to damage sensitive young seedlings.

If nematode species in the field of interest have not previously been identified, consider taking soil samples and sending them to a diagnostic laboratory for identification.

1. Divide the field into sampling blocks measuring between 5 and 20 acres each. Each block should have similar cropping history, soil texture, or history of recent crop injury throughout.
2. Collect several subsamples randomly from a block, and mix them thoroughly to make a composite sample for laboratory analysis, which should be about 1 pint of soil for each block. Use soil from within the root zone (approximately 2 to 14 inches deep) and include samples of suspect plants.
3. Place samples from each block in separate plastic bags, seal them, and place a label on the outside with your name, address, location, and the current/previous crop and the crop you intend to grow.
4. Remove the samples from direct sun exposure, keep them cool (do not freeze), and transport as soon as possible to a diagnostic laboratory.

Contact your local cooperative extension advisor for more details about sampling, laboratories that identify nematodes in soil samples, and how to interpret sample results.

Determine the cropping history of fields to be planted with seed garlic, onion transplants, or onion sets. The host range of the onion and garlic race of D. dipsaci (stem and bulb nematode) includes chives, leeks, certain lettuce cultivars, celery, hairy nightshade, and others. Growing nonhost and poor host crops such as corn in weed-free fields for at least three years will help reduce stem and bulb nematodes. However, such crops might be good hosts for other plant-parasitic nematodes.

Always use clean, noninfested cloves when planting garlic. Private laboratories or the California Department of Food and Agriculture can test garlic cloves to determine if they are infested with plant-parasitic nematodes.

Avoid infesting new fields by thoroughly cleaning machinery and equipment with water. Nematodes do not move far on their own; they are distributed mainly through infected planting material, irrigation or surface runoff water, contaminated equipment, and spread of infested soil.

There are currently no available onion or garlic cultivars with resistance to any nematode species.

If noninfested seeds are not available, treating bulbs with hot water has been shown to reduce stem and bulb nematodes in garlic cloves. However, hot water treatment is unlikely to be 100% effective and may also predispose plants to other diseases.

Preplant fumigation can effectively reduce plant-parasitic nematode infestations in onion fields.