Nematodes

Nematodes are microscopic roundworms that live in diverse habitats. Plant parasitic nematodes live in soil and plant tissues and feed on plants by puncturing and sucking the cell contents with a spearlike mouthpart called a stylet.

Damage caused by nematodes is likely to initially become evident during the first year after planting. Feeding by nematodes can impair root functions such as uptake of nutrients and water. Root lesion nematodes penetrate into the roots and cause damage by feeding and migrating through the cortical tissues. Dagger nematodes feed from outside the roots, but can reach the vascular tissues with their long stylet and are capable of reducing vigor and yield of trees. Xiphinema americanum also transmits strains of tomato ringspot virus to apricots, but the disease is less severe and the symptoms less obvious than on peaches and almonds. Feeding by ring nematode increases the incidence of trees affected by bacterial canker (Pseudomonas syringae). Feeding by root-knot nematode produces root galls, which disrupt root functions.

The symptoms described below are indicative of a nematode problem but are not diagnostic as they could result from other causes as well. Lack of vigor, small leaves, dieback of twigs, and yield reduction are typical symptoms of nematode damage. Orchards infested with ring nematode may exhibit symptoms of bacterial canker and trees with root-knot nematodes may have galls on roots.

It is critical to know the nematode species present and to estimate their numbers to make management decisions. If a previous orchard or crop had problems caused by nematodes that are also listed as pests of apricots, expect nematode numbers to be high enough to cause damage to the young trees.

If nematode species have not previously been identified, take soil samples and send them to a diagnostic laboratory for identification.

1. Divide the field into sampling blocks of not more than 5 acres each that are representative of cropping history, crop injury, or soil texture.
2. Within each block, take several subsamples randomly from the frequently wetted zones at the edge of the tree canopy.
3. Take samples from within the root zone (6–36 inch depth) and include some feeder roots when possible. Mix the subsamples thoroughly and make a composite sample of about 1 quart for each block.
4. Place the samples in separate plastic bags, seal them and place a label on the outside with your name, address, location, information about the current and previous crop, and the crop you intend to grow.
5. Keep samples cool (do not freeze), and transport as soon as possible to a diagnostic laboratory.

Contact your farm advisor for more details about sampling, to help you find a laboratory for extracting and identifying nematodes, and for help in interpreting sample results.

Before fumigating, remove old trunks and large roots brought up by ripping and fallow or plant green manure cover crops for 2 to 4 years. Do not use cover crops that are known hosts of nematodes that feed on the rootstock you plan to plant; contact your farm advisor for additional information. Use certified nematode-free rootstocks or seedlings to establish new orchards. When the orchard is developed, use procedures that improve soil tilth and drainage to help reduce nematode damage.

Use certified nematode-free rootstocks. Among peach rootstocks, Nemaguard is known to be resistant to root-knot nematodes, but it is susceptible to ring and root lesion nematodes. Lovell is somewhat resistant to ring nematode but susceptible to root-knot and root lesion nematodes. The apricot rootstock Royal (Blenheim) seedling is resistant to root lesion nematode but susceptible to ring nematode. Most apricot rootstocks are nonhosts for root-knot nematode. Plum rootstocks Myrobalan 29C and Marianna 2624 are resistant to root-knot nematode, but susceptible to ring and root lesion nematode. Contact your local farm advisor for additional information on rootstock selection.

Trees planted on fumigated orchard sites are generally known to have improved growth and yields compared to those on nonfumigated sites. Preplant fumigate in fall when soils are dry and warm.