Nematodes

Plant parasitic nematodes are microscopic roundworms that feed on plant roots. They live in soil and plant tissues. Of the many genera of plant parasitic nematodes detected in soils from apple orchards, two species of root lesion nematode, four species of root knot and the dagger nematode are believed to be the most economically important ones in California. Both species of root lesion nematode, Pratylenchus vulnus and P. penetrans are widely distributed throughout the state. Meloidogyne hapla is found throughout the state in association with crops such as alfalfa. Meloidogyne incognita, M. javanica, and M. arenaria occur throughout the warmer apple growing regions, with M. incognita having the most common occurrence. Dagger nematodes occur throughout the state, but are believed to be more widespread in the northern part.

Root lesion nematodes penetrate into roots and cause damage by feeding and migrating through the cortical tissues. Interaction of root lesion nematodes with other soilborne organisms can increase injury to roots. They are occasionally associated with the apple replant disease, which is characterized by poor growth of young trees after transplanting. Feeding by root-knot nematodes can impair root functions such as uptake of nutrients and water. Dagger nematodes feed from outside the roots, but can reach the vascular tissues with their long stylet. They are capable of suppressing growth of young trees, but the major problem caused by them is transmission of tomato ringspot virus, which causes apple union necrosis and decline, especially on the variety/rootstock combination Red Delicious and M106 rootstock.

The symptoms described below are indicative of a nematode problem, but are not diagnostic as they could result from other causes as well. Damaged trees generally occur in a circular area within the orchard. Aboveground symptoms of nematode damage are lack of vigor, twig dieback, and decline in growth and yield. Infestation of older trees also results in chlorosis or yellowing of leaves, orange bark, fruit sunburn or sunscald typical in green varieties, and small fruit. Heavy infestation on young trees may result in stunting, and sometimes death. Nematode infestations may occur without inducing any aboveground symptoms.

Below ground symptoms include poor growth of feeder roots or main roots and soil adhering to roots. Root-knot nematode infestation will produce characteristic swelling of roots, called galls.

To make management decisions, it is important to know the nematode species present and their population densities. If a previous orchard or crop had problems caused by the same species of nematode that are listed as pests of apple, population levels may be high enough to cause damage to young trees. If nematode species have not previously been identified, soil samples should be taken and sent to a diagnostic laboratory for identification.

Soil samples should be taken from within the root zone (6- to 36-inch depth). Take subsamples from the frequently wetted zones at the edge of the tree canopy, and include some feeder roots when possible. Divide the field into sampling blocks of not more than 5 acres each that represent cropping history, crop injury, or soil texture. Take several subsamples randomly from a block, mix them thoroughly and make a composite sample of about 1 quart (1 liter) for each block. Place the samples 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. (SeeIntegrated Pest Management for Apples and Pears, UC/ANR Publication 3340, for more details.) Keep samples cool (do not freeze), and transport as soon as possible to a diagnostic laboratory. Request a species identification if either root lesion or dagger nematodes are found. 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.

Remove old roots and plant green manure cover crops resistant to root lesion nematodes (such as the oat cultivar Saia) for 1 to 2 years or fallow the site for 4 years. If Pratylenchus vulnus is present, maintain a poor host for this nematode, such as tall fescue, red fescue, or perennial ryegrass as orchard ground cover. These crops are hosts for Pratylenchus penetrans and should not be used if this species of root lesion nematode is present. Use certified rootstocks or seedlings to establish new orchards. Improve soil tilth and drainage and control other pests. Proper irrigation and fertilizer application will also reduce stress on trees.

Rootstock selection

Most standard rootstocks and some dwarfing rootstocks are believed to have some tolerance to P. penetrans, the root lesion nematode species that is frequently found in apple orchard soils in northern California. However, the dwarfing rootstocks are considered to be susceptible to P. vulnus, which is common in the orchard soils of the San Joaquin Valley.

Always buy trees from California nurseries, which are certified nematode-free; do not purchase trees from out-of-state nurseries unless they are certified nematode-free.

Trees planted in infested orchard sites that have been fumigated generally have improved growth and yields compared to those on nonfumigated sites.