Nematodes

Nematodes are microscopic roundworms that live in diverse habitats. They feed on plants by puncturing and sucking the cell contents with a spearlike mouthpart called a stylet or spear. Of the several types of plant parasitic nematodes detected in California orchard soils, the above species are considered to be the most important.

Damage caused by nematodes may become evident as early as the first year after planting, and the damage they do can reduce growth by as much as 10 to 20%. However, if greater than that amount of damage is observed we attribute it to one or more of the other three components of the replant problem. In replant settings involving peach rootstock, the bulk of first year damage is caused by the rejection component of the replant problem and occurs with or without the presence of nematodes.

Feeding by root knot nematodes can impair root functions such as uptake of nutrients and water. With minor exceptions, root knot nematodes penetrate at the root tip and within 2 weeks stimulate formation of giant cells just as the vascular tissues of the plant are also becoming apparent. Giant cells are enlarged cells that display multiple nuclei and dense cell walls; they become disruptive of normal vascular function as photosynthetic products are disproportionately diverted into these nematode-feeding sites. Root knot nematodes have been implicated in nectarine disease complexes with fungi and bacteria; for example, Meloidogyne javanica is reported to increase the incidence of crown gall on nectarine roots. Lateral roots may fail to develop normally as they emerge because of feeding damage, and there is eventually a notable reduction of roots plus plentiful galling along infected roots.

An important concern with Mesocriconema xenoplax is that it is the ring nematode that predisposes trees of Prunus spp. to bacterial canker. High population levels of this nematode result in greater incidence of bacterial canker, and these elevated population levels are directly related to abundance of large pore spaces within soil. Ring nematodes remain external to the root system as they introduce their lengthy spear into nectarine roots and develop a cellular feeding tube that surrounds the spear. After a week or two the feeding is complete and the smallest roots, their favorite feeding site, are dead. During the first year after planting as much as 85% of the smallest roots can be notably absent. These fine roots do more than supply the tree with nutrients; they are also a source of various plant hormones. Trees infested with ring nematode can also show reduced vigor, but even vigorous trees can display bacterial canker.

Root lesion nematode, specifically Pratylenchus vulnus, penetrates nectarine root surfaces and can also enter roots to cause damage as they feed and migrate through root tissues. At high population levels they create channels within the root as they migrate. This impact can provide easy access for bacteria and fungi, but it is also notable that there are fewer large roots on the tree if the infection occurs in the early years of tree growth.

Dagger nematodes feed from outside the roots but can reach vascular tissues within the root by using their long stylet. If population levels are high enough and there are other stress agents present, they are capable of reducing vigor and yield of trees. However, the main damage caused by the dagger nematode, Xiphinema americanum, is that it vectors a strain of tomato ringspot virus that causes peach yellow bud mosaic, which can debilitate and kill trees. California has few examples of this virus because of regulations that clean nursery stock be provided to growers. At the present time this nematode is not as important to nectarine growers as the others mentioned.

The symptoms described below are indicative of a nematode problem but are not diagnostic because they are general effects of damage to roots and can result from either biotic or abiotic causes.

Symptoms of root knot nematode infestation are reduced vigor and yield, patches of unevenly sized trees, and characteristic galls on roots. Heavily infested trees are more susceptible to moisture stress. Ring nematode infestation can reduce tree vigor but is most noteworthy because predisposes trees to bacterial canker complex, which can cause sudden collapse of limbs or entire trees in spring. Root lesion nematodes reduce overall root presence and tree vigor; upper shoots may dieback as a result. Symptoms of dagger nematode infestation include reduced growth and vigor; if the tomato ringspot virus has been transmitted, yellow bud mosaic disease symptoms may also be seen. Trees newly infected by virus have irregular chlorotic areas and vein clearing (mosaic) on leaf blades. In the second year of infection, the yellow bud phase is expressed in spring as extremely stunted tufts of pale yellow leaves.

It is critical to know which nematode species and the general population levels in order to make rational management decisions before planting. If a previous orchard or crop experienced nematode problems and the intended nectarine rootstock is listed as susceptible to those nematodes, expect population levels to be high enough to cause damage to the young trees and make management decisions accordingly.

If the species of nematodes present in the soil have not previously been identified, take soil samples and send them to a diagnostic laboratory for identification. Divide the field into sampling blocks of not more than 5 acres each that are representative of cropping history, different crop vigor, or soil texture. Within each block, take several subsamples randomly from the frequently wetted zones at the edge of the tree canopy or in the drip zone in the case of drip-irrigated orchards. Take samples from within the root zone (6- to 36-inch depth), and include some feeder roots when possible. Mix the subsamples gently but 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. It is a good idea to place a duplicate label on a moisture-resistant plastic pot marker inside the bag. 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.

In replant settings the available choices include: 1) Fumigate the soil if the prevailing nematodes are damaging and no nematode resistance is available. 2) Fumigate the soil in the planting strips if rootstocks with resistance are available and the setting is a replant. 3) Avoid fumigation by planting 4 years of non-woody crops that are not nematode hosts. 4) Another method currently under study is to utilize 'starve and switch' in lieu of fumigation. Starve the old soil ecosystem by applying glyphosate (Roundup) herbicide to cut stumps and then removing the trees after 60 days. One full year after the glyphosate application, replant on a rootstock with tolerance to the rejection component of the replant problem as well as resistance to prevailing nematode pests. Our best example for Prunus spp is to replant with Hansen 536 rootstock following Nemaguard unless the soil is sandy, which may lead to the development of large numbers of ring nematode and potentially bacterial canker. More research is needed to find alternative rootstocks suitable for nectarine growers to switch to following Nemaguard.

In established orchards there are currently few effective nematicides available. Enzone is effective against ring nematode in highly porous soils at 1000 ppm applied during cool time periods, before May 1 and after mid-October. Focus on nematode control decision-making before replanting. In finer textured soils the degradation rate of Enzone is so rapid (about 50% per day) that penetration among soil particles can be inadequate.

Cultural practices

When replanting, removal of old trunks and any large roots from the previous orchard is important if the fumigation is to be shank applied. If the field is not to be fumigated, one must make extra effort to bring deeper roots to the field surface by ripping and fallow. Smallest roots harbor the greatest number of nematodes within.

Application of a glyphosate herbicide just after harvest can greatly reduce incidence of root knot nematode within roots but will not reduce presence of root lesion nematodes. Root killing will however destroy the food source for ectoparasitic nematodes as well as the food source for the entire soil biology. This starvation tactic followed by one full year of waiting is partially useful when replanting without soil fumigation.

If a cover crop is to be planted during the waiting period, choose one that does not host prevailing nematodes. Sudangrass is a good summer choice. Barley, Merced rye, Blando Brome, or Salina Strawberry Clover are useful `nematode safe' choices for fall seeding. Contact your farm advisor for further information on the nematode host status of cover crops or rotation crops.

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. Where nematodes are a problem and root systems have become reduced, apply irrigations and nutrients more frequently.

In nematode-infested orchards the addition of organic matter can improve water-holding capacity and improve soil tilth; both actions help to alleviate tree stress and thereby the symptoms of nematode damage. The favored organic matter depends on proximity to weed-free feedlots, compost yards, or preference for a particular cover crop. Small amounts of short-lived nematicidal properties may also be present in some of these organic choices, but their primary benefit is soil improvement. By the same token, more frequent irrigations are a side benefit of low-volume irrigation systems, and this tactic can reduce tree stress similar to that achieved with cover crops.

Rootstock Selection

Nematode-free rootstocks are commonly available as a result of the NIPM #7 program of the California Department of Agriculture. Ninety percent of the nectarine industry in California is planted on Nemaguard rootstock. After half a century, Nemaguard's resistance to all root knot nematode species has not been broken apparently because of a mechanism that destroys the ability of root knot nematode females to reproduce after they have entered roots and established their feeding site.

In recent studies of forty-five potential Nemaguard replacements collected from around the world, the ability of Nemaguard rootstock to host root lesion nematode is less than that of two-thirds of these potential replacements. Nemaguard root systems are, however, damaged by ring nematode, and it is among the most difficult to successfully replant because of what is termed the "rejection component" of the replant problem. The rejection component results in very poor orchard development beginning from first leaf and lasting as long as one year. After the first year the growth of the trees unevenly improves unless the trees have been overwatered, organic matter has been added, or damaging nematode pests are present.

Lovell seedling rootstock supports about half the ring nematode numbers as Nemaguard and is notably more tolerant of the bacterial canker complex. Its host status for root lesion nematode is similar to that of Nemaguard, but it is completely susceptible to root knot nematodes unless Dactylella oviparasitica, a fungal biocontrol agent that parasitizes the eggs of Meloidogyne spp., is present and active. If the ground has been fumigated adequately to give several years of root knot nematode relief, this fungus performs quite well in soils south and east of Fresno but not as far south as the Hanford area. While attempts to transport this fungus to other fields have not been successful, when it is present, the population levels of root knot nematode are gone from Lovell seedlings by the fourth leaf.

Marianna 2624 and Myrobalan 29C are resistant to root knot nematode. They impart slight tolerance to root lesion nematode, but as with most plum-type rootstocks, are highly susceptible to ring nematode; therefore, the bacterial canker complex is of great concern where these rootstocks are used and the soil is coarse textured.

Viking is a relatively new rootstock that for most months of the year displays resistance to ring nematode on par with Lovell. However, during the warmest months it becomes a host of ring nematode. In the presence of bacterial canker it offers relief almost as good as that from Lovell. It is almost as resistant as Lovell to root knot nematode and comparable to Nemaguard against root lesion nematode.

When to treat established orchards. Established orchards suffering from nematode problems are those from which fruit size and numbers are reduced or in sandier soils tree limbs are dying because of bacterial canker complex. As new postplant nematicides become available, determine their effectiveness by leaving several trees or rows of trees untreated for comparison.