Nematodes

Plant-parasitic nematodes are microscopic roundworms that live in soil and plant tissues. They feed on plants by puncturing and sucking the cell contents with a spearlike mouthpart called a stylet. Of the several genera of plant-parasitic nematodes detected in California orchard soils, lesion and root knot nematodes are considered to be important factors in limiting plant growth. Dagger nematode can reduce growth, but is more important as a virus vector. Pin nematode is common, but effects on cherry have not been studied and it is not considered a pathogen on other stone fruits.

Damage caused by nematodes is likely to become evident during the first year after planting. Feeding by nematodes can impair root functions such as uptake of nutrients and water. Lesion nematodes penetrate into the roots and cause damage by feeding and tunneling through the root 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. However, X. americanum is more important on stone fruit trees as a vector of the Cherry rasp leaf virus, which causes rasp leaf disease, and strains of Tomato ringspot virus, which cause yellow bud mosaic, cherry mottle leaf, and Prunus stem pitting diseases. These virus diseases can reduce the productivity of the trees significantly, and infected trees may eventually die. Feeding by root knot nematodes causes swellings of the entire root, which impairs normal root functions. Stress resulting from nematode feeding, particularly by ring nematode Criconemoides (=Mesocriconema) xenoplax, can increase tree susceptibility to bacterial canker, Pseudomonas syringae.

Symptoms described below are indicative of a nematode problem, but are not diagnostic as they could result from other causes as well. Symptoms of a nematode infestation are reduced vegetative vigor and fruit yield, and unevenly sized trees. Heavily infested trees are more susceptible to moisture stress. Lesion nematodes may cause reddish brown lesions on roots that later turn dark and ultimately black. Root knot nematode produces characteristic galls (swellings) on roots.

It is critical to know the nematode species present and to estimate their population to make sound management decisions. If a previous orchard or crop had problems caused by nematodes that are also listed as pests of cherry, especially if they are capable of transmitting a virus pathogen of cherry, expect population levels to be high enough to cause significant damage to the young trees.

However, if nematode analysis has not been done, take soil samples to a diagnostic laboratory for identification. Divide the field into sampling blocks of not more than five acres each that are representative of cropping history, crop injury, or soil texture. Within each block, take several subsamples randomly from the frequently wetted zones at the edge of the tree canopy. Take samples from within the root zone (6 to 36 inch depth) and include some smaller roots when possible. Mix the subsamples 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. 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 find a laboratory for extracting and identifying nematodes, and for help in interpreting sample results. For preplant sampling, if any of the nematodes known to cause damage to cherries are present, there is a potential for them to cause problems following planting. The University of California has no established postplant thresholds for nematodes on cherries.

Before fumigating, remove old trunks and large roots brought up by ripping and fallow or plant green manure cover crops for 1 to 2 years (3 to 4 years if lesion nematodes are present). Do not use cover crops that are known hosts of nematodes that damage 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. Both Mazzard and Mahaleb are susceptible to Cherry rasp leaf virus and to the lesion nematode, Pratylenchus vulnus. Colt rootstock is the most susceptible to P. vulnus. Mazzard is immune to M. incognita and resistant to M. javanica. (Immune rootstocks are not attacked by nematodes, whereas resistant or nonhost rootstocks may be invaded by the nematodes and show damage, but do not allow population increases.) Mahaleb is resistant to M. incognita and susceptible to M. javanica. Stockton Morello is immune to M. incognita and susceptible to P. vulnus. Contact your local farm advisor to discuss rootstock options.

When planting or replanting an orchard, be sure to sample for nematodes, especially if the land was previously an orchard or a vineyard. If sampling indicates that any of the pest nematodes of cherry are present, plan to preplant fumigate using the following time schedule:

• Summer to Fall: Remove trees or vines, destroy residues, and deep cultivate to remove residual roots and break up cultivation pans or soil layering.
• Winter to Spring: Fallow or plant grains.
• Spring to Summer: Level (if necessary), cultivate, and do other operations required for next year's planting. Dry the soil.
• Late Summer to Early Fall: Rip the soil. You will be required to have surface moisture if applying Telone II. Fumigate preferably in September or October but before November 15. Do not apply chloropicrin or Telone II after mid-November.
• Winter to Spring: Observe waiting period on fumigant container label; plant young trees on resistant rootstock if root knot nematode is present.

Make a solid application of methyl bromide if the rootstock to be used has no resistance to ring nematode (Criconemoides xenoplax) or root lesion nematode (Pratylenchus vulnus) and sampling indicates either of these species is present. A solid application, when done properly, can provide control for up to 6 years.

If sampling indicates that only root knot nematode is present, or the orchard has soils that are not conducive to the development of high populations of ring nematode, or the rootstock being used is resistant to these nematodes, a strip or spot fumigation can be made. Strip or spot applications provide about 6 months of control.

Apply nematicides at the rates listed in the table below. Formulations with chloropicrin may be used where other diseases are present or because chloropicrin's odor helps to indicate the presence of the gas. Use the highest rate recommended for the soil conditions within the profile. For example, if a soil has a loamy sand surface layer with 5% soil moisture and a subsurface loam layer with 10% moisture, use the higher rate given for the loam. Do not plant for one month after tarps have been removed. If soils become cold (below 50°F) soon after treatment, an additional 30– to 60–day waiting period before planting may be necessary. Observe the waiting period on the fumigant container label, then plant young trees on resistant rootstocks, when available.

To determine application rates:

1. Using the soil and moisture chart, determine the number code based on your soil type and its percent moisture. Use the top number if treating with 1,3–D and the bottom number for methyl bromide treatments.
2. In the second table, use the number code to determine application rate. For example: If your soil is a loamy sand with 5% moisture, the number codes are 1 and 8. If using methyl bromide, the application rates for soil in the temperature range of 40–77°F are 200 lb/acre or 400 lb/acre, depending on the type of control desired.