Nematodes

Plant-parasitic nematodes are microscopic, unsegmented roundworms that feed on plants by piercing and feeding on cell contents with a spearlike mouthpart called a stylet. The nematodes that parasitize plum and prune live in the soil and roots. Multiple species may occur in the same orchard and can feed on other plants too. Pin nematodes (Paratylenchus spp.) are also frequently found in plum and prune orchards, but do not appear to cause problems.

Ring nematodes feed on roots, particularly on small feeder roots, which stresses the trees and predisposes them to bacterial canker (Pseudomonas syringae). Dagger nematodes reduce tree vigor with their feeding, but more importantly they can vector tomato ringspot viruses, which cause prune brownline. Root lesion nematodes damage roots by moving through cortical tissues and feeding in these areas. Root-knot nematodes occupy a single feeding site inside a root and, after triggering the plant response of galling, remain there for the rest of their life.

The symptoms below may suggest a nematode problem but are not diagnostic because they can be confused with other problems.

Nematode feeding stunts roots, reduces the tree’s ability to take in water and nutrients, and creates entry points for diseases. Nematode-infected trees may have poorly developed root systems which reduces the overall vigor of the tree. Above ground symptoms of nematode damage on individual trees include reduced overall vigor, small leaves, twig dieback, and yield reduction.

Nematodes are usually unevenly distributed throughout an orchard resulting in patches of low vigor trees. This growth pattern may indicate a nematode infestation. Infestation can be compounded by soil texture differences that may stress the tree for abiotic reasons. For example, trees in a sandy streak across the orchard may be challenged for availability of water and nutrients, but in addition ring nematodes and root-knot nematodes thrive in these areas and cause additional damage. Orchards infested with high numbers of ring nematodes are at high risk for symptoms of bacterial canker including blighted buds, blossoms, and leaves. Cankers can develop and cause girdling, which can kill limbs and entire trees. Trees on plum rootstocks tend to host higher numbers of ring nematodes. Peach rootstock ‘Lovell’ has some tolerance to ring nematode infection but is highly susceptible to root-knot nematodes.

When managing nematodes, it is important to identify the species present. If a previous orchard or crop had problems with one of the nematode species listed as pests of plum or prune it is likely that without remediation the problem will persist in a subsequent orchard. If the species present has not been identified, take soil samples and send them to a diagnostic laboratory for identification.

To sample for nematodes (see video Soil Sampling for Nematodes in Walnut and Almond Orchards, Note, this video is completely relevant to plum and prune orchard sampling for nematodes):

1. Communicate with the chosen diagnostic laboratory that the nematodes listed above as pests need to be determined. The laboratory can use appropriate extraction techniques and can also give you guidance if your soil type requires specialized handling. Request a species diagnosis if root lesion or dagger nematodes are found.
2. Divide the orchard site into sampling blocks that represent differences in soil texture, drainage patterns, or cropping history, but are no larger than 5 acres in size.
3. Take soil and small feeder root samples from within the root zone with soil moisture. Typically, the drip line of the tree canopy is a useful guide to actively growing parts of the root system where the highest nematode activity is expected. Take samples between 4 and 24 inches below the soil surface. For diagnostic samples, collect subsamples from the soil around trees that show symptoms and separate subsamples from the soil around adjacent, healthy looking trees for comparison.
4. In a fallow field, collect subsamples from at least 5 to 10 locations within the sampling block depending on the uniformity and size of the area. Depending on the length of the fallow period sampling depth needs to be determined. If extended dry periods during a summer fallow have preceded the sampling, soil from depths that contain soil moisture needs to be collected.
5. In a fallow field, take the subsamples randomly, attaining a representation of the sampling block and mix them thoroughly in a container to make a composite sample. About one quart of soil is needed for each sample.
6. Place the samples in separate plastic bags, seal them, and place a label on the outside with your name, address, location, the current or previous crop, and the crop you intend to grow.
7. Keep the samples cool (do not freeze) and out of direct sunlight, as drying out and heat exposure will prevent proper extraction of nematodes from soil. Transport the samples to a diagnostic laboratory as soon as possible.

The following measures help prevent the spread of nematodes to uninfested fields:

• Use certified planting stock.
• Clean soil from equipment before moving between orchards.
• Do not reuse irrigation tail water.

Whenever possible, plant new orchards in land that has previously been planted with nonwoody crops for at least 5 years. Take care with crop choice because of the wide array of alternate hosts. For example, California-bred alfalfa cultivars can be a good choice except where ring nematode develops well, such as in highly porous soils. Many legumes are hosts for ring nematodes. Root lesion nematodes will survive within dead tree root tissue or soil, with 5 percent remaining 5 years after tree removal. Root-knot nematodes feed on a wide diversity of crops. While proper crop choice may reduce some nematode numbers, the high reproductive potential of most species renders this strategy incomplete. Monitoring for nematode numbers remains recommended.

Consider the use of resistant rootstocks. Because rootstocks available for plum and prune differ in their resistance to specific nematodes, it is important to select rootstocks that are resistant to the species of nematode present in the soil.

‘Nemaguard’ peach rootstock is resistant to root-knot nematodes (Meloidogyne incognita, M. arenaria, M. javanica) but susceptible to root lesion and ring nematodes. Peach rootstock ‘Lovell’ has some tolerance to ring nematode but is highly susceptible to root-knot nematodes. Plum rootstocks (‘Marianna 2624’ and ‘Myrobalan 29C’) tend to be least damaged by root lesion nematodes but are susceptible to damage by ring nematode. The newer rootstock ‘Krymsk 86’ offer some vigor benefits, but it is susceptible to nematodes.

For a nematode-infested site where a plum or prune orchard will be planted following a previous woody crop, follow the following procedure to prepare the area for fumigation. If the previous crop was an annual crop, the procedure starts in the second year after soil sampling for nematodes.

Orchards Following Woody Crops

• Summer to Fall: If plant-parasitic nematodes are suspected, soil samples should be taken from the rootzone of the existing crop, before its removal, to increase chances of detection. In the traditional process, plant material including as many residual roots as possible are removed from the field. Newer research in almond has demonstrated that the tree residues can be shredded after drying and placed back into the orchard. Considerations need to be given to fertility (nitrogen requirements), but disease issues have not been found to be exacerbated. The effect on nematode diseases is still under investigation. Currently this strategy has not been tested for plum or prune. To achieve optimum nematode control, deep cultivate to 5 feet to break up cultivation pans and soil layering, cultivate, and level.
• Winter to Spring: Both sandy soils and finer-textured soils may benefit from a cover crop to suppress weeds and add organic matter to the soil. In fine-textured soils a summer crop of deep-rooted grasses (e.g., sudangrass ‘Piper’) enables deep drying in preparation for soil fumigation. Crop species susceptible to the nematode populations known in a particular field should be avoided. Sudangrass ‘Piper’ was found to be resistant to root lesion nematode and may be a good choice for sites infested with this species.
• Summer: If grass or another cover crop was planted, harvest it. Cultivate the land to produce the friability and consistency desirable for soil treatments. Relevel the land (if necessary) and do other operations required for orchard planting.
• Late Summer to Fall: If the soil is compacted, rip to a minimum depth of 24 inches for ease of fumigation shank penetration. This may bring up clods from the subsurface making the structure of the soil unfit for soil fumigation. In areas where light rains (less than 1 inch) occur in summer or fall, or pretreatment irrigation is required, this may provide an opportunity for breaking up clods. There are strict guidelines about the required moisture conditions of the soil at fumigation treatment. Make sure to follow DPR requirements.

At sites where sampling confirmed the presence of ring nematode or root lesion nematode and a nonresistant rootstock is to be used, use a broadcast or strip application of 1,3-dichloropene. Under optimal fumigation conditions, a broadcast application can provide up to 6 years of control. A strip application can strongly support tree growth during establishment, but nematode monitoring should be initiated after the first growing season.

Where sampling has confirmed that only root-knot nematode is present, a strip or spot fumigation may be made in soils that are not favorable to ring nematode, or when resistant rootstocks are being used. Strip or spot applications provide about 6 months of control.

If surface moisture is insufficient for DPR requirements arrange for irrigation or time applications with natural precipitation. Fumigate in September or October when soil temperatures and soil moisture conditions allow. Follow DPR and Agricultural Commissioner rules on when to complete 1,3-dichloropropene fumigation, before November 15 is best. Soil should be 50° to 80°F to a 12-inch depth before fumigating. When fumigating:

• Use the highest label rate recommended for the soil conditions within the profile.
• When following the same tree crop (or the same rootstock) the inclusion of chloropicrin in the fumigation mix may be indicated. This fumigant is known to have activity against the fungi and bacteria implicated in replant disease.
• If the ground was tarped during fumigation, do not plant for one month after the tarps have been removed. If soils cool below 50°F soon after treatment, an additional 30- to 60-day waiting period before planting may be necessary.
• Observe the reentry interval of at least 5 days and the waiting period on the fumigant label. Plant young trees on resistant rootstocks, if available. Fumigation does not remove the need to plant on a nematode-tolerant/resistant rootstock, especially when the field had nematode problems previously.