How to Manage Pests
UC Pest Management Guidelines
Plant parasitic nematodes are microscopic, unsegmented roundworms. Those that parasitize plums are obligate plant parasites that live in soil and/or roots. Two or more species may occur in the same orchard. They feed on other plants in addition to plums. Pin nematodes (Paratylenchus sp.) are another group that are frequently found in prune and plum orchards, but they are not thought to cause problems in these orchards.
Ring nematodes spend their lives in soil feeding on roots. Feeding by ring nematodes, particularly on small feeder roots, predisposes trees to bacterial canker (Pseudomonas syringae). Dagger nematodes reduce tree vigor with their feeding but mostly are important because they vector tomato ringspot viruses. Root lesion nematodes damage roots by moving through cortical tissues and feeding in these areas. Root knot nematodes take up a single feeding site within a root where they remain for their lifetime.
The symptoms described below are indicative of a nematode problem but are not diagnostic because they could result from other problems as well.
Belowground. Nematodes puncture and remove the contents of plant cells. This activity stunts root growth and reduces the tree's ability to take in water and nutrients. Because of this, nematode-infested trees may have poorly developed root systems. With root knot nematodes in particular, feeding reduces the overall energy of the tree. Nematode feeding also creates entry points for other disease organisms.
Aboveground. Lack of vigor, small leaves, dieback of twigs, and yield reduction are typical symptoms of nematode damage. Nematodes are usually distributed unevenly throughout an orchard resulting in patches of low vigor trees. Orchards infested with high population levels of ring nematodes frequently exhibit symptoms associated with bacterial canker including blighted buds, blossoms, and leaves, and cankers that occur on and can result in the girdling and death of limbs and/or trees. Trees on plum rootstocks tend to host higher population levels of ring nematode than those on Nemaguard peach rootstocks.
To make management decisions, it is important to determine which nematode species are present. If a previous orchard or crop had problems with one of the nematodes listed as a pest of plums, it is likely a subsequent orchard will have problems as well. If species present have not previously been determined, soil samples should be taken and sent to a diagnostic laboratory for identification.
Visually divide the orchard site into sampling blocks that represent differences in soil texture, drainage patterns, or cropping history, but are no larger than five acres in size. Take a separate sample from each block so that each can be managed separately. In a fallow field, collect subsamples from several locations within the sampling block. In an established orchard collect separate subsamples from the soil around trees that show symptoms and from the soil around adjacent, healthy looking trees for comparison. Subsamples should include feeder roots, when possible, and be taken in frequently wetted zones at the edge of the tree canopy. Samples should be taken from within the root zone of the tree. Mix subsamples well and place about 1 quart of soil and roots in a plastic bag. Seal bag, place label on outside of bag, keep samples cool (do not freeze), and transport as soon as possible to a diagnostic laboratory. Inform the laboratory that you want to know if the nematodes listed as pests above are present so that they can use appropriate extraction techniques. Request a species diagnosis if root lesion nematodes are found.CULTURAL CONTROL
Whenever possible, plant new orchards in land that has previously been planted in nonwoody crops for several years. California-bred alfalfa cultivars can be a good choice except where ring nematode develops well (i.e. highly porous soils). Root lesion nematodes survive well within dead root tissue or soil, with 5% lasting 5 years after tree removal.
Prevention. The following measures will help to prevent spread of nematodes to uninfested fields:
Consider the use of resistant rootstocks. Because rootstocks for plums differ in response to various plant parasitic nematodes, it is important to select rootstocks that are resistant to the species of nematode present in your soil. Nemaguard peach rootstock is resistant to root knot nematodes but susceptible to root lesion and ring 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.
Preplant preparations. For a nematode-infested location that is to be planted with prunes or plums following a previous orchard or vineyard, a year-long procedure is suggested to prepare the area for fumigation with methyl bromide. Beginning in summer/fall, remove trees or vines along with as many residual roots as possible, destroy plant residues, deep cultivate, and break up cultivation pans and soil layering. Also, sample for nematodes and obtain an accurate identification of the plant parasitic species present. Next, leave sandy soils fallow during the year but finer-textured soils may need a summer crop of sudangrass to enable deep-drying in preparation for soil fumigation.
If you will be planting in a field following an annual crop, a shorter procedure can be used to prepare the area for fumigation. Plant the annual crop in spring, use it to dry the soil, and harvest it in summer. Sample for nematodes and obtain an accurate identification of the plant parasitic species present.
Following harvest of either the grass or annual crop, level the land (if necessary), cultivate, and do other operations required for planting. Finally, in late summer/fall, rip the soil at least to a minimum of 24 inches. If the subsurface soil is dry, surface clods are a problem, and you are in an area where light rains (less than 1 inch) occur in summer/fall, you may wish to wait to fumigate until after a light rain that would help to break up surface clods. The label for Telone requires that the field surface be moist at the time of fumigation. Either wait for early fall rains or sprinkler irrigate; do not use furrow irrigation to meet this requirement. Complete Telone fumigation before November 15. If surface clods are not a problem, fumigate in September or October when soils are very dry. Soil should be warm (50° to 80°F) to a 12-inch depth before application of dichloropropene or methyl bromide. Observe the waiting period on the fumigant container label before planting. Consider planting young trees on resistant rootstocks.
UC IPM Pest Management Guidelines: Plum