How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PESTS

Plant parasitic nematodes are microscopic, unsegmented roundworms. Those that parasitize prunes and 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 prunes and plums. Pin nematodes (Paratylenchus sp.) are another group that are frequently found in prune orchards, but they are not thought to cause problems in these orchards.

DAMAGE

Ring nematodes spend their lives in soil feeding on roots. Feeding by ring nematodes stresses trees and makes them susceptible to bacterial canker (Pseudomonas syringae). Dagger nematodes reduce tree vigor with their feeding as well as 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 take up a single feeding site within a root where they remain for their entire life. Feeding by ring nematode reduces the number of feeder roots.

SYMPTOMS

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. 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 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.

FIELD EVALUATION

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 prunes, 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 5 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 or dagger nematodes are found.

Prevention. The following measures will help to prevent spread of nematodes to uninfested fields:

1. Use certified planting stock.
2. Clean soil from equipment before moving between orchards.
3. Do not reuse irrigation water.

Resistant Rootstocks
Consider the use of resistant rootstocks. Because rootstocks for prunes 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 and the plum rootstocks Marianna 2624 and Myrobalan 29C are resistant to root knot nematodes, but susceptible to root lesion and ring nematodes. Marianna 2624 is also resistant to tomato ringspot virus, which is vectored by dagger nematode and causes prune brownline. Other rootstocks are susceptible to this virus.

COMMENTS ON CONTROL

Preplant preparations. For a nematode-infested location that is to be planted with prunes 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, during winter/spring plant deep-rooted grasses (e.g., sudangrass) to help dry the soil and harvest the grass in summer.

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. Complete fumigation prior to 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.

Common name		Amount to Use	R.E.I.+	P.H.I.+
(trade name)			(hours)	(days)
When choosing a pesticide, consider information relating to environmental impact Not all registered pesticides are listed. Always read label of product being used.
PREPLANT
A.	METHYL BROMIDE*	300–600 lb	see label	NA
	COMMENTS: Must be applied under a Critical Use Exemption. Use methyl bromide for fine-textured soils. Apply methyl bromide: as a broadcast fumigation using tarps; by fumigating the soil with 300 lb/acre, inverting the top 12 inches of soil, and refumigating in 14 days with 150 lb/acre; or by fumigating a 10- or 11-foot strip down each planting row where soil is too moist to effectively apply Telone and there is resistance to the prevailing nematodes in the new rootstock. Fumigants such as methyl bromide are a source of volatile organic compounds (VOCs) but are not reactive with other air contaminants that form ozone; methyl bromide depletes ozone. Fumigate only as a last resort when other management strategies have not been successful or are not available.
B.	METAM SODIUM*
	(Vapam HL, Sectagon, etc.)	75 gal	48	NA
	COMMENTS: Metam sodium can effectively reduce populations of nematodes to 5-foot depth if applied properly as a drench in large volumes of water, but it does not penetrate and kill plant roots deeper than 3.5 feet. This product is best applied in springtime or to pre-moistened soil. Its usefulness is limited to sandier soils or soils that infiltrate 6 to 8 inches of water within 12 hr or less. Can be applied via a series of small level basins (e.g., one tree row at a time) if there is adequate water supply for complete filling of the basins within 1-2 hours. But, for best tree growth, do not replant any Prunus spp. within one year after the drenching of the basins. Fumigants such as metam sodium are a source of volatile organic compounds (VOCs) but are minimally reactive with other air contaminants that form ozone. Fumigate only as a last resort when other management strategies have not been successful or are not available.
C.	1,3-DICHLOROPROPENE*
	(Telone II)	33.7 gal/broadcast acre	5 days	NA
	COMMENTS: This restricted use product may be applied only by professional fumigation companies. It is effective at 33.7 gal/acre rate (top label rate for broadcast applications) if applied to dried sandy soils or sandy loam soils with no more than 12% soil moisture content anywhere in the surface 5 feet of soil profile. In California the applications must be applied to soils having a moist surface; this task is difficult to achieve without use of sprinklers unless there is a fortunate rainfall. Do not flood irrigate prepared lands to achieve this surface moisture requirement. Broadcast apply where nematode resistance is unavailable for prevailing nematodes. Strip applications are permitted at higher treatment rates and effective where resistant rootstocks are available, the clay loam soil profile contains no more than 19% soil moisture, the field has been pre-ripped to 4- or 5-foot depth, and the delivery shank is winged to limit off-gassing. Fumigants such as 1,3-dichloropropene are a source of volatile organic compounds (VOCs) but are minimally reactive with other air contaminants that form ozone. Fumigate only as a last resort when other management strategies have not been successful or are not available.
POSTPLANT
A.	SODIUM TETRATHIOCARBONATE
	(Enzone)	750–1,000 ppm	4 days	NA
	COMMENTS: Liberates carbon bisulfide soon after soil contact and its half-life may not exceed 24 hours. Thus, performance is limited to soils that quickly infiltrate 2 to 3 inches of water within several hours. Enzone is quite effective against nematodes external to the roots, particularly ring and dagger nematodes in coarse textured soils applied via low volume during a 4-hour irrigation. Apply during cooler months before May 1 or after October 15 and no more than twice per year. Fall applications can halt bacterial canker incidence the following spring.
+	Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.
*	Permit required from county agricultural commissioner for purchase or use.
NA	Not applicable

PUBLICATION

UC IPM Pest Management Guidelines: Prune
UC ANR Publication 3464
Nematodes
M. V. McKenry, Kearney Agricultural Center, Parlier
B. B. Westerdahl, Nematology, UC Davis

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