Description of the Pest
Nematodes are true roundworms that are microscopic. Several species of nematodes are damaging to olive, including the citrus nematode (Tylenchulus semipenetrans), root lesion nematode (Pratylenchus vulnus), and root-knot nematodes (Meloidogyne spp.).
Damage
All root-invading nematodes cause roots to be less efficient in transporting water and nutrients, which leads to reduced growth, reduced fruit production, and decline of the tree. Root-knot or citrus nematodes may be damaging to the root system if relatively high numbers are present, but root galling is only associated with root-knot nematodes.
Root-knot nematodes
Root-knot nematodes are endoparasites, invading the root tips and establishing permanent feeding sites inside the root. Their feeding stimulates the root tissue to enlarge, forming giant cells called galls on which they continue to feed. Since galls are essentially enlarged root tissue, they cannot be rubbed off. Eventually individual galls will coalesce giving the root a bumpy swollen appearance, without any fine feeder roots. Trees become slightly weaker, especially in sandy areas, and fruit size on infected trees is reduced.
Citrus Nematodes
Citrus nematodes are semi-endoparasites. They bury into the root cortex with their anterior end while the posterior end remains outside the root. Roots have a dirty and enlarged look, because soil adheres to the gelatinous matrix that protects the eggs. The surface of infected roots appears rough and irregular. Feeder roots shorten and slowly disappear.
Root Lesion Nematodes
Root lesion nematodes are migratory endoparasites. They invade and move within roots. When there is too much feeding damage to the root, lesion nematodes leave and continue feeding on noninvaded roots. Their feeding destroys root cells, causing lesions to form along the root. Lesions are usually narrow and dark and are often invaded by rot-causing pathogens.
Management
Management of nematodes with soil fumigation must be done before planting. Proper water and cover crop management can help to avoid damage from these nematodes in established plantings.
Cultural Control
Irrigate infested, weakened trees with greater frequency to avoid plant stress. Most cover crops will host root-knot nematode, but few cover crops host root lesion and citrus nematodes. Fortunately, olive trees have hardy root systems.
Chemical Control
New orchards sites can be fumigated before planting to reduce nematode numbers. There are no postplant treatments.
| Common name | Amount to use | REI‡ | PHI‡ | |
|---|---|---|---|---|
| (Example trade name) | (hours) | (days) | ||
| Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide’s properties and application timing, honey bees, and environmental impact. Always read the label of the product being used. | ||||
| A. | 1,3–DICHLOROPROPENE* | |||
| (Telone II) | Label rates | See label | NA | |
| COMMENTS: Dry the soil by withholding water during summer and using cover crops such as sudangrass or safflower in finer-textured soils. This will usually require one year of field preparation time when following a nematode-infected perennial crop. The drier the soil, the better for deep penetration. Deep-tilling the area can aid the drying process. Surface moisture is required at the field surface before application. Apply no more than 1.5-acre-inches of surface moisture; this can be accomplished by waiting for a fall rain or by sprinkler application. Apply before mid-November. Inject 18 to 30 inches deep with chisels. Increasing the dose or depth will increase the depth of penetration but 33.7 gal/acre is currently the maximum California rate (i.e., 332 lb a.i./acre). This treatment rate applied broadcast will provide adequate control unless the soil exceeds 12% soil moisture content within the surface 5 feet of depth. In clay loam soils with moisture contents up to 19%, increase the application rate and apply only in 8- to 12-foot-wide planting strips rather than as a broadcast application. Strip applications will only enable 1 year of nematode relief, whereas broadcast applications can provide 6 years of nematode relief. 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. | ||||
| B. | 1,3–DICHLOROPROPENE + CHLOROPICRIN* | |||
| (Telone C35) | Label rates | See label | NA | |
| COMMENTS: Dry the soil by withholding water during summer and using cover crops such as sudangrass or safflower in finer-textured soils. This will usually require one year of field preparation time when following a nematode-infected perennial crop. The drier the soil, the better for deep penetration. Deep-tilling the area can aid the drying process. Surface moisture is required at the field surface before application. Apply no more than 1.5-acre-inches of surface moisture; this can be accomplished by waiting for a fall rain or by sprinkler application. Apply before mid-November. Inject 18 to 30 inches deep with chisels. Increasing the dose or depth will increase the depth of penetration but 46.7 gal/acre is currently the maximum California rate. This treatment rate applied broadcast will provide adequate control unless the soil exceeds 12% soil moisture content within the surface 5 feet of depth. In clay loam soils with moisture contents up to 19%, increase the application rate and apply only in 8- to 12-foot-wide planting strips rather than as a broadcast application. Strip applications will only enable 1 year of nematode relief, whereas broadcast applications can provide 6 years of nematode relief. 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. | ||||
| C. | METAM SODIUM* | |||
| (Vapam, Sectagon 42, etc.) | Label rates | See label | NA | |
| COMMENTS: For adequate results, apply sufficient quantities of water to saturate the soil, and be sure all the water applied to the field during treatment has some of the product mixed into it. It is important to add product to the water at a point in the irrigation system where turbulence allows mixing. Apply to soils during springtime when soil moisture is at field capacity but not saturated. After application, wait 14 to 21 days before planting if soil is coarse to medium in texture, well drained, and warm. When soil is cold (below 60°F), wet, fine textured, or high in organic matter, a waiting period of 30 to 60 days may be necessary. Water-mediated applications can be effective as deep as the water penetrates into the soil. These treatments will not kill old roots below 3 ft of depth whereas 1,3-D can kill roots at 4-foot depth. In fields where soil is commonly too moist for a 1,3-D application, metam sodium or metam-potassium can be used. In finer textured soils, apply in 6- to 8-acre-inches of water across the field surface. Top label rate is 75 gal/acre for Vapam, Sectagon, etc. and 55 gal/acre for K-Pam. Fumigants such as metam potassium 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. | ||||
| ‡ | Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) 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. |