Symptoms and Signs
Roots infected with Armillaria mellea have white to yellowish fan-shaped mycelial mats between the bark and the wood. Dark brown to black rhizomorphs sometimes can be seen on the root surface.
Comments on the Disease
The fungus survives on dead roots.
Management
Avoid planting plum orchards where forest or oak woodland has recently been cleared or where there is a history of Armillaria root rot. All stone fruit rootstocks are susceptible to Armillaria root rot but some, such as Marianna 2624, are less affected than others and may be useful in some situations. Maintain the vigor of the trees to help resist Armillaria attack. Infested sites can be fumigated, but often this procedure will not prevent recurrence of the disease. Physical barriers to contain infection centers have been used successfully in orchards. A 4-foot trench is dug around the infected trees and plastic tarp is laid inside the trench wall from bottom to top before the soil is replaced. The tarp prevents healthy roots from coming in contact with diseased ones, thus preventing spread of the disease.
Cultural Control
Research on other tree crops has indicated that exposing an infected crown and upper root area of a tree infected with Armillaria mellea may help to slow the development of the fungus into the crown area. Remove soil from around the base of the tree to a depth of 9-12 inches. Leave the trunk exposed and keep the upper roots and crown area as dry as possible. During winter, provide drainage so that rain doesn't collect in the hole. Recheck the hole every few years to make sure it has not filled in with leaves, soil, and other matter; the hole must be kept open and the crown and upper roots exposed. Some rootstocks may produce suckers when exposed, and these will need to be removed.
Organically Acceptable Methods
Cultural controls are acceptable in organically managed orchards.
Chemical Control
Before fumigation, remove all infected trees, stumps, and as many roots greater than 1 inch in diameter as possible. Healthy-appearing trees adjacent to those showing symptoms are often infected also. Removal of these adjacent trees and inclusion of that ground in the soil fumigation may be advisable. Infected trees, stumps, and roots should be burned at the site or disposed of in areas where flood waters cannot wash them to agricultural lands. Complete eradication is rarely achieved, and retreatment may be necessary in localized areas. If the soil is wet or if it has extensive clay layers to the depths reached by the roots, fumigant treatment may not be successful. The greatest opportunity for eradication occurs on shallow soils less than 5 feet in depth. Fumigate from late summer to early fall.
| 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. | ||||
| PREPLANT | ||||
| A. | METHYL BROMIDE* | Label rates | see label | see label |
| COMMENTS: For preplant fumigation; must be applied under a Critical Use Exemption. Before fumigating, dry soil by withholding water during summer and using cover crops such as sudangrass or safflower. The drier the soil, the better for deep penetration. Deep-till the area after drying. If the soil is dusty, wait for an early rain before ripping and fumigation. Ripping a dry soil that is silty can result in large clods on the surface. Inject methyl bromide 18–30 inches deep with chisels and cover with gas-proof cover. Increasing the dose tends to increase the depth of penetration, but it cannot be relied upon to penetrate wet soils, especially if soils are high in clay. Do not remove the cover for at least 2 weeks and aerate 1 month before planting. 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. | SODIUM TETRATHIOCARBONATE | |||
| (Enzone) | Label rates | 4 days | see label | |
| MODE OF ACTION: Unknown. A thiocarbonate insecticide. | ||||
| COMMENTS: Trees must be in the ground at least in the ground 1 year before treatment or injury may occur. See label for treatment timing. | ||||
| ‡ | 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. |