Agriculture: Almond Pest Management Guidelines

Armillaria Root Rot (Oak Root Fungus)

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 can sometimes be seen on the root surface. Infected trees develop pale foliage with small leaves, a lack of new growth, and a thin canopy, usually followed by sudden death when the first hot weather of early summer arrives.

Comments on the Disease

The fungus survives on dead roots. It spreads from one tree to another through close contact of diseased roots with healthy roots. All stone fruit rootstocks are susceptible to Armillaria root rot. The plum rootstock Marianna 2624 is the most resistant to the fungus, but it is not immune. Use of this rootstock is the only practical alternative if almonds are to be grown in soils where Armillaria has infected roots and killed trees on other rootstocks. Wet soil conditions resulting from heavy rainfall or excessive irrigations can exacerbate the disease.

Management

Management of the disease is based on prevention. There are no currently available postplant fungicides for this disease. Preplant fumigation can reduce, but not eliminate, the occurrence of the disease. Complete eradication is rarely achieved, and re-treatment may be necessary in localized areas.

  • Before preplant fumigation, remove all infected trees, stumps, and as many roots greater than 0.5 to 1 inch in diameter as possible. Healthy-appearing trees adjacent to those showing symptoms are often infected also. Remove these adjacent trees and include that ground in the soil fumigation.
  • Burn infected trees, stumps, and roots at the site or dispose of them in areas where flood waters cannot wash them to agricultural lands.

The greatest opportunity for eradication occurs on shallow soils less than 5 feet in depth. Treat for Armillaria root rot from late summer to early fall. 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.

Cultural practices such as deep plowing or subsurface ripping may move inoculum throughout the orchard and subsequently spread the disease when new trees are planted. Marianna 2624 is the most tolerant almond rootstock, although ongoing screening trials indicate that Krymsk 86 may also be tolerant. No rootstock is immune.

Common name Amount per acre 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. 1,3 DICHLOROPROPENE/CHLOROPICRIN*
  (Telone C-35) Label rates See label See label
  COMMENTS: This restricted use product is applied only by professional fumigation companies and is a multi-purpose liquid fumigant for preplant treatment of soil to help manage certain soil-borne diseases and to control other pests (e.g., plant parasitic nematodes, symphylans) in croplands. It is effective at 39-46.7 gal/acre rate (labeled for shank 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 and chloropicrin are a source of volatile organic compounds (VOCs) but are minimally reactive with other air contaminants that form ozone.
 
B. CHLOROPICRIN
  (Pic-clor 60) Label rates See label See label
  COMMENTS: Fumigants such as chloropicrin are a source of volatile organic compounds (VOCs) but are not reactive with other air contaminants that form ozone.
 
C. 1,3 DICHLOROPROPENE *
  (Telone II) Label rates See label See label
  COMMENTS: This restricted use product is applied only by professional fumigation companies and is a multi-purpose liquid fumigant for preplant treatment of soil to help manage certain soil-borne diseases and to control other pests (e.g., plant parasitic nematodes, symphylans) in croplands. 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.
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.
1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of action. Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode-of-action group numbers 1, 4, 9, 11, or 17 before rotating to a fungicide with a different mode-of-action group number; for fungicides with other group numbers, make no more than two consecutive applications before rotating to a fungicide with a different mode-of-action group number.
Text Updated: 08/17
Treatment Table Updated: 08/17