Armillaria Root Rot (Oak Root Fungus)

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.

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 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.