Foreboding Fungus Vexes Almond Growers

Phellinus tuberculosus conk, growing in the scaffolds of a prune tree. (Photo Credit: Bob Johnson)

In parts of California, almond growers have recently become concerned about Ganoderma root and butt rot. To put this concern in context, it becomes necessary to understand wood decay fungi in general and how wood decay affects orchards.

Wood decay fungi are present wherever there are trees and may be recognized by mushrooms and conks (shelf mushroom). They form on trees, downed logs, and other woody debris. Most species of wood decay fungi are non-aggressive and opportunistically colonize and decay dead wood. These species pose little threat to the overall productivity and longevity of almond orchards.

However, some species of wood decay fungi can be extremely damaging to orchard crops. These fall into three general categories based on the type of damage they cause and the portion of the tree they infect: Armillaria root rot, heart rots, and butt rots.

A Look at Root and Heart Rot
Root rot caused by Armillaria mellea, also known as oak root fungus, survives on dead wood in the soil. The pathogen spreads from tree to tree when healthy roots come into contact with infected roots or other dead wood, resulting in characteristic disease centers. Armillaria initially infects the cambium (just under the bark) of the tree first, eventually girdling the tree at the crown. The pathogen will then continue to decay the wood of the roots. Armillaria is the most studied of the orchard wood decay fungi, and additional information and management strategies can be found at the University of California IPM website.

Active Ganoderma adspersum conk, notice bright white edge and reddish spores covering top surface. (Photo Credit: Bob Johnson)

The other types of wood decay fungi found in orchards infect, colonize, and decay the heartwood and sapwood of living trees, reducing the wood’s structural integrity. Heart rot is generally limited to above-ground portions of the tree. In almonds, Phellinus spp. are the most common type of heart rot fungi, but there are a number of other species that cause similar symptoms.

Infection is generally initiated by spores at pruning or other types of wounds. Wood decay is often accompanied by a spore-producing fruiting structure (conk) in the scaffold or on the trunk. Extensive decay eventually results in limb breakage. This can be especially problematic when decay occurs at a primary scaffold or near the crotch of the tree. Because of limited pruning in almonds, heart rot-associated limb breakage is less prevalent than in other stone fruits.

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