Wood decay fungi are a widespread occurrence in many older orchards. Their appearance may seem concerning, but they are often seen as part of the aging process. However, wood decay fungi in young trees may be a sign of tree stress such as a wounding event that provided an entry point for a wood decay fungus. Wood decay fungi can weaken a tree, contribute to decline in productivity, and ultimately lead to tree death. Many different fungal species cause wood decay, and these fungi can often be found on other tree species near an orchard.

Wood decay fungi can infect the roots, trunks, scaffolds, and branches of trees. Most of these fungi lead to a slow decline, but some species such as Armillaria and Trametes are more aggressive and may cause a more rapid deterioration. The most common wood decay fungi of apple and pear include Armillaria, Chondrostereum, Schizophyllum, Stereum, and Trametes species. Breakage of tree limbs or trunks are common symptoms of wood decay.  

The fungal fruiting bodies (see photos below) that form on the surface of the bark are often a sign of an advanced infection. Fruiting bodies are the reproductive structures of the wood decay fungi and they contain spores that may spread and cause new infections. The fruiting bodies often appear as shelf-like conks emerging from the bark. Wet weather is favorable for fruiting body formation.

The distinct characteristics of a wood decay fruiting body often enables identification of the fungus. There are two categories of wood decay: brown rot and white rot. Brown rot fungi typically decay the carbohydrate components in wood (cellulose and hemicellulose) resulting in rotted wood that is dry and crumbly. White rot fungi represent most wood decay fungi associated with pome fruits like apple and pear, and they decay both cellulose and lignin. This category of fungi results in soft or spongy decay and in some cases, bleaching of the wood.

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Most wood decays are caused by opportunistic fungal pathogens. These fungi infect trees that have been exposed to some type of stressor, usually wounding. Wounds to the bark may be caused by winter injury, wind, or sunburn. Pruning wounds also provide an entry point for wood decay fungi to infect trees.

Management of wood decay fungi can be difficult. Minimizing injury to trees will help reduce the impact of wood decay. Remove and destroy dead and diseased limbs to prevent infection and spread of wood decay fungi. If fungal fruiting bodies are present, they should be removed, and infected branches should be pruned out. Pruning should be done during dry periods as spore inoculum is more likely to spread during rainfall. When pruning, avoid stub cuts and leaving large wounds. Tree paints and sealants are not generally recommended due to limited efficacy and interference with natural wound healing processes of the tree. Overall, management practices should focus on encouraging healthy, vigorous trees.

References:

English H. & Ogawa J.M. (Eds.). (1991) Diseases of Temperate Zone Tree Fruit and Nut Crops. University of California, Division of Agriculture and Natural Resources.

Sutton T.B., Aldwinckle H.S., Agnello A.M. and Walgenbach J.F. (Eds.). (2014) Compendium of Apple and Pear Diseases and Pests, Second Edition. APS Press. https://doi.org/10.1094/9780890544334

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