In A Nutshell- Fourth Quarter 2010

Katherine S. Burnham, Sanderlin, Randy S., Graham, Charles J.  |  5/20/2011 7:44:13 PM

Wood Rot of Pecan Trees: A slow and invisible killer

Spore producing structures (conks) of a wood rotting fungus on trunk of

December 22, 2010 Fourth Quarter Newsletter

Wood Rot of Pecan Trees: A slow and invisible killer
R. S. Sanderlin

There are numerous entities and events that can kill mature pecan trees. Some can happen quickly such as lightening strikes while other events may take years to finish a tree off. In this article I will discuss one of the problems that typically take years to kill a pecan tree: wood rot fungi. This article is based on orchard observations and literature review. I have not conducted research with wood rot of pecan, and very little, if any, literature exists that is specific for pecan. Consequently, much of the information presented is general for hardwood trees of all types.

Wood rot of hardwood trees has been described under several names, including wood decay, white rot, brown rot, and heart rot. Typically, wood rot is a problem only of mature trees at least 30 years old, and as trees get older wood rot tends to become more common. Younger pecan trees usually have a vigor and rate of growth that apparently prevents wood rot fungi from becoming established in them or prevents the fungi from spreading throughout the trunk of trees.

There are several genera of fungi that can cause wood rot of hardwood trees. These fungi are sometimes loosely put into two groups based on the type of decay they cause in wood; white rot fungi and brown rot fungi. In my personal experience, white rot decay fungi are the most common in pecan trees. White rot fungi produce enzymes that break down both cellulose and lignin components of wood. Cellulose (made up of a long chain of glucose units) and lignin (an even more complex polymer), are the first and second most abundant components of wood and provide most of the structural strength of wood. White rot fungi give digested wood a stringy whitish or yellowish appearance and the wood is often spongy because of the loss of strength.

The wood rotting fungi cannot penetrate through the outer intact bark of trees. They usually gain entrance into a tree through an open wound, such as a broken limb or pruning cut. Airborne spores of the fungus produced in spore producing structures formed on other infected wood sources enter trees through these wounds. These spore generating structures are usually most abundant in the fall and early winter months during wet periods but can occurred anytime environmental conditions are right for their growth.

Once a wood rot fungus has gained a foothold in a tree, a long running battle begins between the tree’s defense system and the fungus. A healthy tree with strong wood is usually able to seal off the fungal invader from the rest of the tree by forming both physical and chemical barriers. The fungus may stay confined in the area where it entered the tree and eventually die. If the tree is not able to completely seal off the fungus, an ongoing scenario results in limited growth of the fungus through newly formed defense walls each year followed by more wall formation may occur and fungal growth. This pattern can repeat over years until the tree is killed by the slowly encroaching wood rot fungus. As the structural components of the wood are eaten away, wood will become too weak to support the weight of the tree and collapse will. Often the first visible distinctive sign of wood rot in a tree is the appearance of the spore producing structures called conks on the side of the tree trunk (Figure 1). The conks are mushroom-like structures that can appear as “shelves” sticking from the side of a tree. They will differ in shape and color depending on the species of fungus causing the wood rot. Conk color usually varies from white to various shades of brown or occasionally red on pecan. The conks are most prominent in late summer through fall especially during wet periods but can occur any time of year if environmental conditions are right for their formation. Unfortunately, by the time these structures are produced by the wood rotting fungus, extensive damage has already occurred within the tree.

Even when the conks of wood rotting fungi begin to appear it may be several years before the tree is killed by the fungus and during this time infected trees can still produce good crops of quality nuts. But as the saying goes the hand writing is on the wall, or in this case the tree trunk in the form of the spore producing conks. During periods of tree stress, particularly severe winters, I have observed slow moving wood rot situations that seem to suddenly speed up and several trees will die in one growing season. Perhaps the wood rot fungi are able to grow much quicker than normal when trees are placed under certain types of stress.

Some pecan cultivars appear to be more susceptible to wood rot development than others. The cultivars Schley, Success, Elliott, and Desirable appear to be very susceptible to wood rot while the Stuart cultivar appears to be quite resistant to wood decay fungi. It is common in old orchards that contained a lot of these three cultivars for the Schley and Success to be all but destroyed by wood rot while the Stuarts still appear to be healthy trees after many decades. The original orchard planted at the Pecan Station around 1930 contained a lot of Stuart, Schley, and Success
trees. At the time these trees were removed during the 1980-90s, almost all of the Schley and Success had a significant amount of wood rot and loss of trees from wood rot was a primary factor that required removal of these trees. On the other hand, I do not recall signs of wood rot in any of the original Stuart trees in the Station orchard.

Because pecan trees are going to have limb breakage, there is little that can be done to prevent wood rot fungi from setting up housekeeping in older trees. One avenue of fungus entry can be pruning wounds; however, wood rot fungi are apparently not able to infect young vigorous trees so this is not a concern during shaping trees to a central leader by pruning. Regardless of the age of a tree, pruning cuts should never be made that leave stubs (partial limbs) behind the pruning cut. Limb stubs are excellent places for wood rot fungi to start infections that eventually can move through the entire tree. Pruning cuts should be made at the outside border of the junction of a limb to another limb or the main trunk of the tree. Sometimes (but not always) there a slight but visible ring or collar of enlarged tissue at this junction and cuts should be made on the outside border of the enlarged ring of tissue. In this area of limb growth, trees are able to more quickly form the defensive walls to prevent fungal growth and more quickly heal wounds to reduce the amount of time that vulnerable tissue is exposed to fungi. I know from my own experience that saying this is a lot easier than doing it, especially if you are working from a lift with chain saws trying to remove hundreds of broken limbs caused by a storm. Cleaning up trees after a storm will take precedence over worrying about potential wood rot and not all cuts can be made in textbook fashion. To the extent that is practical do not leave stubs when pruning, and remove stubs formed by broken limbs on large trees. There are products on the market for sealing pruning wounds. These are not needed on young trees, and most research indicates that they are of no value in keeping fungi out of cuts and breaks of older limbs. Some of these products may actually slow wound healing time, so the use of these products is not recommended.

Regrettably, wood rot in older pecan trees is going to occur to some extent. About the only preventive measure you can take is to try to prune limbs close to the trunk to remove stubs whenever this is practical.

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