What I learned about oaks

As promised here’s a report on what I learned at Rancho last weekend about oaks – diseases and pruning. The class was taught by Susan Sims, master arborist (Sims Tree Health Specialist, Inc. and Sims Tree Learning Center) and Chuck Owen, certified arborist and pruner of heritage trees (Chuck’s Complete Tree Service, ph: 909-484-1373). Susan covered pests and diseases, while Chuck discussed pruning.

Chuck describing proper pruning, while his crew works on an oak in the Cultivar Garden.

Pruning
First of all, I cannot stress enough that it is absolutely true: I am not an arborist, and furthermore, I probably should not play one at home. Pruning is an art and a science but I did learn some things last weekend.

1. Prune trees at the proper time of year. For evergreen oaks this is in late summer/early fall. If you plan to do it soon, don’t wait any longer, do it now. Though coast live oaks don’t drop their leaves all at once, they are pretty much dormant in late summer. Pruning now will avoid mildew and the growth of multiple branches called witches broom. Pruning live wood during other times of the year can definitely stress an oak – don’t do it unless it is dead wood or is necessary for safety reasons. (And as an aside: Do not prune live wood on pine trees during the summer. The sap is running and a cut or wound will ooze sap that attracts bark beetle, a very destructive pine disease.)
2. Smaller cuts are better than large ones so remove problematic branches before they get too big.
3. Cut the branch outside the branch collar, roughly perpendicular to the branch you are cutting. This makes the smallest cut so it heals fastest. Do not make the cut parallel to the parent branch since will be a larger diameter cut and the cut may go through the branch collar. Cut properly to avoid tearing the bark.

There is a lot of information on the web about pruning trees. Read, study, take classes, and learn before cutting. If it is an old specimen tree, hire a certified arborist whom you trust. A poorly pruned tree is susceptible to disease, breakage, and will probably never return to its former glory.

Diseases and Pests
Susan Sims gave a powerpoint presentation on oak diseases and pests, and then pointed out some of them in the garden.

Class listens as Chuck and Susan share their knowledge of oaks.

1. Oak trees are a habitat unto themselves. They have many, many different insects, fungi, and bacteria that live near, on, and off of them.
2. For a healthy, non-stressed tree most of these are not a problem. Many of these pests are opportunistic and can only attack the tree when it is stressed or unhealthy.
3. There is a concept about plant diseases called the disease triangle. It says that there must be three conditions for a plant to become diseased: susceptible host, pathogen, and environmental conditions. Timing is a forth element important to the occurrence of disease.
   Example: armillaria is a root fungus present in most soils that causes disease in oaks. It is the pathogen. The oak is the susceptible host. Environmental conditions favoring the development of disease would be anything that the stresses the oak, such as watering during the summer, planting beneath an oak, construction near the oak or heavy pruning. Clearly the timing of horticultural practices is important and can be a major cause of stress.
4. Once the tree is stressed it is susceptible to many diseases and often it is difficult to tell which one attacked first, or which one was ultimately responsible for its demise. Many plant diseases cannot be cured, but even for curable diseases, once we are aware of the problem, it is often too late. The message is clear – providing proper care is the most important thing one can do to keep a tree healthy and disease free.

Susan demonstrates checking the base of a stressed oak for cavities and problems.

Much more was covered during the class. We had a lively discussion on mycorrhizae (symbiotic soil fungi that have been shown to be necessary for the uptake of nutrients and elements by plant roots). Although I have found many articles describing these relationships, I am having trouble finding studies that show that inoculating soil with mycorrhizae is helpful. Susan Sims is a big advocate of the practice. I would love to read results from good research that is not being done by the producers and sellers of mycorrhizae. Anyone have anything on this?