Last time, writing on this year’s rash of leaf diseases, I closed with this: Trees that have good vitality tend to be more resilient. Pay close attention to growing conditions in the tree root zones and correct any problems as much as practical.
Considering “resilience,” self-defense from pests and diseases is not the first priority in trees. They have several other functions to take care of first, using their limited available energy.
It is called energy allocation. Trees move energy according to priorities. When energy becomes limited, a tree will reduce energy spending beginning with the lowest priority. A tree with less stored energy becomes vulnerable to insects and diseases. Here are the priorities:
1. Maintain respiration of all parts.
2. Produce fine roots and leaves.
3. Produce flowers and seed.
4. Extend branches and roots.
5. Store energy-rich chemicals.
6. Add wood to stem, roots and branches.
7. Create anti-pest chemicals for defense.
(Adapted from “Michigan Forests Forever” Teachers Guide)
After a disease attack, like the leaf diseases fostered by disease-friendly weather in spring, trees will put available energy into the higher-priority functions. Any efforts we make should aid respiration, fine root growth and leaf production, assuming weather conditions no longer favor infection.
Given time and low stress, the tree has a chance to recuperate.
So, treatment should ensure:
— The tree is getting the simple necessities: Sunlight, water and oxygen for root respiration, and minerals that are available in good soil.
— The tree is not being further stressed by pests, diseases, root disturbances or excessive pruning.
For the California native blue oak hit hard by powdery mildew, mentioned in my previous column, our summer weather is not likely to favor further infection, so treatment with a fungicide this summer is probably not a useful option.
This tree species is supremely adapted to dry summers and rainy winters, about six months of each per year. Intermittent winter rainfall followed by good drainage allows for adequate moisture plus adequate oxygen diffusion into the root zone. Dry summer root zone conditions suppress root diseases and the natural layer of oak leaf mulch fosters good soil health and root growing conditions.
A good approach to treatment is to emulate those conditions.
Now that we are in midsummer, it is not the best time to irrigate a California native oak. At this time, a light irrigation to moisten the mulch and top few inches of soil well away from the base of the trunk would probably be helpful. It might even help the tree generate some fine absorbing roots and a new crop of leaves yet this summer.
If you apply irrigation like this, using a portable sprinkler, or soaker hose, it is critical to estimate the appropriate amount of time, apply the water, then check the depth of soaking with a probe of some sort.
Later on, around late October or early November, is when a more thorough soaking should be applied if no soaking rains have yet fallen by then.
For monitoring soil moisture, shovel or spading fork will do. I recently acquired a nice little, inexpensive 14-inch aluminum spike soil probe (indoorflowerpots.com) that should serve this purpose quite well. And for deeper, yet easy soil moisture monitoring, I’m now using an AMS Brown Moisture Probe. It can reach down to a depth of 3 feet in reasonably soft, not rocky soil. Its drill bit tip recovers a small sample of soil, enough to judge moisture content by feeling the soil with your fingers.
Give the tree time to build up energy reserves, remembering they live on slower pace than we do!