After 20 years and nearly 500 columns in this newspaper, I’ve covered just about everything I see as important in tree care. And I have skipped a few topics that might be of limited interest, like the “marmorated stink bug” and the “bow-legged fir aphid.”
Considering our transition to a new year and what our local trees have been through, let’s have a look at where we have been and what we might expect.
Here are some of the tree problems I covered last year: Lack of winter chill hours, spring arriving earlier and trees blooming out of season, low vigor in drought-stressed trees, thousand canker disease killing black walnut trees, foamy canker appearing as a new threat to live oaks, and Sudden Oak Death (SOD) disease as a continuing problem.
All of those issues share a vital, common factor: changes in the environment.
It brings to mind that old saying — Mark Twain, quoting author Charles Dudley Warner — “Everybody complains about the weather but nobody does anything about it.”
I am convinced that mankind has, unintentionally, done something with the weather.
I know there are many who don’t buy that thinking. But nevertheless, we need to be practical and deal with the changes as they affect our trees.
Tree pests and diseases depend on environmental conditions; one corner of the disease triangle:
1. The presence of the disease organism.
2. A susceptible host plant.
3. An environment that favors the disease.
With the drought, now relenting here in Northern California, I expect to see cases of dieback in live oaks, as we have seen after previous droughts.
Opportunistic fungi, like Botryodiplodia, generally present in the environment, kill branches rendered susceptible by drought stress.
When the dieback appears, we can prune to remove dead and dying branches, look at the growing conditions in the root zone and take steps to improve them, if practical.
With thousand cankers disease in walnuts and foamy bark canker in live oaks, again, drought-stressed trees attract the tiny beetles that vector the diseases. In some cases, it might be practical to intervene by improving growing conditions in the root zone.
Some property owners may choose to apply protective insecticide sprays to highly valued specimen trees.
In contrast, SOD is strongly dependent on plentiful moisture. Lingering rainfall in the spring has proven to favor infection rates. If you have susceptible oak species under your care, assess the risk of disease and implement preventative measures before the symptoms appear. Guidance is available through suddenoakdeath.org.
The lack of chill hours in recent winters has had multiple effects. First, many trees that need a certain number of chilly hours through winter, to fulfill their dormancy requirement, have been weakened. These include many fruit trees and walnuts. Second, fireblight infection rates, it appears, were favored by warmer winter temperatures that allowed the bacteria to proliferate on susceptible trees, especially on pears and apples.
This winter, so far, seems to be providing the necessary chill hours. That, and the improved amount of rainfall will be beneficial. Fireblight might be suppressed somewhat by colder winter temperatures, but it is not going away. Protective measures should be maintained, including eradicative pruning and spray applications in accordance with product labels, of course.
Future years may determine the fate of certain fruit tree varieties that need lots of chill hours, survival rates of trees confused by climate changes, and the infection rates of new diseases and pests that have appeared in our ecosystems.
I wonder if we can do anything about the weather, but I know we can do a lot for trees under our care.