I spoke at the annual Sonoma County Regional Parks Pest Control Applicators Seminar last week in Santa Rosa. Speaking to about 150 landscape pest control people including more than 40 Certified Arborists, my topic was “The Top 10 Pests and Diseases for 2015.”
Following the adage “Write what you know,” I told the audience I was speaking from my recent, limited, local experience as a consulting arborist in and near Napa Valley, not summarizing all of the current knowledge on the subject.
My source material was hundreds of photographs I took on assignments this year.
I found only five insect problems, and only one is of serious concern: Western oak bark beetles spread foamy canker disease. I might have included Dutch Elm Disease in this category, but I did not have any calls involving that problem this year.
Considering biotic diseases, those caused by living organisms, I noted 13 caused by fungi and bacteria. These are of more concern. I might have included Sudden Oak Death disease, but I did not see any cases of it in my direct experience this year.
For abiotic diseases, those caused by environmental conditions, I identified four including mineral deficiencies, sun injury on tender stems, drought effects on trees and other plants, and the lack of winter chill hours last winter. I might have included toxic effects of irrigation with reclaimed water, but I have not yet seen this directly in my practice here in Napa.
Human-caused problems comprise a subset of abiotic disease including tree damage caused by construction work, poor planning for construction, poor practices in nursery production, improper use of herbicide, poor planting practices, ill effects of discontinuing irrigation for trees during drought, and man-made climate change affecting temperatures and precipitation patterns.
Forced to simplify a huge subject into a short list of concerns, I ended up forming 10 categories rather than selecting 10 specific diseases. I came up with the following:
— 1. Phytophthora diseases: Tree— and plant-killing organisms sometimes referred to as water molds akin to the one that causes sudden oak death disease. In my experience, the most common disease in this group is crown rot, the death of the “crown,” the uppermost portion of the root system at the base of the tree or plant. Excessive irrigation is a major contributor.
— 2. Oak root fungus (Armillaria): Various species of Armillaria are native to temperate climate forests and woodlands throughout the world, and common locally. Armillaria becomes an aggressive disease when woody plants are predisposed by low vitality, excessive irrigation and cohort diseases such as crown rot.
—3. Bacterial diseases such as fireblight and Pseudomonas diseases like blossom blast on pears, gummosis on fruit trees, and bacterial knot on olive trees. These seem to be increasingly destructive of late. Mild winter temperatures and moisture influence infection rates. Timely pruning, copper solution sprays and other disease control sprays are needed to manage these.
— 4. Fusarium, a fungal killer of palms is of serious concern, even here in Napa. Arborists must be up to date on prevention practices.
— 5. Foamy canker, a fungal disease of coast live oaks, is caused by the fungus Geosmithia morbida and spread by the western oak bark beetle. Drought predisposes the trees to attack by the beetle. Once the trunk is infected, the tree dies. I know of only two confirmed cases in Napa County so far.
— 6. Drought has direct and indirect harmful effects. We can manage it somewhat with cautious irrigation for highly valued trees.
— 7, 8, and 9 are all people-related diseases or problems such as improper pesticide usage, poor design and construction planning, improper irrigation, planting and maintenance practices and poor communication.
— 10. In the big picture, climate change is happening and it is having ill effects on trees and plants. As tree and plant managers, we need to be alert and ready to act locally, changing our practices as the world changes around us.