A series of calls, emails and personal observations prompts me to address this topic: an unusual outbreak of leaf diseases.
First, I noticed my Erfurt rose, a reliably disease-resistant hybrid musk, speckled with rust. It’s a common fungal disease causing yellow to orange spots on the upper surface, with raised orange bumps on the under-surface of the affected leaf.
I have never sprayed it with fungicides through the 20 years it’s been growing in my garden, and it has shown consistent good health with an outbreak of rust only rarely.
With respect to trees, I’ve noticed a number of cases of rust on weeping willows in various regions of the valley. Symptoms are very similar to rust on roses.
An otherwise healthy, very large blue oak, out in the Wild Horse Valley-Green Valley Road area showed about 90 percent of the leaves turning off-color and falling with symptoms of powdery mildew.
And then two more: In the Stag’s Leap region, a group of valley oaks with leaves all turning yellow and showing rust on the undersides of the affected leaves. And in Gordon Valley (at the eastern boundary of Napa County) a section of blue oak woodland with leaves dying. Photos I received show foliar symptoms of two leaf diseases, anthracnose and powdery mildew.
In the latter case, the property owner was concerned about Sudden Oak Death. I was able to reassure him that blue oaks (Quercus douglasii) are not susceptible. There is at least some comfort in knowing the blue oaks are not at risk from that tree-killing disease.
Fortunately, the diseases in this outbreak do not kill oaks outright. They damage foliage to the extent that the trees literally sever ties and shed the infected leaves.
That can be an exhausting waste of energy for a tired old tree, but, in otherwise vigorous trees, backup systems are in place in the form of stored energy and a healthy new bud at the base of every leaf.
Any explanation of the outbreak is complicated by the fact that the different diseases require different conditions to grow and infect leaves.
Rust and anthracnose diseases require free moisture on the leaves and a certain temperature range.
One common species of rose rust requires at least four hours of moisture on the leaves with a temperature range of 64.4 to 69.8 degrees. Other rust diseases on trees have similar requirements.
Looking at weather records for this spring (www.wunderground.com), I found the mean temperature for April was 64 degrees (when my roses were leafing out and coming into bloom). And there was 0.58 inches of rain during that month.
Anthracnose diseases have similar requirements. In oaks, the ideal temperature range for one common anthracnose species is 60.8 to 68 degrees with moisture on the leaves for several hours.
It looks like the weather provided ample opportunities for rust and anthracnose diseases to take hold.
But powdery mildew is different. Rain and freestanding moisture on the eaves inhibits it. They thrive in warm days, high humidity and cool nights.
High temperatures this May reached up to the 90s while last May they reached only as high as 74 degrees, a possible explanation for the increase this year.
Considering treatment, infected leaves cannot be cured. Various fungicide spray treatments can be applied to key trees before or at the onset of symptoms. And there is at least one systemic fungicide that can be injected into trees well in advance of infection of some of these diseases.
It takes a watchful eye and a qualified pest control company to do high-tree spraying and injection applications, although some systemic materials are available retail for home use.
There are serious drawbacks. Fungicides can have severe unintended consequences in the environment, and the diseases being treated tend to become resistant to the chemicals. On large scales, it is not practical to treat a whole woodland area.
Trees that have good vitality tend to be more resilient, so pay close attention to growing conditions in the tree root zones and correct any problems as much as practical.
More about that soon.