Recently, an unknown species of Phytophthora fungus was discovered as an underlying cause of Sudden Oak Death. A tree may be infected with Phytophthora for a number of months or years before exhibiting a sudden change in foliage…
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Various species of caterpillars chew on the leaves of local oak trees. These species include: Western tussock moth ( Orygia vetusta), California oakworm (Phryganidia californica ), and Fruittree leafroller (Archips argyrospilus) to name a few. There are other larvae that may chew oak leaves. Certain conventional insecticides or formulations of Bacillus thuringensis may be used to control larvae populations. Conventional insecticides are generally non-specific to their targets, while the Bacillus thuringensis sprays are target specific to moth and butterfly larvae.
One possible reason is that a fungus called anthracnose is infecting the leaves.
Various species of anthracnose (Apiognomonia, Discula, Gleosporium, Glomerella, and Gnomonia) cause partial to total defoliation of trees in late spring or early summer. When symptoms are severe, we do not recommend treatment. Resistant plant material, good sanitation practice, fertilization to stimulate vigor and fungicide applications may provide some control but will not eradicate an existing infection. Moist weather in April and May increases susceptibility to this pathogen.
The crew was excellent—professional, pleasant and hard working. And they left the yard cleaner than when they arrived. I was very impressed. Ted, Palo Alto