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California live and valley oaks and Armillaria mellea (oak root fungus) both evolved in a climate of winter rains and summer drought (Mediterranean climate).
In a yard and home landscape situation your control alternatives are very limited. In commercial situations several fumigant materials can be used to suppress Armillaria mellea infected sites. However, none of these materials are registered for use in residential landscape situations.
With the late and often heavy spring rains, Botrytis gray mold has been an unusually severe problem. Rose flowers have in many instances been severely blighted. Jasmine, geranium plus other garden flowers and vegetables have also been affected.
Once a plant, any plant, is infected with a virus, it is infected for life. Science has not found a way to eradicate viruses from growing plants. In the laboratory it is possible to take certain plant tissues and through manipulation produce virus-free progeny.
There are several fungi that can produce a flower rot in camellias. The Botrytis gray mold fungus and a Sclerotinia white mold fungus are the most likely causal agents. Many treatments have been tried to manage and control camellia flower blight. To date none have been completely successful.
There are three viruses reported to infect hydrangea - Hydrangea Ring Spot V., Tomato Spotted Wilt V., and Hydrangea Latent V. Without doing some expensive laboratory testing, it is not possible to be specific as to which virus is present. All three viruses are present in California.
This fungus is commonly called a shelf fungus. The main body of the fungus is inside the tree slowly digesting (rotting) the heartwood. The fungus will not kill the tree, but over time can cause a substantial decay of the heartwood, which results in a structural weakening of the tree.
The small balls you see on the ground under oaks are oak galls, and they are produced by the activity of a tiny wasp, called Dryocosmus sp. The wasp spends the winter in the gall on the ground.
Pale, yellow eruptions are caused by a fungus which infects the leaf as it is emerging from the bud and expanding in the spring. The fungus is present at low levels in oaks every year.
