Hi Wanda,

I wonder if you have an insect feeding on your oak trees and producing what is called "honeydew." It's a sugary substance. Mold tends to grow on honeydew. We call that mold "sooty mold."

Have you noticed if any of the surfaces seem sticky? 

Article on sooty mold. 

There are some good photos of sooty mold in this article. 

Take a look at these links and see what you think. If it is sooty mold, then water should wash it off. However, what is creating the honeydew is another matter. You might try using binoculars and looking at your oak tree and other trees to try and see if you have a pest attacking your trees. 

Lindsey K. Kerr, MS, MHP (she, her, hers)
Consumer Horticulture Educator
Michigan State University Extension

Have not noticed insects.  I will go look closely.  This year we have had aphids in many colors.  Not just white.

On many of our plants.

Hi Wanda,

Aphids are very small so you might not see them on your trees. 

On the hand, I was doing some more reading, and it is uncommon for oak trees to create honeydew. So it may be that you have aphids or scale another pest feeding on your plants elsewhere in your garden. 

Here is an article from MSU on honeydew. It could be that the weather has been favorable for the type of insect that secrets honeydew, but the rain might wash them away. Next year, they may not be a problem at all. 

Here is a relevant part of the article: 

More often than not, heavy infestations of these little sap-feeders cause problems for people. The last couple of years, in particular, have seen locally high numbers of scales and resulting sooty molds. This sticky, black material adheres to lawn furniture, house siding, cars, driveways, and most anything else kept under the canopy of scale-infested trees. These conditions are visually unattractive. Sooty mold can be removed by using a mild soap (about 3-4 ounces per gallon) in a sprayer. However, this does not eliminate the source of the problem, which is honeydew-producing insects and ubiquitous sooty mold.

I hope this helps!