In the course of writing this comment, I checked the Wikipedia page for Oak Savannas to see if it had any interesting insights. I didn't realize that California oaks formed oak savannas as well. Hopefully that means that research in the Midwest and Californian savannas can be shared and used to draw insights into one another.
It's thought that a major route of infection is late spring rain, when the weather is warm. As rainwater drips through the canopy of infected trees, the pathogen releases it's spores into the water, which then drip on neighboring trees and into the soil. Matteo Garbelotto of the UCB forest pathology lab claims that four years after an especially late spring rainfall event, you usually see a wave of dead trees (it takes about four years for the pathogen to kill a tree).
As a second point, this demonstrates the danger presented by monoculture. The industrialization of agriculture horticulture has been unquestionably beneficial for us as a species. However, we do an absolutely terrible job of managing existential risks in horticulture. When we grow crops, we grow millions of acres of the same crop. When we plant trees, we plant thousands of the same trees. City streets in the US used to be lined with Ash trees, as they were inexpensive and fast growing. Now there are areas where almost every tree has been cut down because of the Emerald Ash Borer.
I think the blame can be assigned as much to the consumer as it can with the individuals growing the plants. Consumers, (myself included) are extremely picky about what they will eat or buy. Varieties of crops are selected for their looks, and varieties of crops that are not consistently good looking fall by the wayside.
I don't think this will cause a famine in the first world in the near future, but the poor genetic diversity of our horticulture is definitely an area of concern.