According to the Weather Channel, ongoing drought conditions have contributed to the 29 million tree deaths in California, a number that is still on the rise. In addition to millions of oak trees in the state being killed off by sudden oak death disease, bark beetles have also played a large role in taking out the timbers. Drought weakens the trees, which then are more vulnerable to other stressors like the beetles.
You can read more about this and see a slide show of the devastation at https://weather.com/science/environment/news/california-sudden-oak-death-epidemic-killing-trees-bark-beetles-drought.
