A deadly disease is killing oak trees in California IMAGE BY hjl

What is killing California's trees?


Drought and disease are ravaging one of California’s most precious resources – its trees. 

Water-starved pines have been infested with beetles, which have killed up to 40 million of the towering trees. And as many as 10 million oaks have succumbed to disease in a death spiral that is past the point of eradicating, according to research from scientists at several universities. Their study was published in May in the Proceedings of the National Academies of Science.

A statewide task force of scientists, fire officials and land managers is working feverishly to contain the problem and combat the oak dieoff – pines forests will regrow, but some species of oaks may be lost for good. And Gov. Jerry Brown has declared a state of emergency.   

The issue poses a serious fire threat and can compromise California’s vital watersheds. In addition, it has potentially widespread commercial implications for the state’s nursery trade.

CALmatters is looking into the problem and recently interviewed David Rizzo, a plant pathologist at the University of California, Davis. Rizzo was one of the first researchers to identify the Sudden Oak Death disease and a co-author of the recent study.

Here’s an excerpt of our interview:

Q: When was Sudden Oak Death first noticed?

A: Prior to 1995, it was an unknown disease. People started noticing dying trees in Santa Cruz and Marin Counties. It wasn’t quite clear what was killing the trees. The problem kept getting worse, through the ‘90s.

Then, by the summer of 2000, it exploded -- dying trees everywhere, particularly in Marin County. It takes two to three years to kill an oak. Most of those were probably infected in 1998. What we found was a pathogen never described before.

Q: How is the disease spread?

A: The pathogen makes its spores on non-oaks. California bay laurel is the Typhoid Mary. In the rainy season, pathogens splash off the laurel leaves and infect oaks. The El Nino [weather pattern] of 1998 was a monster; it rained into June. Parts of Marin County got 100 inches of rain. 

Q: What other “host” plants are there?

A: A number of years ago we discovered that rose bushes can be hosts. Our paper was published two weeks before Mothers’ Day. That didn’t go over well.

Q: How does the disease kill the tree?

A: It is a parasite. The pathogen releases enzymes that kill tissue, and the spores feed off the dead tissue. The spores look like translucent little footballs, which can only be seen under a microscope. Rain storms spread the spores. We have evidence that, in rare events, spores can be blown several kilometers.

Q: But we are in a drought. Why is this disease spreading?

A: We have been getting just enough rain to keep it going. And in the northern part of the state, where it’s spreading in Humboldt and Mendocino Counties, there hasn’t been a dramatic drought. That’s the area we are most worried about. The drought will knock it back, but it will take an epic drought to wipe it out.