A couple of weeks ago I wrote about the rain events early this month, when many locations on the north coast saw 3-5 inches of rainfall within a few days. I speculated that since this rain coincided with relatively warm conditions, we might soon see symptoms of P. ramorum infection on a wide variety of host plants.
As a follow-up, here are some photos of symptoms I saw the week after that post.
On Douglas-fir
On evergreen huckleberry
On western starflower
Several of these samples did yield P. ramorum in the laboratory (the huckleberry wasn't sampled, but the other two here were, as well as several other species on the site).
A couple of cautionary notes: First, this is a site with exceptionally high levels of the pathogen. The UC Davis Rizzo Lab consistently recovers the pathogen from the soil on this particular site year after year. I haven't seen these kinds of symptoms at other sites yet, so there is hope that this big rain event did not contribute to a new, large-scale spread event for P. ramorum.
Second, even if this rain event did contribute to the spread of spores to the pathogen's tree hosts (oaks and tanoaks), we probably won't see the results for a year or two. It often takes at least that long for the pathogen to kill trees.
I just thought it would be interesting to share some new photos of symptoms, even if they are not very consequential. Here in the north coast, we haven't entirely escaped the rainy season yet. This could be good news for many animals and plants, but bad news for tanoaks if we have another big, windy rain event.
The Eureka Times-Standard reported yesterday morning that the rain we received on the north coast over the last weekend brought our rainfall totals up to 78% of normal for the water year. As far as water reserves go, this isn’t great, but it’s better than it was back in February. Still, it would take a whole lot of water over the rest of the spring to boost our levels up to anywhere close to normal.
Phytophthora ramorum, the pathogen that causes Sudden Oak Death, thrives in moist conditions, so the wetter the winter/spring, the more pathogen spread we usually see. After the wet winter of 2005-06, tree infections exploded across the north coast, and we are still seeing the legacy of those infections showing up three years later in the form of newly dead trees. Since the winter/spring periods between then and now have been relatively dry, the mortality has been tapering off.
The total amount of precipitation is not the only factor influencing the level of P. ramorum infection, however—when the rain comes is just as important. Rainfall that comes late in the season, especially in the relatively warm months of April, May, and June, tends to be followed by higher levels of infection, since the pathogen thrives not only in moist conditions but also in warm ones. It’s in those years, when rainfall coincides with budbreak and new leaf development in a variety of plant species that are hosts for P. ramorum, that we see the symptoms that many people think of as classic P. ramorum symptoms, such as “shepherd’s crooking” on the branch tips of host trees such as tanoak.
This year, I suppose, we’ll have to cross our fingers that sufficient precipitation will happen before that warm period. We need the rain—it’s important to so many parts of our natural and agricultural environments.
rain gauge
