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.
Janice mentioned on Monday that the rainy weather last weekend and this week may encourage new P. ramorum infections in our coastal forests. I guess we'll have to wait and see how the rest of the rainy season shapes up to get an idea of just how extensive or damaging this pathogen activity might be. But since the rain this week was long-lasting, windy, and wet (we got 3.5 inches in southern Humboldt County over the past few days), I will be keeping my eyes open as I go out in the field for some of the interesting non-lethal symptoms that show up on plant foliage during wet springs.
Two hosts of particular interest to me are tanoak and Douglas-fir. This is because in southern Mendocino and southern Humboldt Counties, these two species have just begun to put out new foliage. The level of development varies between sites--in southern Mendocino County, for example, new, succulent tanoak leaves were bursting out at the beginning of last week, whereas in southern Humboldt County the buds were still at the point of opening up.
Although P. ramorum only kills tanoaks and selected oaks in our West Coast forests, and its ability to infect other plant tissues varies wildly from species to species, it seems to be pretty good at infecting and blighting brand-new, succulent tissues of most any plant that its sporangia or zoospores fall on. So when we get a lot of rain over the course of a winter, or particularly late rains, I look for these kinds of symptoms:
On Douglas-fir
On tanoak
On redwood
On canyon live oak
None of these symptoms is lethal. On the two conifers pictured above and on canyon live oak, the pathogen apparently simply kills the new growth and is unable to penetrate into older tissue. It may cause a minor growth slowdown, but this is probably little more than an inconvenience to the plant. The important exception to this statement is in the case of conifer plantations where tree form is very important, as in the case of Christmas tree farms. If such plantations are surrounded by symptomatic bay laurel trees, rainy winds can blow spores onto the emerging leaders of the growing fir or Douglas-fir trees below. If the leader is killed, the tree's growth form may be compromised, leading to a bushier tree or one with lower branches competing to become the new leader. Obviously, the timing of infection has to be just right for this scenario to occur.
And what about tanoak? We're still not completely sure what role pathogen persistence on tanoak branches and foliage plays in continued branch or twig dieback, inhibition of sprout growth, or pathogen sporulation in forests containing tanoak. A subject for more research!
