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!
Hosting a field trip in May, at least in the Bay Area, usually doesn't require a back-up plan due to bad weather. Such was not the case last Friday, as I stood outside with 20 folks in varying states of water-resistance.
The farmers we were with that day relished the rain, even as the wind picked up and turned the drops horizontal. The lack of water this spring, as well as the past few springs, helps water keep its status as “liquid gold” here in semi-arid
During cyclical El Nino years, we can expect lots of precipitation all the way through late spring. It has typically been these years that have also led to the most Phytophthora ramorum infections in our forests. Anytime that abundant water and mild spring temperatures overlap, it is a recipe for fungus and mold, and P. ramorum is no exception. Mortality due to Sudden Oak Death usually follows 1-3 years later, or longer, depending on the species of tree infected.
Thus, this week caused me to shift my own predictions of how prominent Sudden Oak Death may be in the coming year. Given the weather up to last week, I was ready to write 2008-2009 off as a “bad year” (or a “good year” depending on your perspective) for the pathogen. Now, with warm rains and healthy winds all up and down the north and central coasts, I’m wondering if this weather will be just enough to create a new round of infections. While we likely won’t see the kind of tree mortality that El Nino years typically bring, there’s no doubt that we’ll still be dealing with Sudden Oak Death for the next few years to come.
