As we slide into Memorial Day weekend, I thought it would be worthwhile to look back at an earlier forest disease epidemic that galvanized foresters and forest pathologists for half a century and changed the look, makeup, and function of forests in the eastern U.S. for perpetuity: chestnut blight.

This disease, caused by the fungus Cryphonectria parasitica, practically eliminated the American chestnut, Castanea dentata, from the eastern forests to which it is native. In these forests, the chestnut tree was a stately dominant that provided an important mast crop for wildlife and also durable, rot-resistant lumber. Its abundant tannins were used for tanning leather. Because of these qualities, you could think of chestnut trees as having been analogous in different ways to both the redwoods (stateliness, wood quality) and tanoaks (tannins, wildlife food) of our western forests. It was a very important tree that has now been replaced in dominance throughout much of its range by oak species.

Canker on chestnut stem caused by C. parasitica (photo courtesy of Linda Haugen, USDA Forest Service, bugwood.org)

Two particular features of chestnut blight are interesting to me in relation to Sudden Oak Death: (1) its spread was both fast and slow, and (2) it changed the ecological niche of the American chestnut tree without eliminating it entirely.

Chestnut blight was discovered in 1904 and had taken out most mature American chestnut trees by 1940. Looking back, this is swift work! But if we look at it from the perspective of someone learning about chestnut blight in, say, 1913, it might not command the same sense of urgency. At that point, one could not predict its ultimate effects on the east's hardwood forests. This is roughly the same point we are at today with P. ramorum and Sudden Oak Death. P. ramorum was named nine years ago, and we are trying to learn more about it, wondering what its ultimate effects will be. If it does eventually eliminate a majority of its primary host, tanoak, from western forests, this too could take several decades. This is slow according to a human perspective, but from a forest perspective, even a couple of centuries is the blink of an eye.

Above, I said that C. parasitica practically eliminated the American chestnut from its former range. I inserted that "practically" because plenty of American chestnut trees still exist. Their root collars and root systems are fairly resistant to the fungus, and the chestnut can resprout (again, like tanoaks). But before the sprouts can mature, the fungus kills them. Thus, chestnut is maintained as an understory plant rather than as the mature tree it once was. This bears interesting resemblances to the case of tanoak and P. ramorum, in which P. ramorum can kill the main stem and then somehow hang around and continue to cause problems to the new sprouts that re-emerge from the old root system. Whether it does this in every case, or even in a majority of cases, is yet to be seen. Perhaps this won't be as big a problem for tanoak as it was for chestnut. But if it is, tanoak, too, could be reduced to an understory plant that never grows large enough to produce the mast crop that so many wildlife species depend on.

There's a lot more that could be said about chestnut blight--for example, about hopeful efforts to breed resistant trees (this link has a great picture of a historic chestnut stand) and provide biological control of C. parasitica--but for this Memorial Day weekend, thinking about what it has done so far is enough for me. Looking at this historical case through the lens of our uncertainty about the possible future effects of P. ramorum is enough to inspire some thoughtfulness about the future of our coastal Western forests.

 

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!

 

Nicole’s last blog entry about tomato late blight, caused by Phytophthora infestans, was pretty interesting, and it inspired me to look up some more information about the disease and its management. Given that both tomato late blight and sudden oak death are caused by members of the Phytopthora genus, one might expect some common themes to show up. And they do. Following are what I think are some of the more interesting parallels, with quotes about late blight (in italics) from the University of California Integrated Pest Management Program:

 

“Late blight is found when humid conditions coincide with mild temperatures for prolonged periods. When humidity is above 90% and the average temperature is in the range of 60° to 78°F, infection occurs in about 10 hours.” As I noted in an earlier blog post, researchers at UC Davis have observed that the greatest quantities of P. ramorum spores are detected in the field when we have rains during April, May, and June. (Incidentally, these conditions can also promote tree infection by a variety of other fungi.)

 

“The fungus overwinters in potatoes, tomatoes, hairy nightshade, and possibly in the soil. Spores of the fungus are easily spread by wind to other plants . . . . Remove any nearby volunteer tomato and potato plants and nightshades . . . . Disc tomato fields in fall to eliminate a winter reservoir for the fungus.” The resemblance between late blight and the diseases caused by P. ramorum is pretty striking here. Whereas P. infestans overwinters in the members of the nightshade family mentioned here, P. ramorum appears to overwinter—and, perhaps more importantly, oversummer—mostly in leaves of the California bay laurel. Because of this, experiments on controlling P. ramorum at a larger scale in the field have primarily involved removing the hosts that play the largest role in harboring and transmitting the pathogen (bay laurel and tanoak). Note that this doesn’t necessarily entail the removal of all of these hosts in a given area—rather, strategic removal of the ones near tanoaks or true oaks, within the distance that spores can be expected to fly in rainy weather, is warranted.

 

 

P. ramorum survival structures (chlamydospores) in bay laurel leaf (chlamydospores are blue). Photo courtesy of Jennifer Parke, Oregon State University

 

“Apply a protectant fungicide before disease development begins; once an outbreak occurs in a field, it is important to apply additional applications at regular intervals. Coverage must be thorough for applications to be effective.”  In both the nursery and forest situations, fungicides are also primarily useful as prophylactics, rather than cures, for P. ramorum infection. When applied to nursery plants that are already infected with P. ramorum, most of the common contact fungicides (such as mefenoxam or metalaxyl) will suppress symptoms but not eliminate the pathogen from the plant. Similarly,  the systemic fungicide phosphonate, which is useful as a preventative, is usually ineffective after oak or tanoak trees are infected with P. ramorum. Only in cases of extremely small infections on coast live oaks can phosphonate sometimes slow down the course of infection.

 

 

Doug Schmidt from UC Berkeley injecting tanoak tree with phosphonate fungicide. Photo courtesy of Radek Glebocki, UCCE

 

“Tomato varieties resistant to certain races of the late blight fungus are grown where the disease occurs regularly.”  Nicole mentioned the availability of these resistant varieties in the case of tomatoes. The concept of growing resistant trees is one that has also occurred to people who research P. ramorum. However, since our understanding of this particular Phytophthora is still young, there is a long way to go. Because of the multiple steps required to identify the genetics behind resistance and to breed and test seedlings for resistance both in the greenhouse and in the field—complicated by the longevity of trees as compared to field crops—the identification of tanoak and oak individuals that are resistant to P. ramorum could easily be 15-30 years away.

 

Not that this is meant to strike a note of helplessness or doom. Rather, it just goes to show that versatility, toughness, and adaptability seem to be traits that many species of Phytopthora share—imperiling tomatoes and tanoaks alike!

 

 

P. infestans infecting a potato sprout touching an already-infected tuber. Photo courtesy of D. Inglis

 

 

 

Here on the north coast, Phytophthora ramorum kills tanoaks almost exclusively. We don’t have coast live oaks, and our California black oak population has steadily been decreasing because of land-use changes accompanied by corresponding changes in forest cover (a topic for another blog entry sometime). This year I’ve had the opportunity to see more majestic tanoaks than usual because of some projects we’ve been involved in—projects to save large tanoaks by applying a systemic fungicide that prevents infection by P. ramorum.

 

What we often see in our part of the world are dense thickets of small tanoaks, usually growing in clumps where it is obvious that all the trunks sprouted from the same mass of root tissue. Often these clumps are legacies of cutting in the past, either to clear land or to harvest the trees for the tannin in their bark (most of this occurred around the turn of the 20^th century). When tanoaks are cut, the mass of budding root tissue at the base of the tree (called the “burl”) is stimulated to produce a multitude of new sprouts to replace the lost trunk. Although they still have the old root system, these areas of young, dense tanoaks haven’t established their dominance in the forest yet with expansive crowns, so they usually compete for forest floor space with lots of huckleberry and other shrubs.

 

Here’s an example:

 

 

However, here and there on the landscape you can find some groves of “old-growth” tanoak. These specimens easily compete for beauty with any of the true oaks California is famous for. Native Americans valued these groves for their abilities to produce vast quantities of acorns and took care of them to keep them accessible and healthy. Here's a big one:

 

 

There’s a common misconception out there that foresters and land managers don’t value tanoak or think of it as a “weed” tree because it lacks economic value in the wood markets of today. It is true that land managers sometimes try to discourage tanoak growth in certain stands of trees so that they can reforest those areas with conifers such as Douglas-fir. But this doesn’t mean that they always think of tanoak as a weed. In these cases, as with all land management activities, it’s important to think about management goals for the land and how best to work with the land to enhance an abundance of values.

 

 In many cases, by attempting to control tanoak, foresters are trying to return to its historic condition land that was cut over years ago and left without any attempt to replant the Douglas-firs that originally grew there. In other cases, the soils, topography, and moisture conditions on the site are perfect for growing Douglas-firs, so the property foresters are trying to maximize the capabilities of that land to grow what is marketable. Tanoak is a beautiful wood with a lot of character, and if people come to value wood with these qualities, industrial forestry attention could shift away from its strong focus on Douglas-fir.

 

This also fits in with a larger societal picture of trying to take responsibility for our own needs for wood in California by growing and buying local rather than importing all our wood from places like Canada and Brazil that lack California’s stringent environmental regulations (check out this report--thanks to Yana Valachovic for the reference). As we try to puzzle out all the pieces of this complex picture by fitting needs for wood products into just-as-compelling needs for ecosystem stability and connectivity, aesthetic beauty, and wildlife habitat, we come to realize that tanoak is an important piece of the puzzle—and we see the damage that non-native, invasive species can potentially do when they rob us of such pieces.