Friday, November 26, 2004

Oak Mortality Syndrome, an early report

A report from the Santa Cruz Mountains Bioregional Council meeting, Sept. 28, 2000



Map showing known infestations of "Sudden Oak Death" in 2000.

Dear friends,

Oak Mortality Syndrome is the new name for what has been called "sudden oak death".

Today the Santa Cruz Mountains Bioregional Council hosted a panel presentation and discussion: "Sudden Oak Death -- Dealing With a Potentially Devastating Forest Disease, and Lessons Learned from Pine Pitch Canker". The panelists were David Rizzo, the plant pathologist who recently identified the water mold causing the disease, Steve Tjosvold who conducts local field research on it, and Rick Hawley who studied the effectiveness of control methods for the unrelated Pine Pitch Canker disease.

There are about 60 known species of Pytophthora, and the one causing the disease had not previously been described. As recent news stories tell us, it seems to be related to Phytophthora lateralis, which has caused losses of many Port Orford Cedars in recent decades. The common name for Phytophthora is "water mold" and it is related to the brown algae, but basically acts like what is commonly known as fungus, so everybody calls it a fungus. It is believed to be either exotic [possibly from the Orient] or a new hybrid.

Exactly how it spreads is not known, though the organism has a waterborne stage, and mud is a major suspect. It seems somewhat like other species in the Pytophthora genus that may be vectored by blowing in the wind in a dormant stage. If that proves to be so, it is very bad news, impossible to contain it.

It seems to grow best at temperatures of about 50 to 60 degrees, hardly at all over 80 to 90 degrees.

It took so long to discover it because the usual methods of taking samples and submitting them for analysis were too slow, and the organism died before an attempt was made to culture it. Dr. Rizzo figured out what it was when he went out, collected a fresh sample and immediately drove back to his lab to culture it. The ususal method of taking a sample, keeping it a day, putting it in the mail, etc. resulted in the organism responsible for the disease dying before the culture was attempted.

It is not believed to be spread by the beetles; they come after the disease develops. The mystery is that it attacks always above ground, low on the trunk of the tree. Can't figure out how it gets there. One theory is animals rubbing the bark. It was noted, if so, feral pigs are really bad news!

Worst case scenario is that it could spread to all the black and red oaks thoughout North America.

Best case scenario is that some freak event infected all the trees now dying several years ago. The "sudden death" is noted when the foliage begins to turn color. But I saw coast live oaks in China Camp State Park that were cracked with the black domes of the Hypoxlyon thouarsianum fungus [one of the secondary attackers typical of the syndrome, which involves a number of native diseases and pests going after the weakened trees] well up their trunks which still had green foliage. It may be that the coast live oaks can hold off the Phytophthora longer than the tanbark oaks; that has proven to be the case in greenhouse studies where young trees were deliberately infected.

The Port Orford Cedars are being protected in part by preventing access to them in the rainy season, particularly keeping vehicles on dirt roads from getting near them, which seems to be preventing new infections, though it is difficult to get full cooperation from the public; some people crash through the barriers.

It is now believed that any infected wood should be allowed to dry out and shouldn't be moved any further than necessary. The previous recommendation to tarp it with clear plastic is considered possibly a bad idea that would favor the disease more than hurt it. A new term that came up is "waste shed" -- infected wood probably shouldn't be allowed to leave a watershed. The infection is almost entirely in the bark and cambium, and only in the aboveground parts. It may sometimes enter the wood just a little, never more than an inch.

The discussion was largely about what we don't know. The causitive organism was only discovered in June, and never seen before. The disease is basically only really active in cool, wet weather, so we will begin to learn much more as we get into the rainy season.

What we don't know:

  • Is is native or exotic? [though it certainly seems to be exotic]
  • How is it dispersed? How far?
  • Does it survive in soil or litter?
  • Does the pathogen survive in dead wood?
  • Does the pathogen survive in chipped wood?
  • How long does it actually take to kill a tree?
  • How does it interact with other diseases and pests to kill a tree?
  • Will fungicides control the pathogen?
  • What will the ultimate mortality be and what species will it spread to?

So far the disease has only been seen in wildlands, not in street trees or in yards [where it appears near homes it has always been in trees that are functionally part of adjacent open space]. There are other diseases that also occasionally kill trees, particularly those that have been stressed through injury, compacted soils, overwatering, air pollution, etc, but the causitive organism for the oak mortality syndrome has not been identified as occurring except in wildlands so far. Wildlands with much human use, like China Camp State Park on the shore of the Bay in Marin County, do seem to be a possible correlation, however.

It is difficult to make any recommendations yet. The main ones are: don't move infected wood to new areas, and don't move mud from the vicinity of infected trees to new areas. Dead wood should be allowed to dry out.

It has been shown that the disease comes first, then the insects, so insecticides are not considered promising for control.

New understandings are expected to be coming along in the coming weeks and months, since we now have a much better idea what we are dealing with and it is being studied intensively by many people.

Some arborists present at the meeting said they are seeing a marked increase also in wetwood infections -- another disease, caused by bacteria. Part of the problem is that we have so many very old oaks, not so many middle-aged or young ones. Old oaks seem to be more vulnerable to disease.

I thought it was a great idea to bring in an expert on the pine pitch canker disease containment problem, Richard Hawley of Greenspace The Cambria Land Trust, though it is caused by a totally different organism in different ways, the lessons learned are worthwhile. He brought a great publication "Programs for Capturing, Handling, Utilizing, and Disposing of Infected Pine Material in San Luis Obispo County". One little tidbit I picked up from his discussion is the problem of pallets. Packing materials are typically built crudely of unfinished wood, and sometimes still have the bark on them -- and thus are a probable vector for spreading disease from continent to continent, sort of the botanical equivalent of ballast water, which has brought so many foreign organisms to San Francisco Bay that an estimated 80%+ of the biomass of the Bay consists of introduced organisms.

All three panelists gave us some historical perspective on other tree declines, of which there have been many in the last 100 years, most notably the loss of the American Chestnut, which began in 1904 and was complete by the 1950s. The first slide was a very old photograph of American Chestnuts -- so big their trunks looked like old growth redwoods -- the trees grew to 10' in diameter and over 100' tall, and over about 45 years all were at least top-killed in their extensive native range, where they were once a dominant tree, with huge ecological effects. In North America in this century there have been significant declines of oaks, maples, birches, ashes, sweetgums, beeches ... also in trouble in recent decades the Monterey pines, some Colorado pines, aspens and dogwoods. Western Australia suffered a particularly serious decline of Eucalyptus, involving many species with very high mortality, turning some forested areas into grassland.

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