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Pest AlertAugust 29, 2001

Sudden oak death

The death of a large number of trees in the coastal region of California has received a lot of attention out west. But those of us further east are beginning to take notice as the disease spreads and the host list for the newly described fungus lengthens to include tree and shrub species important to us. Recent lab work has demonstrated that at least two eastern red oak species, Q. rubra red oak and Q. palustris, pin oak are highly susceptible to this disease. Thus far, members of the white oak group are not highly susceptible to the causal fungus. Now, Oregon reports finding the disease within its borders, clearly demonstrating the capacity of the fungus to move over long distances. So what does that mean to us? Why should we be concerned?

The Fungus

Sudden oak death is caused by a newly described fungus, Phytophthora ramorum. While little is known about this fungus, it is in the genus of 'water molds' that includes many root and bleeding canker pathogens as well as potato late blight. Most are soil borne and spread by movement of soil and/or water (or anything with soil on it, like nursery stock, vehicles or hiking boots). Many have motile spores that can actually swim in search of a suitable host. Most also have long-lived resting spores that can survive in the soil for many, many years. As a result, most fungi in this genus are extremely difficult to control once they become established in an area. Many also have very wide host ranges, which seems to be the case with this fungus.

P. ramorum produces zoospores (motile spores) and chlamydospores (resting spores). The zoospores, produced in sporangia (balloon-like structures) are short-lived, but can swim through water film to infect new areas of the host. The sporangia themselves can infect susceptible hosts without producing zoospores. While the fungus prefers cool moist climates, the chlamydospores are very drought and heat resistant. It takes temperatures more than 110 degrees Fahrenheit to kill them. In the U.S., the fungus has been found only in the coastal regions of California and Oregon. However, it occurs on native rhododendrons in the Netherlands and Germany under climates similar to the Lake States.

Host Species and Symptoms

There are many hosts (other than those seen in California and Oregon - see list below) according to Dr. Rizzo, the California scientist who first isolated the fungus. (Note: The fungus was first isolated from Rhododendron in Germany in 1993.) Most of these are Ericaceous shrubs like the rhododendron species. On these plants, the fungus causes leaf lesions and tip-dieback characterized by blackened areas with indistinct margins. The fungus does not kill the plants. Of the known hosts, only the tanoaks, true oaks and evergreen huckleberries produce bleeding stem lesions that are generally fatal. Stem lesions or cankers generally occur near the lower part of the stem, are characterized by dark red to black oozing, and include dark discolored wood beneath the bark. See the web sites below for more detailed descriptions.

Host List (list from

Plant Family Scientific Name Common Name(s) or Cultivar(s)
Caprifoliaceae Viburnum x bodnantense Arrowwood
Ericaceae Arbutus menziesii Pacific madrone
Ericaceae Rhododendron spp. 'Catawbiense Grandiflorum', 'Colonel Coen', 'Elegans' (R. Roseum), 'Gomer'
Ericaceae Vaccinium ovatum Evergreen huckleberry
Fagaceae Lithocarpus densiflorus Tanoak or tanbark oak
Fagaceae Quercus agrifolia Coast live oak
Fagaceae Quercus kelloggii California black oak
Fagaceae Q. parvula var. shrevei Shreve's oak
Hippocastanaceae Aesculus californica California buckeye
Lauraceae Umbellularia californica California bay laurel or Oregon myrtle

The Threat to MN

So back to Minnesota and why we should care! While the means of spread is not fully understood because the fungus is so new to us, it acts much like the more common Phytophthora spp. So it is likely but not yet proven that it can be moved over long distances with soil. That means hikers, farm and recreational vehicles, nursery stock and anything else that can pick up soil and move it around may carry the resting spore stage of the fungus to new areas. The fungus currently is found in some of the most popular tourism areas of the west coast visited by thousands of people annually. The fungus also occurs as an inconspicuous spot on common ornamental shrubs that are produced on the west coast and shipped all over the United States. So the potential for new introductions is huge!!

To date, tree losses have been confined to coastal species not found in MN. Experiments to test the susceptibility of other hosts have been somewhat slow because of the difficulty associated with moving host material around. But the list has been expanded to include two red oak species important to our forest resources. These two species are also the ones most susceptible to oak wilt and oak decline as a result of local environmental stresses. If the fungus were to be introduced into MN, tree losses could be substantial.

We don't know how the fungus would respond to our weather conditions. So far it has not moved much beyond the moist coastal region of California and Oregon. But its occurrence in Germany and the Netherlands suggests that it could do just fine here.

So . . . just to be safe, let's do everything we can to keep the fungus out.


Prevention is the best means of control. You can help keep the fungus out of MN by not moving the fungus or any soil that may contain the fungus. Here are some tips how to do that.

Avoid importing infected nursery stock by dealing with reputable nurserymen and/or asking to see certification that the material is disease-free.

Clean all vehicles and tires thoroughly when traveling into or out of areas where the disease is found. Do so before coming home.

Clean shoes and hiking boots after visiting these areas.

Clean equipment or tools that come into contact with soil from these areas.

Check the source of newly planted shrubs and watch for symptoms of the fungus.

Because there are many other leaf spot diseases on ornamental shrubs and organisms that can kill oak trees, be sure to have any problems diagnosed before you assume you have the new fungus. But if you suspect a new introduction, contact your local Dept. of Agriculture representative, Extension agent, or the DNR forester in your area. They will help you identify the problem and determine where to go next. Remember, prevention is the key.