Tree Diseases


Although a control was discovered for the Late Blight of Potato and Downy Mildew of Grapes diseases, both began with the inadvertent introduction of the disease causing fungus. As devastating as these diseases were, most countries, including the United States, still did not implement preventive measures to stop introduction of potential diseases and pests. It was not until 1912 that the Plant Quarantine Act was passed and that was only after Cryphonectria parasitica (=Endothia parasitica), the cause of the Chestnut Blight, had destroyed millions of Castanea dentata, American Chestnut trees. Even after this policy was adopted, Ulmus americana, the American Elm, has almost been driven to extinction by Ophiostoma ulmi, the cause of the Dutch Elm Disease. Despite good intentions, quarantine has not had great success in excluding plant pathogens from entering United States borders. Even in recent time fungal pathogens of trees have found their ways across borders that have had strict quarantine procedures. Some examples that have occurred in recent years include Phytophthora ramorum, the cause of Sudden Oak Death and Ceratocystis fagacearum, the cause of Oak Wilt.

We will go over the story of these diseases of trees that are native to North America. Unlike crop diseases, treatment for tree diseases is far more difficult. Breeding for resistance take far longer and utilization of chemical treatments is not efficient because wide girths of most forest trees make penetration of fungicide into the host extremely difficult. Each type of the trees are valuable in various ways.  The American Chestnut was a forest tree logged for its wood, but its fruit was also an food for people, as well as animals in chestnut forest. The American Elm while not sought after for its wood is desirable as an ornamental shade tree. Oaks are valuable hardwood trees for lumber and are an important part of the ecosystem for animals that reside in their forests. All three are of value beyond what can be evaluated in terms of dollars and cents and are a part of our American heritage.

The Chestnut Blight

Castanea dentata, The American Chestnut

In its natural habitat, at the turn of the century, the American Chestnut's distribution ran from New England to the Mid-Atlantic States to the Midwest and down the Appalachian highlands to the Gulf States.

Map of American Chestnut distribution prior to Chestnut Blight. Map from:

Trees were typically up to 100 feet fall, with a columnar growth, 3-4 feet in diameter. Large trees could be found that were 10-12 feet in diameter, with crowns that were up to 100 feet wide. The wood of the Chestnut was utilized for a variety of purposes. Its appearance was like that of oak, but not as strong or dense, making it a more workable wood for the building of furniture. Because of its resistance to decay, it was also valued for usage as railroad ties, utility poles and coffins as well as for its tannin. In these usages, no single species has been able to replace the chestnut. However, the Chestnut was more than a species that was logged for its wood. It is part of our American heritage. The fruit of the chestnut was an important food for the Native American Indians that lived within the distribution of this species and for the Europeans who later colonized North America as they moved westwards. Before the blight, it was very much part of Thanksgiving and Christmas. The nuts of the Chestnut were used for part of the turkey dressing and was food for the turkey and other animals that lived in the forests, as well. Even though this generation of children have probably never seen a Chestnut, it is still a well known tree because it has become part of our culture. This is the tree that Nat King Cole sang about in his Christmas Song (click here for excerpt) and it is the tree that had inspired Henry Wadsworth Longfellow to compose his poem, The Village Blacksmith: (an excerpt for your edification)

Under a spreading chestnut-tree
The village smithy stands;
The smith, a mighty man is he,
With large and sinewy hands;
And the muscles of his brawny arms
Are strong as iron hands.
Left image: Large, uninfected, 120 year old American Chestnut, in Oregon from Right image: Leaves and fruit of American Chestnut from

The Chestnut Blight

The story of the Chestnut Blight is can be found in various books (Carefoot and Sprott, 1967; Schumann, 1991; Hudler, 1998) Cryphonectria parasitica, the cause of the Chestnut Blight, was first detected in 1904, by Mr. Herman Merkel, the head forester, at the Bronx Zoo. Merkel took immediate action when the disease was discovered. He sprayed, he cut down the dead trees and thought that he the disease under control. However, the following year, more trees became infected and the blight was now spread further. His report on this disease was sent to the Department of Agriculture, in Washington and plant pathologists were soon dispatched to New York. Although, they could see that it was a serious and virulent disease, they didn't feel that there was any cause for alarm. It was, after all, a simple matter of removing the dead and infected trees and replanting. Unfortunately, the disease did not end in New York. The following year, reports were received from outside the park, and chestnuts were dying throughout New York State. Now people were becoming alarmed about the disease, as they should be, and although pathologists now gave the disease top priority, the disease continued slowly through the eastern forests.

Although the disease was first reported from the Bronx Zoo, in 1904, it seems likely that the outbreak of the disease did not originate there. It was only because of the watchful eyes of Mr. Merkel that the disease was detected immediately. According to Dr. Sandra Anagnostakis, the introduction of the causal organism must have occurred much earlier, possibly during the 1880's and from Japan, not China, as was first believed. Also, the introduction occurred not once by many times by importers of different species of chestnuts trees. A review as to what Dr. Anagnostakis believes to have occurred can be read here. According to Anagnostakis, an earlier introduction is more likely since the blight would otherwise have been unable to spread as rapidly as it did.

An adequate means of controlling the disease, in the United States, has never been found and by 1915, the government had even given up trying to eradicate this disease. As far as the government was concerned, the American Chestnut would either survive on its own or become extinct. The government did, however, pass the Plant Quarantine Act, in 1912, in order to reduce the likelihood of such a catastrophe happening again.  By 1923, the blight had spread to approximately 80% of all American Chestnuts throughout their natural distribution. By the 1950s, 80% of all had succumbed to the blight. It is now impossible to find an uninfected tree in its natural range of distribution. Yet, the species has managed to survive. The disease does not affect the roots of the tree and each time a tree dies, sprouts grow from the old stumps of the diseased trees, but growth does not occur for more than 15 feet. At that time the sprouts eventually are infected and die from the blight. Nevertheless, as long as the Chestnuts continue to sprout in this matter, there is hope that perhaps some means will be discovered that will save this species or that a mutation will arise that will be resistant to the disease.

Unfortunately, the blight did not end in the United States. Once the disease destroyed the American Chestnut forests, it was inadvertently introduced across the Atlantic, sometime during the 1930s, where it found a suitable host in the European Chestnut, which made up approximately 15% of the forests of Italy and southern European countries.


Infections normally occur on young twigs, entering through open wounds in the bark caused by feeding insects. The fungus then grows into through the bark and works its way to the food conducting tissues and the cells that give rise to it. This will eventually lead to the death of the twig. The infected twig can be recognized by the formation of cankers where brightly colored red or yellow reproductive structures are borne that may form around the circumference of the twig, eventually leading to the death of the twig. Infections occur from twig to twig and death occurs rapidly in these terminal branches and when the infection reaches the main trunk, death of the entire tree will follow.

Upper left image sexual fruiting bodies, where asci and ascospores are borne. Lower left, asexual fruiting bodies where conidia are borne. Both images from: Right image of canker of Chestnut Blight from:

Unlike the crop plants which we have discussed, superficial spraying with a fungicide is useless, since the infection occurs deep within the tree, below the bark. In theory, the disease can be controlled in a single tree or perhaps a small number of trees, if all the infected twigs are pruned and destroyed while the infection is just beginning. However, with the millions of trees that that were once present, this was not practical.

 In the 1950s, some trees in Italy were found with cankers of the Chestnut Blight, but were not being killed by the disease. Studies have demonstrated that isolates from these trees were a new strain of the fungus that was much weaker than the common strains. These strains have been designated as hypovirulent. Not only are these weaker, but when they come in contact with a virulent strain, i.e. one that will kill the tree, the hyphae fuse and the factor that causes the hypovirulence is transferred to the virulent strain. This phenomenon appears to be caused by virus in the fungus that is transferred from fungus to fungus. Extensive research was carried out in an attempt to utilize this hypovirulent strain of this fungus to control the disease, which has been successful in parts of Europe. Although in theory this method should control the blight in the United States, it has not done so. For reasons not understood, the hypovirulent strain of the blight seems to remain localized and does not disperse.

There Is Still Hope

Although the original, giant American Chestnuts trees are no longer with us, as a result of the blight, all is not lost. When the old trees died off, new sprouts emerged from the base of the dead trees. These sprouts will also eventually die off as a result of the disease, but new sprouts continues to grow new trees and a shrubby growth of American Chestnuts has maintained itself in native forests. 

Left Image: Blighted Chestnuts from Right Image: Sprouts growing from base of dead stump of Chestnut from

Thus, the genetic diversity of the American Chestnut is still with us. Hybridization of the American Chestnut is now being carried out with the Chinese Chestnut, Castanea mollisima, which is resistant to the Chestnut Blight, and the planting of the hybrid is now taking place that may yet save the American Chestnut. An early program that began in 1925 involved hybridization of the American Chestnut with both the Japanese and Chinese Chestnut. By the late 1940's, thousand of hybrids had been generated that were tested annually for resistance until 1978 and the results written up by Berry (1980). The conclusion of the report was that none of the the hybrid developed into resistant strains although some showed promise. In 1983 the American Chestnut Foundation was founded that started a new hybridization program that they now believe has developed a hybrid that has incorporated the resistant gene(s) from the Chinese Chestnut into the American Chestnut while still maintaining 15/16 of the genetic integrity of the latter. Planting of the hybrid is now being carried out and it will be several more generations before it will be known if a truly blight resistant hybrid has been developed. Trying to develop a hybrid resistant tree that is long lived takes a long time! Even if a successful hybrid is developed, replanting the native forests will prove to be a major task. A different means of developing a resistant American Chestnut involves that "dirty method" creating a genetically modified organisms (GMO). A summary describing the last two methods of developing a blight resistant American Chestnut can be found here.

Dutch Elm Disease

Ulmus americana

Ulmus americana, the American Elm is native to Eastern North America, occurring in its northern most distribution from Nova Scotia, west to Saskatchewan, South to Central Texas and east to Florida. Tree typically up to about 30 m tall, with a spreading canopy, used primarily as an ornamental shade and street tree. 

Left Image: Map of distribution of Ulmus americana, American Elm. From Right Image: Pathway shaded with American Elms in New York's Central Park. From Mike Rollinger

Dutch Elm Disease

The Dutch Elm Disease (DED) is now known to be comprised of three species of Ascomycota, Ophiostoma himal-ulmi, O. ulmi and O. novo-ulmi. Ophiostoma ulmi (=Ceratocystis ulmi) first appeared in Holland in 1919 where it devastated the elm trees and would later spread throughout Europe. In 1927, it crossed the English Channel and attacked the elm trees in England. In a few short years, the disease would find its way across the Atlantic where it would become established and cause the first DED pandemic. It came in on imported European Elm logs on at least three separate occasions. Its presence was first confirmed in Ohio, in 1930. The occurrence of this fungus in the United States was discovered by two Dutch plant pathologist, Christine Buisman and Bea Schwarz who simultaneously discovered the disease in Cleveland and Cincinnati, respectively. They were all too familiar with this fungus since it had already laid waste to the elms from their native Holland. The more agressive O. novo-ulmi would come later and cause a second pandemic. It is uncertain when it became established or where its origin is, but is believed to have arrived sometime in the 1940's or 1950's and is believed to be responsible for the higher mortality rate that occurred to the 1970's. During this period, O. ulmi has become rare in nature. Thus, the enactment of Plant Quarantine Act did not prevent another catastrophe as serious as the Chestnut Blight.

The spores of O. ulmi and O. novo-ulmi are dispersed by the American Elm Bark Beetle, Hylurgopinus rufipes, and the European Elm Bark Beetle, Scolytus multistriatus. The female beetles carry the spores of the fungi to the elm trees where they dig a series of tunnels into the host where they lay their eggs. The beetles also will cultivate the fungi in the tunnels that will serve as the food source for the larvae when they hatch. The larvae will mature into adults and will further spread the DED when they emerge from the host. However, during the period before the larvae hatch, the mycelium grows into the food and water transport tissue and causes a vascular wilt that will lead to the death of the infected area. More will be said about these beetles later in the semester. For now it is important to understand that without the beetles the DED would not have been able to spread as it did. The infected trees normally grow in close proximity with other trees and as their roots come in contact with one another, they will be grafted to each other and the disease can spread from tree to tree in this matter as well. 

Left Image: Hylurgopinus rufipes from Right Image: Scolytus multistriatus Image: 


As in the case of the Chestnut Blight, the internal nature of the infection makes treatment difficult. However, control of the disease is possible. Since beetles carry the spores of the disease, one means by which we can reduce infection is to control the beetles by using insecticides. Removal of infected plant material will also aide in reducing the opportunity for infection since the source of the DED will be removed. Spraying infected trees with fungicide can also be used to treat the disease, but as in the case of the Chestnut Blight, it is practical only for one to a few trees since it would have to be sprayed into the water and food conducting tissue to get at the fungus.. 


Because DED clogs the water and food conducting tissue, as the leaves wilt, they will turn red and then brown. Infections occurring in branches will have wilting within the crown of the tree. While those infections that occur as a result of root grafts will have the symptoms lower in the crown and on the side of the tree where the root graft has occurred. Where infection occurs in the water and food conducting tissue, a dark discoloration of the wood will occur. This symptom can readily be observed by cutting a section into the infected wood. 

Left Image: Discoloration of sections through stem in first two section twigs. Most prominent in second twig. Third twig is uninfected. From Right image is an infected elm tree with discoloration of leaves. From

Saving the American Elm

Today, the only hope seems to be hybridization between the various species of elms. However, because of the American Elm's use as a shade tree, the characteristic of a large canopy and crown must be maintained.  The Asiatic elm does not have this since characteristic, but efforts to hybridize it with the American Elm has met with some success. Unlike the American Chestnut, resistant trees of the American Elms have been found. With these resistant trees, it has been possible to come up resistant cultivars of the American Elm that are not hybrids. Whether these new elms are hybrids or are American Elms, they are not completely resistant to DED, but with sound management of these new trees will have an excellent chance of survival. The biggest danger is that there are not a lot of DED resistant cultivars. Thus, if unfavorable conditions should come about, large populations of elm trees could conceivably be lost.

Sudden Oak Death


The disease seemed to appear suddenly from several disparate localities in Europe and the United States during the 1990’s. In 1993, an undescribed Phytophthora species caused a twig blight disease on Rhododenron, in nursery plants and gardens, and less commonly, on Viburnum (Werres et al. 2001). It would not be until Werres et al. (2001) that it would be described as a new species, Phytopthora ramorum. The same species would later be found in Mill Valley, California, in 1995, where large numbers of Lithocarpus densiflorus, tan bark oak and Quercus, oak species were dying from the disease, thus the common name for the disease sudden oak death (SOD). The distribution of this P. ramorum would spread along the California coast, south to Monterey County and north, to Oregon and inland and would later move inland and has been demonstrated to be established on the University of California, Berkley campus. By 2002, it had been responsible for the death of tens of thousands of oak trees (White, 2002). Among species of Quercus, it appears to be restricted to three species of oaks: Q. agrifolia Née (coast life oak), Q. kelloggii Newb. (California black oak), Q. parvula E. Greene var. shrevei (C.H. Muller) Nixon (shreve oak) and Q. chrysolepis Liebm. (canyon live oak). However, its host range in California is not restricted to oaks or to members of oak family, Fagaceae. Instead, P. ramorum has now been isolated from numerous host plants that include agricultural crop plants, nurseries plants, as well as native and naturalized plants. The disease has now also been observed to make a shift to softwood, lumber trees, such as Sequoia sempervirens, the Coast redwood and Pseudotsuga menziesii, Douglas fir (White, 2002). A list of plants that have been infected can be found in (USDA, 2010). Although, infection may not be fatal to many of these other hosts, they serve as carriers of the disease that can potentially infect susceptible hosts where it may prove fatal. The dispersal of P. ramorum has expanded by natural means wind and water. Despite expanded quarantine efforts, targeted mostly at nurseries, the disease continues to expand. In 2004, two nurseries in Southern California had inadvertently extended the range of P. ramorum beyond California, to 21 states. Other means by which the disease spreads is by infected plant debris adhering to shoes or clothing of hikers being in infected area. 

Symptoms of Disease

The disease has been described in various publications (Garbelotto and Rizzo, 2006; Rizzo, et al. 2002; USDA, 2010). Its symptoms can be recognized in oak species by discoloration of the trunk by cankers that are “bleeding” red or black saps of what would seem to be otherwise healthy oak trees. The canker would eventually spread around the entire circumference of the tree. As the disease advanced, opportunistic organisms were also commonly observed. The stroma of Xylaria thouarsianum are commonly observed, as well as species of ambrosia beetles, Monarthrum scutellare and M. dentiger, and Pseudopityophthorus pubipennis, a bark beetle. More will be said of ambrosia beetles in a later lecture.

Treatment of Disease

At this time there is little that can be done to stop the continuing range expansion of P. ramorum. Preventive measures, such as early detection, quarantine of nurseries and educating the public, i.e. cleaning hiking shoes, clothing and tires of vehicles that have traveled to areas of infections, and burning of infected plant material at site of infection are the only means of control at this time. Many web sites have suggestions by which the community may help in the prevention of further spread of this disease. Potassium phosphite, sold commercially as Agri Fos® has had some success if applied to uninfected trees may prevent infection or suppress trees that are infected. More information of this treatment can be found here. It is, however, costly and not practical for saving large oak forests since its application must be carried out for individual trees.

The above images were from:, and, respectively.

Oak Wilt

An oak diseased caused by Ceratocystis fagacerum, a disease closely related to Ophiostoma ulmi. As with DED, the disease is a wilt characterized by discoloration of leaves and is spread by insects and root grafts. Oak Wilt occurs in the Midwest, mostly on red and black oaks, but all species of oaks are susceptible. Its distribution range is Michigan, Pennsylvania, Maryland and West Virginia. 

Left Image: Distribution of Ceratocystis fagacerum from Right Image: Discoloration of leaves in Oak Wilt symptom from

Literature Cited

 Agrios, G.N. 2005. Plant Pathology. 5th. Edition. Elsevier Academic Press, Burllington, MA

Berry, F.H. 1980. Evaluation of Chestnut Test Plantings in the Eastern United States. Forest Service Research Paper. NE-454.

Carefoot, G.L. and E.R. Sprott. 1969. Famine on the Wind. Angus & Robertson Ltd., London.

Garbelotto, M. and D. M. Rizzo. 2005. A California-based chronological review (1995–2004)of research on Phytophthora ramorum, the causal agent ofsudden oak death Phytopathol. Mediterr. 44: 127–143

Haugens, L. "How to Identify and Manage Dutch Elm Disease". Retrieved September 12, 2011, from 

Hudler, G.H. 1998. Magical Mushrooms and Mischievous Molds. Princeton University Press. Princeton,  New Jersey.

Kavaler, L. 1965. Mushrooms, Molds, and Miracles. The John Day Company, New York.

Large, E.C.  1940. Advance of the Fungi. Henry Holt & Company, New York.

Littlefield, L.J. 1981. Biology of the plant rusts: an introduction. Iowa State University Press, Ames.

Rizzo, D. M., Garbelotto, M., Davidson, J. M., Slaughter, G. W., and Koike, S. T. 2002. Phytophthora ramorum as the cause of extensive mortality of Quercus spp. and Lithocarpus densi-florus in California. Plant Dis. 86:205-214.

Rizzo, D.M., Garbelotto, M., Davidson, J.M. & Slaugter, G.W. 2002. Phytophthora ramorum as the cause of extensive mortality of Quercus spp. and Lithocarpus densiflorus in California. Plant Dis. 86: 205-214

Schumann, G. L. 1991. Plant Diseases: Their Biology and Social Impact. The American Phytopathological Society. St. Paul, Minnesota

United States Department of Agriculture (USDA), (2010, August 10).  Sudden Oak Death. Retrieved September 11, 2011, from

University Of California, Berkeley (2006, April 21). Sudden Oak Death Introduced To U.S., Study Finds. ScienceDaily. Retrieved September 11, 2011, from­ /releases/2006/04/060420235514.htm

Werres, S., Marwitz, R., Man In't Veld, W.A., De Cock, W.A.M., Bonants, P.J.M., De Weert, Themann, K., Ilieva, E. & Baayen, R.P. 2001. Phytophthora ramorum sp. nov., a new pathogen on Rhododendron and Viburnum. Mycol. Res. 105: 115-1165.

White, J.L 2002. SOD pathogen hits coast redwoods, Douglas fir. California Agriculture 56(6):182-182.

Terms and Concepts

American Elm Bark Beetle: One of two beetles responsible for dispersal of spores of Ophiostoma ulmi and O. novo-ulmi, the cause of Dutch Elm Disease.

Ceratocystis fagacerum: Species of Ascomycota that causes Oak Wilt disease.

Cryphonectria parasitica: Species of Ascomycota that causes the Chestnut Blight disease. Disease is responsible for loss of the Castanea dentata (American Chestnut) forest throughout North America.

Cultivar: A short hand for (culti)vated (var)iety of plants that have been developed from a natural species and maintained under cultivation  

European Elm Bark Beetle: One of two beetles responsible for the dispersal of spores of Ophiostoma ulmi, cause of the Dutch Elm Disease.

Hypovirulent Strain of Cryphonectria parasitica: Strain of Cryphonectria parasitica that are weaker than the common strains of this species and will not cause the death of death of Chestnuts. When mycelia of this strain come in contact with the virulent (=common) strains of this species, it is redered hypovirulent.  

Oak Wilt: Disease of oak, similar caused by Ceratocystis fagacerum and related to Dutch Elm disease.

Ophiostoma novo-ulmi: One of two species of Ascomycota that causes Dutch Elm disease. A more aggressive, virulent disease. 

Ophiostoma ulmi: One of two species of Ascomycota that is the cause of the Dutch Elm disease that is responsible for the elm forests in North America and Europe.  

Phytophthora ramorum: Species of Oomycota that causes Sudden Oak Death, but many other hosts can be infected.

Plant Quarantine Act: Following the introduction of the Cryphonectria parasitica, law enacted that would require plants coming into country to go through a quarantine period to ensure that they are not carrying diseases.

Sudden Oak Death: Disease of oaks and wide variety of oak caused byi Phytophthora ramorum.

Virulent Strain of Cryphonectria parasitica: Common strain of Cryphonectria parasitica that normally will eventually kill Chestnut once infection occurs.

Questions to Think About

  1. How did the American manage to survive the Chestnut Blight and was even able to maintain its genetic diversity despite the fact there apparently was no resistant gene to the disease in this species?
  2. Why is treatment with chemicals so difficult in tree diseases?
  3. What is the means of spore dispersal in the Dutch Elm Disease?
  4. The American Elm has been saved from the Dutch Elm Disease by breeding for resistance and hybridization. The American Chestnut also appears that it may be saved by breeding for resistance as well. However, it has taken a long time for developing resistant cultivars. Why has it taken so long?