Lecture 08: Tree Diseases

Introduction

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 (Murrill) Barr [=Endothia parasitica (Murrill) P.J. Anderson & H.W. Anderson], the cause of the Chestnut Blight, had destroyed millions of Castanea dentata (Marsh.) Borkh., The American Chestnut trees. Even after this policy was adopted, Ulmus americana L., The American Elm, has almost been driven to extinction by Ophiostoma ulmi (Buisman) Melin & Nannf., 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 Werres, De Cock & Man in 't Veld, the cause of Sudden Oak Death and Ceratocystis fagacearum (Bretz) Hunt, 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 (Fig. 1).

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. Very few mature trees, such as those shown in Fig. 2, from Oregon are seen anymore. 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 (Fig. 3) 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 the  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 The Christmas Song, probably the favorite Christmas song of all time (give a listen below).

It is the tree that had inspired Henry Wadsworth Longfellow to compose his poem, The Village Blacksmith: 

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.

Symptoms

Infections normally occur on young twigs, entering through open wounds in the bark caused by feeding insects. The fungus then grows into 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 (Fig. 4) where yellow (Fig. 5) asexual reproductive structures that produce conidia and red (Fig. 6) sexual reproductive structures that produce asci and ascospores 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.

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.

Grente (1965) discovered 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 a virus in the fungus that is transferred from fungus to fungus (Day, et al., 1972). 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 controlling the disease in France (Grente, J. and Berthelay-Sauret, S., 1978). Unfortunately, control of The Chestnut Blight, in the United States, utilizing the hypovirulent strain was unsuccessful. Although the hypovirulent strain was successful in causing remission of the disease to occur in the trees in which it was inoculated, further transmission to other trees did not occur, or occurred only poorly due to vegetative incompatibility of the Italian hypovirulent mycelium with those of the virulent mycelia in the United States. Because of the incompatibility, the two mycelia do not fuse and the virus in the hypovirulent strain did not transfer to the virulent mycelia in the United States or would do so only poorly (Anagnostakis, 1977). However, further research have discovered a mitochondrial DNA mutant that carries a stable hypovirulent virus that can be transferred to the ascospores of the fungus, thereby making possible the dispersal of this hypovirulent strain to virulent strains on diseased trees (Anagnostakis, et al., 1998).

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 (Figs. 7-8). 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. 

Thus, the genetic diversity of the American Chestnut is still with us. With the inoculation of hypovirulent strains of the fungus into trees with sprouts, it has made possible the growth of these sprouts to an age where they will flower and produce seeds. This provides genetic diversity of the American Chestnut that can now be used in Hybridization of the American Chestnut now being carried out with the Chinese Chestnut, Castanea mollisima Blume, 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 (Fig. 9). Trees typically up to about 30 m tall, with a spreading canopy, were used primarily as an ornamental shade and street tree (Fig. 10). 

Dutch Elm Disease

The Dutch Elm Disease (DED) was once limited to be a single species belonging to the phylum Ascomycota, Ophiostoma ulmi (=Ceratocystis ulmi). This species 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 in the United States, in 1930, in Ohio and became a pandemic. 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. However, additional species whose host are also elms have since been introduced to Europe and the United States. Ophiostoma novo-ulmi was described by Brasier (1991) and is a more aggressive than O. ulmi.  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 and the cause of the second pandemic of DED. Since its discovery, O. ulmi has now become rare in nature and O. novo-ulmi has been the cause of even greater destruction to the elm forests.Thus, the enactment of Plant Quarantine Act did not prevent another catastrophe as serious as the Chestnut Blight. During the time that O. novo-ulmi was discovered, another species, O. himal-ulmi was described by Brasier and Mehrotra (1995) on elms, in the Himalayas.  

The spores of O. ulmi and O. novo-ulmi are dispersed by the American Elm Bark Beetle, Hylurgopinus rufipes (Fig. 11), and the European Elm Bark Beetle, Scolytus multistriatus (Fig. 12). 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 manner as well. 

Treatment

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.. 

Symptoms

Because DED clogs the water and food conducting tissue, as the leaves wilt, they will turn red and then brown (Fig 13). 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 (Fig. 14). 

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 due to lack of genetic diversity.

Sudden Oak Death

Introduction

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, Phytophthora 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. Instead, P. ramorum has now been isolated from numerous unrelated 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 leaves becoming pale green and later red in color (Fig. 15), discoloration of the trunk by cankers that are “bleeding” red or black saps (Fig. 16) 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 Annulohypoxylon thouarsianum (Fig 17) 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.

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. Its place of origin is unknown, but is known to be an invasive species. 

Rapid Ohia Death

Metrosideros polymorpha Gaudich. (ʻōhiʻa lehua or ʻōhiʻa) is an evergreen tree in the myrtle family that is endemic to Hawaiʻi and the dominant tree on the major Hawaiian Islands (Metrosideros polymorpha, n.d.), comprising about 62% of the total forest area (Hodges, et al. 1986), occurring in a wide range of habitats. It grows at sea level and up to 8,200 ft elevation, and in moist and dry forests. It varies in appearance, as the specific epithet "polymorpha" indicates, varying from being a tall tree, ranging from 66-82 ft under favorable conditions to an almost prostrate shrub when growing in boggy soils.

Because of the hard texture of the wood, in the native Hawaiians had previously utilized it for building houses, heiaus, poi boards, various weapons, outrigger canoes, as well as statues and idols (Metrosideros polymorpha, n.d.#Uses).  Currently, the tree is little used commercially, but it is ecologically significant in that it provides watershed protection and major habitat for several species of native birds, some of which are endangered (Hodges, et al. 1986). 

The ʻōhiʻa lehua populations of trees throughout Hawaiʻi, as is true of many species of native flora and fauna, have long been in decline. The forests have been invaded by various alien species, including Psidium littorale (strawberry guava) Falcataria moluccana (albizia), Miconia calvescens (purple plague) and others. Although these threats have caused some decline in the species, none has been as serious as the most recent fungal pathogens that cause what is commonly referred to as Rapid Ohia Death (ROD).

The disease was first observed by landowners on Hawaiʻi Island, in Puna, in 2010 and spread rapidly from there to tens of thousands of acres of ʻōhiʻa trees in a short time (University of Hawaiʻi News, 2018, April 16). It was not until 2014 that the pathogenic fungus was tentatively identified as Ceratocystis fimbriata, but later determined to be two new species by Barnes, et al. (2018) that were named  Ceratocystis huliohia I. Barnes, T.C. Harr. & L.M. Keith and C. lukuohia I. Barnes, T.C. Harr. & L.M. Keith. Of the two species, C. lukuohia is the more aggressive and deadly, causing a systemic wilt that will choke off the water supply to the tree quickly, causing death in a relatively short period of time. Ceratocystis huliohia also causes a systemic wilt, but spreads more slowly throughout the tree and kill off localized parts of the tree, but will also eventually lead to the death of the tree. Infected trees can readily be identified by the browning of tree crowns (Fig. 20). From Hawaiʻi Island, ROD was eventually dispersed to three other islands. The second island on which it was was documented was on Kauaʻi (Hurley, 2018), and then on Maui, but fortunately, the infection was only found on a single tree (Star Advertiser Staff, 2019).  Oʻahu was the last island on which ROD has been dispersed (Hurley, 2019). To date, there has not been any reports of ROD occurring on Molokaʻi and Lānaʻi. 

The of ROD has occurred from Hawaiʻi Island to other islands despite the strict quarantines that have been enforced with respect to transport of soil samples and ʻōhiʻa trees, as well as public outreach asking that clothes and shoes be cleaned by people before and after leaving  ʻōhiʻa forests. It has recently also been suggested that ambrosia beetle frass that contain spores from ROD fungi are readily dispersed by wind (Kylle, R., et al. 2018). However, I believe that these authors are not seeing the whole picture. Ambrosia beetles are known to be dispersers of path pathogen that have devastated numerous economically important trees including avocados, poplars, oaks and elms (Huler, J. and R.R. Dunn. 2011). The dutch elm disease, which has decimated elm species in Europe and United states is an example of how ambrosia beetles has contributed to the near extinctions of various elm species. We will go over the latter example when we cover the topic of insect-fungi symbiosis. 

Literature Cited

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

Anagnostakis, S. L. 1977. Vegetative incompatibility in Endothia parasitica. Experimental Mycology 1:306-316.

Anagnostakis, S. L., Chen, B., Geletka, L. M., and Nuss, D. L. 1998. Hypovirus transmission to ascospore progeny by field-released transgenic hypovirulent strains of Cryphonectria parasitica. Phytopathology 88:598-604.

Barnes, I., A. Fourie, M.J. Wingfield, T.C. Harrington, D.L. McNew, L.S. Sugiyama, B.C. Luiz, W.P. Heller, and L.M. Keith. New Ceratocystis species associated with rapid death of Metrosidros polymorpha in Hawaiʻi. Persoonia. 40: 154-181.

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

Brasier, C. M. 1991. Ophiostoma novo-ulmi sp. nov., Causative Agent of Current Dutch Elm Disease Pandemics. Mycopathologia 115:151-161

Brasier, C. M., Mehrotra, M. D. (1995). "Ophiostoma himal-ulmi sp. nov., a new species of Dutch elm disease fungus endemic to the Himalayas". Mycological Research 99 (2): 205–215.

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

Day, P. R., Dodds, J. A., Elliston, J. E., Jaynes, R. A., and Anagnostakis, S. L. 1977. Double-stranded RNA in Endothia parasitica. Phytopathology 67:1393-1396.

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

Grente, J. and Berthelay-Sauret, S. 1978. Biological control of chestnut blight in France. IN: "Proceedings of the American Chestnut Symposium." W. L. MacDonald, F. C. Cech, J. Luchock, and C. Smith, eds. pp. 30-34. West Virginia University, Morgantown.

Haugens, L. "How to Identify and Manage Dutch Elm Disease". Retrieved September 12, 2011, from http://www.na.fs.fed.us/spfo/pubs/howtos/ht_ded/ht_ded.htm. 

Hodges, C.S., K.T. Adee, J.D. Stain, H.B. Wood and R.D. Doty. 1986. Decline of Ohia (Metrosideros polymorpha) in Hawaii: a review. United States Department of Agriculture. Forest Service. General Technical Report PSW-86.

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

Huler, J. and R.R. Dunn. 2011. The sudden emergence of pathoenicity in insect-fungus symbiosis threatens native forest ecosystems. 278: 2866-2873.

Hurley, T. 2018. Ohia tree disease spreads to Kauai. Star Advertiser. June 12, 2018. Section B, pg. 1.

Hurley, T. 2019.

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

Kylle, R. C.P. Ewing, M.A. Hughes, L. Keith and G.M. Bennett. 2018. Presence and viability of Ceratocystis lukuohia in ambrosia beetle frass from Rapid Ohia Death-affected Metrosideros polymorpha trees on Hawaii Island. Forest Pathology. 49 (1): 1-4

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.

Metrosideros polymorpha (n.d.) In Wikipedia. Retrieved September 18, 2019, from https://en.wikipedia.org/wiki/Metrosideros_polymorpha

Metrosideros polymorpha (n.d.) In Wikipedia. Retrieved September 18, 2019, from https://en.wikipedia.org/wiki/Metrosideros_polymorpha#Uses

Monteiro-Vitorello, C. B., Bell, J. A., Fulbright, D. W., and Bertrand, H. 1995. A cytoplasmically transmissible hypovirulence phenotype associated with mitochondrial DNA mutations in the chestnut blight fungus Cryphonectria parasitica. Proceedings of the National Academy of Science 92:5935-5939.

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

Star Advertiser Staff (2019). Rapid ohia death found on Maui for the first time. Star Advertiser. July 3, 2019. Section B, pg. 2

United States Department of Agriculture (USDA), (2010, August 10).  Sudden Oak Death. Retrieved September 11, 2011, from http://www.aphis.usda.gov/hungrypests/suddenOakDeath.shtml

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

University of Hawaiʻi News (2018, April 16). Two new species of fungi that kill ʻōhiʻa trees get Hawaiian names. Retrieved September 18, 2019, from https://www.hawaii.edu/news/2018/04/16/ohia-killing-fungi-get-hawaiian-names/

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.

Important Terms and Concepts

Chestnut Blight: Disease of the American Chestnut caused by Cryphonectria parasitica, a member of the Ascomycota, that has decimated the species in its natural distribution. The disease also occurs in other species of chestnuts throughout the world, except those in China.  

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.  

Dutch Elm Disease: Disease of the American Elm caused mostly by two species Ophiostoma novo-ulmi and O. ulmi  that has decimated the species in its natural distribution. The disease also occurs in other species of elms throughout the world, except in China.

Hypovirulent Strain of Cryphonectria parasitica: Strain of Cryphonectria parasitica that is infected with a virus that weakens it, such that it not cause the death of death of Chestnuts even though cankers will still form on the trees. When mycelia of this strain come in contact with the virulent (=lethal) strains of this species, it is redered hypovirulent.  

Ophiostoma novo-ulmi: One of two species of Ascomycota that causes Dutch Elm disease. This is the more aggressive and virulent disease. The origin of this species is unknown.

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. This species was the one that originated from Europe and was responsible for the initial decline of the American Elm.

Phytophthora ramorum: Fungus causing the Sudden Oak Death. Despite its name, this species can infect a wide range of species of plants. The disease is now known to have spread by inadvertent transport of nursery plants, infected with the fungus, to various parts of the United States. This is also likely the means by which it was first introduced to this country.

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. The plant is inspected for disease organisms, and inspection of soils in which they are planted are also inspected. 

Rapid Ohia Death (ROD): A fungal disease of Metrosideros polymorpha that is rapidly spreading and killing trees of this species. If no treatment can be discovered, the ʻōhiʻa may become another of Hawaii's extinct species. There are actually two pathogens involved, Ceratocystis huliohia and C. lukuohia.

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

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

xOx  Gene: Gene that is derived from wheat and inserted into the American Chestnut genome that prevents infection of Chestnut Blight. Essentially, making it a GMO tree. It is the most recent means that has been proposed to treat the Chestnut Blight disease.

Questions to Think About

1.       What problems do we encounter with tree diseases that is not generally encountered in crop plants?

2.       What is quarantine, with respect to diseases and how effective is it?

3.       What is the Chestnut blight and what damaged did it do to the American Chestnut forests in the United States?

4.       The American Chestnut forest is barely surviving the Chestnut Blight, but has maintained its genetic diversity. How is that possible?

5.       What is the hypovirulent strain of the Chestnut Blight and why is it important?

6.       In breeding resistant hybrids of American Chestnut with the Chinese Chestnut, what fraction of its genetic makeup is actually that still American Chestnut?

7.       How is the OxO gene suppose to save the American Chestnut?

8.       What is the cause of the Dutch Elm Disease and what damage did it do to the American Elm forests of the United States?

9.       What is the means of spore dispersal in the Dutch Elm Disease?

10.   Why will it be easier to restore the American Elm forest than the American Chestnut forest

11.   The Sudden Oak Death disease infects many economically unimportant plants. Why worry about these other plants?

12.   What is currently the most effective means by which Sudden Oak Death is treated?

13.   What species of fungi cause the Rapid ʻŌhiʻa Death (ROD) disease?

14.   What is probably the primary means by which ROD has spread so rapidly?

15.   Generally, what is the major problem in breeding resistance in tree diseases that was not encountered in crop plants?