Diseases of Plants and Animals
Bacteria and fungi (together with insects) play an important role in natural communities because they help break down material from dead plants into nutrients that benefit living plants. But non-native bacterial, viral and fungal pathogens can kill entire populations of plant species that have no natural resistance.
Explore this page:
- Where Do Plant Diseases Come From?
- Plant Diseases
- You Can Make a Difference!
Where Do Plant Diseases Come From?
Many of the most threatening plant diseases spreading in the U.S. today were brought there on plants or wood products from around the world. (In the same way, some of the most threatening plant diseases spreading in Asia today were brought there from the U.S.!) Ecobit: Why are catastrophic plant diseases usually non-native?
Today, there are virtually no mature American chestnuts remaining in these forests.
What killed the chestnuts? A fungus called chestnut blight (Cryphonectria parasitica).
Before the blight, the fast-growing American chestnut provided foliage, fruits, and living wood as food for bears, turkeys, and native insects and many more animals. Its dependable, abundant crops of chestnuts were an important food for Native Americans, and later, European immigrants who incorporated them into Christmas traditions.
It easily survived wildfires and outbreaks of native insects and diseases. Its durable, high quality wood was widely used in construction and carpentry. But since the 1930s, it has all but disappeared. Store-bought chestnuts today come from European stock or hybridized strains.
The chestnut blight fungus was apparently unwittingly brought to the U.S. on highly resistant Japanese and/or Chinese chestnut trees in the late 1800s, and spread undetected for many years throughout the U.S. via the nursery trade.
The fungus enters wounds on the tree and grows in and under the bark, forming cankers on the stems. The tree eventually dies back to the roots. Chestnut blight is easily spread on the feet of any insect or other animal that walks across the cankers.3
In some cases the roots continue to send up sprouts, which succumb to the fungus before they are old enough to reproduce. After the loss of American chestnut, drought-tolerant oaks appear to have increased where American chestnuts once grew.4 5 Oaks can host the fungus without getting sick, so there is a continual source of the fungus to infect new chestnut sprouts.
Scientists are working to develop blight controls and to breed a resistant strain of American chestnut, with the goal of restoring the majestic chestnut to eastern forests of North America.
Butternut canker (Sirococcus clavigignenti-juglandacearum) is currently eliminating entire populations of butternut trees (also called white walnut) throughout its range in the United States.
The origin of the fungus is unknown, but it was probably introduced from outside the United States in the mid 1900s.6
Infected trees typically have dead branches or a dying top. Cracked, sunken, discolored bark may ooze black inky fluid in spring, or appear sooty with whitish margins in summer. Where the bark has come loose, dark oval stains may be visible on the wood beneath.7 Controls currently do not exist.8
Rain splash and wind (and possibly insects and other animals) spread the spores to other butternut trees and to black walnut. However, black walnut, which is related to butternut and overlaps part of its geographic range, appears to be resistant to the fungus.
Butternut has prized edible nuts. Never a very common tree, butternut is becoming hard to find. Since some trees in the U.S. seem to display resistance, there is hope that the species can outlive the disease.
Dogwood anthracnose (Discula destructive) is a fungus that appeared simultaneously on both coasts of the United States in the mid-1970s, origin unknown.
It forms purple-rimmed tan blotches on the leaves, and cankers on the twigs of our beautiful flowering dogwood. Lower branches often die first, and the trunk may produce a lot of sprouting as a last-ditch (but ill-fated) attempt to survive.9
Healthy trees may be able to survive a bout with the fungus, but trees stressed from drought or other problems can die within a few years—sometimes within months. Early removal of infected parts may help the tree survive (taking care to disinfect the pruning tools with rubbing alcohol between cuts).
The spores of the fungus can be splashed by rain or carried by other means, such as insects, birds, and pruning tools. Some trees seem to be resistant.
Flowering dogwood trees are valued for their beauty in early spring and because their fall berries feed many birds.
Dutch Elm Disease
Dutch elm disease (Ophistoma ulmi) is a fungal disease from Asia, not Holland. Its common name comes from the fact that it was first described by Dutch scientists, not many years before it arrived in North America in about 1930.
It can affect all of our native elm species, and has swept through much of the United States, killing a majority of the elms formerly present. The fungus is moved from infected elm trees to healthy ones by spores carried on the elm bark beetle. The disease can also be passed from tree to tree by their interlocking roots.10
When the elm bark beetle comes to feed and reproduce in the trunk of a tree, it drops fungal spores, which the tree attempts to quarantine by closing off newly-infected parts of its circulatory system—effectively shutting off the flow of water and nutrients to any limb that depends on that part of the tree’s vascular system.
The visual result is a tree with individual dead branches with brown, wilted leaves. As the fungus spreads, this symptom eventually progresses throughout the tree and kills it.
One option in the battle against Dutch elm disease is to inoculate individual elm trees against the fungus—an expensive option at $100 per tree.
To decrease the risk of attracting elm bark beetles (which move the fungal spores from tree to tree), elm trees can also be pruned of their dead branches—but it should be done in winter when there is no smell of fresh sap to attract more elm bark beetles. Elm bark beetles are more attracted to weak and stressed elm trees, so watering elm trees during a dry summer helps deter the beetles.
Elms growing within 50 feet of one another risk infecting each other through their roots. Trenching around a tree exhibiting early symptoms will help to protect nearby trees. It is critical to the survival of other elms that trees infected with Dutch elm disease be immediately destroyed, so that the elm bark beetles cannot continue to reproduce in its trunk; some estimates suggest that, left in place, a diseased tree can release upwards of 400,000 beetles within two years!11
Elm Yellows (or Elm Phloem Necrosis)
Elm yellows is caused by a bacteria that is spread by a sap-sucking insect, the leaf hopper. Unlike Dutch elm disease, elm yellows typically affects the entire crown of an elm tree all at once, turning the leaves yellow in late summer and causing them to drop prematurely.12 Trees may develop witch's broom—dense sprouts in tiny patches in the crown.
By the time symptoms have appeared, the tree is infected systemically, and may die right away, or within a year or two. When infected, the inner bark of several species of elms develops the odor of wintergreen; that of slippery elm acquires the smell of maple syrup.13
Beech Bark Disease
Beech bark disease (Nectria coccinea var. faginata or Nectria galligena) arrived in the United States around 1890, and is slowly spreading throughout the eastern part of the country, killing large American beech trees. Beech bark disease has been found in Virginia forests14 and Maryland forests.15
It is a canker disease caused by a fungus that enters trees through tiny wounds in the bark, killing patches of inner bark. If enough cankers form, ringing the entire tree trunk, the tree will die.16
The wounds by which the fungus enters are created by a yellow beech scale insect that inserts its needle-like mouth to feed on the inner bark. This scale should not be confused with the generally harmless beech blight aphid.
American beech plays an important role for wildlife, providing food—beechnuts—and den habitat for numerous animals.
There is currently no cost-effective way to manage this disease in a large forest stand. Fortunately, there are some American beech trees that seem to have less feeding injury from the beech scale. These resistant trees suffer less of an attack by the fungus. Over time, the offspring of these healthy trees may be able to replace the trees that succumb to beech bark disease.17
Sudden Oak Death
The origin of sudden oak death (Phytophthora ramorum) is uncertain, but it appeared on the west coast of the U.S. in the mid 1990s. Natural resource managers everywhere should be watchful for early signs of the disease because of nationwide shipments in 2004 of millions of potentially infected host plants (rhododendrons and camellias) from west coast nurseries. The pathogen prefers cool, moist air, putting drier natural communities at lesser risk.
The pathogen weakens the oak tree, causing reddish or black bleeding ooze from lesions on the bark, and later a rapid browning of the leaves. It renders the tree vulnerable to bark beetle and fungus invasions.18
Many plant species can host the sudden oak death pathogen without dying. Unfortunately, this creates an ongoing source for the air-borne pathogen.19 Such hosts include many ornamental nursery plants as well as forest trees and shrubs such as mountain laurel, azaleas, rhododendrons, viburnums, witch hazel, and Solomon's plume.20
Hikers, researchers, and horses may unintentionally move spores from one forest to another, unless care is taken to clean their shoes, hooves, tires, equipment, etc. when leaving an infected area, especially in areas of muddy soil.
Various chemical treatments, including injections to the tree trunk, can be used preventatively in an area known to be at risk of an epidemic, or with very early detection. Laboratory tests show northern red oak and southern red oak to be highly susceptible to the disease.21 The pathogen prefers cool, moist air.
You Can Make a Difference!
"An ounce of prevention is worth a pound of cure." The sooner someone notices telltale signs of plant disease outbreak and speaks up, the better, whether it's in your own neighborhood or in a park! Parks keep a vigilant watch, and ask visitors to do the same, so that suspected disease outbreak can be identified early and curbed where possible.
When visiting a park, you can help keep natural communities healthy by being observant and sharing what you see with park staff. Ecobit: An Extra Set of Eyes
- 1. . 1910. The ecological plant geography of Maryland; coastal zone; eastern shore district.
- 2. . 1974. Plant communities on a quartzite ridge in Chester County, Pennsylvania. Proceedings of the Pennsylvania Academy of Science . 48:101-106
- 3. . 1997. Chestnuts and the Introduction of Chestnut Blight.
- 4. . 1973. Distribution of major forest species in southeastern Pennsylvania. Ecological Monographs. 43:303-327
- 5. . 1978. A study of forest communities in southern Chester County, Pennsylvania. Proceedings of the Pennsylvania Academy of Science 52:37-44.
- 6. . 1997. Three American tragedies: chestnut blight, butternut canker, and Dutch elm disease. In: Britton, Kerry O., ed. Proceedings, exotic pests of Eastern forests; 1997 April 8-10; Nashville, TN. Tennessee Exotic Pest Plant Council: 45-54
- 7. . 1995. Pest Alert: Butternut Canker.
- 8. . 2009. Major forest insect and disease conditions in the United States 2007.
- 9. . 2001. How to identify and control dogwood anthracnose.
- 10. . 1998. How to identify and manage Dutch elm disease.
- 11. . 2003. European elm bark beetle and Dutch elm disease in Seattle. Accessed 2015
- 12. . 1998. How to identify and manage Dutch elm disease.
- 13. . 2008. Plant Answers.
- 14. . 2017. Major Forest Insect and Disease Conditions in the United States: 2015.
- 15. . 2008. Beech Bark Disease. Accessed 2018
- 16. . 2008. Forest Health Fact Sheet: Beech Bark Disease .
- 17. . 2008. Forest Health Fact Sheet: Beech Bark Disease .
- 18. . 2008. Sudden oak death.
- 19. . 2008. Sudden oak death.
- 20. . 2008. Sudden oak death.
- 21. . 2008. Sudden oak death.