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An Index of Common Tree Diseases

The Major Disease of Trees in the United States


21. Laminated Root Rot

Laminated Root Rot
Canadian Forest Service
The disease Phellinus weirii occurs in patches (infection centers) sporadically distributed in clusters throughout its range. The most susceptible hosts are Pacific silver fir, white fir, grand fir, Douglas-fir, and mountain hemlock.

22. Littleleaf Disease

Littleleaf Disease
Littleleaf disease is the most serious disease of shortleaf pine in the Southern United States. Affected trees have reduced growth rates and usually die within 6 years. The disease is caused by a complex of factors including the fungus Phytophthora cinnamomi Rands, low soil nitrogen, and poor internal soil drainage. Often, microscopic roundworms called nematodes and species of the fungal genus Pythium are associated with the disease.

23. Lucidus Root and Butt Rot

Lucidus Root and Butt Rot
Lucidus root and butt rot disease is one of the most common root and butt rots of hardwoods. It has a wide host range including oaks, maples, hackberry, ash, sweetgum, locust, elm, mimosa, and willows, and is found throughout hardwood forests. Host trees normally decline for a variable period of time and then die.

24. Mistletoe (Phoradendron)

Mistletoe on Broadleaf
USFS Photo
Members of the genus are parasites of conifer and hardwood trees and shrub in the Western Hemisphere. There are seven species of native true mistletoe that are found on hardwoods in many parts of Eastern, Western, and Southern United States. The one most commonly known and widespread is P. serotinum (also known as P. flavescens) which occurs mainly in the East and Southeast.

25. Oak Wilt

Oak Wilt
Robert L. Anderson, USDA Forest Service, United States
Oak wilt, Ceratocystis fagacearum, is a disease that affects oaks (especially red oaks, white oaks, and live oaks). It is one of the most serious tree diseases in the eastern United States, killing thousands of oaks each year in forests and landscapes. The fungus takes advantage of wounded trees - the wounds promote infection. The fungus can move from tree to tree through roots or by insects. Once the tree is infected there is no known cure.

26. Powdery Mildew

University of Illinois Extension
Powdery mildew is a common disease that appears as a white powdery substance on the leaf surface. The powdery appearance comes from millions of tiny fungal spores, which are spread in air currents to cause new infections. It attacks all kinds of trees.

27. Scleroderris Canker

Scleroderris canker, caused by the fungus Gremmeniella abietina-Scleroderris lagerbergii (Lagerb.) Morelet, has caused extensive mortality in conifer plantations and forest nurseries in the northeast and north central United States and eastern Canada.

28. Sooty Mold

Sooty Mold on Holly
University of Florida
Sooty mold appropriately describes the disease, as it looks just like chimney soot. Although unsightly, it seldom damages the tree. The pathogens are dark fungi growing either on the honeydew excreted by sucking insects or on exuded material coming from leaves of certain trees.

29. Sudden Oak Death

Sudden Oak Death
A phenomenon known as Sudden Oak Death was first reported in 1995 in central coastal California. Since then, tens of thousands of tanoaks (Lithocarpus densiflorus), coast live oaks (Quercus agrifolia), and California black oaks (Quercus kelloggii) have been killed by a newly identified fungus, Phytophthora ramorum. On these hosts, the fungus causes a bleeding canker on the stem.

30. Thousand Cankers Disease

Missouri Agriculture

Thousand cankers disease is a newly discovered disease of walnuts including black walnut. The disease results from the walnut twig beetle (Pityophthorus juglandis) hosting a canker producing fungus in the genus Geosmithia (proposed name Geosmithia morbida). The disease was thought to be restricted to the western United States where over the past decade it has been involved in several large scale die-offs of walnut, particularly black walnut, Juglans nigra. Unfortunately, it has now been found in eastern Tennessee.

31. Verticillium Wilt

Jody Fetzer, New York Botanical Garden, Bugwood.org
Verticillium wilt is common in many soils and affects several hundred herbaceous and woody plant species. Ash, catalpa, maple, redbud and yellow poplar are most frequently infected trees in the landscape but rarely in natural forest conditions. This disease can become a serious problem on susceptible hosts in infested soils but many tree varieties have been developed with some resistance.

32. White Pine Blister Rust

White Pine Blister Rust
The disease attacks pines with 5 needles per fascicle. That includes Eastern and Western white pine, sugar pine and limber pine. Seedlings are in greatest danger. Cronartium ribicolais a rust fungus and can only be infected by basidiospores produced on Ribes (current and gooseberry) plants. It is native to Asia but was introduced to North America. It has invaded most white pine areas and is still making progress into the Southwest and into southern California.

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