Part 6

The fungus gains entry into susceptible hosts primarily through wounds or dead and dying twigs. Small oval cankers on stems or branches are the first symptoms of infection. As the fungus continues to attack and kill the cambium, the sunken cankers enlarge, eventually girdling and killing the branch or stem above the cankered area. In the spring and early summer, cankers on living portions of the host often produce an exudate. Infected sweetgums generally produce the exudate in great quantities, to which the common name of bleeding necrosis has been applied. Reproductive structures called stroma are produced by the fungus on dead cankered stems and branches during moist periods of the spring and summer.

No practical method of control is known. Diseased trees seldom recover. Infection of high value shade and ornamental trees may be prevented to some degree by avoiding mechanical damage. Dead limbs and branches should be pruned and wounds covered with a suitable tree paint. Infected trees should be removed and burned.

SEPTORIA CANKER

Septoria canker is caused by the fungus _Septoria musiva_. Although this is a disease of poplars, native poplar species are not severely attacked. However, this is an important problem wherever hybrid or introduced poplars are grown. With the ever-increasing emphasis on poplar planting, this will probably become a much more important problem in the near future.

Young stem cankers usually develop around openings such as wounds, lenticels, or leaf scars, appearing first as sunken, dark areas of the bark. The infected area later becomes more depressed and darker as tissue dies, and often a black margin will be formed around the canker. Small, pink, hair-like spore tendrils are produced by the fungus around the canker margin, especially during moist weather. These tendrils contain spores which can cause further infection, and arise from small dark fungal fruiting structures called pycnidia. This cankered area is often invaded by insects and other fungi and is also a weakened area at which wind breakage may occur. The fungus also causes a gray to black leaf spot, which usually has a light colored center. These spots may coalesce on a severely infected leaf and involve the entire leaf surface. This leaf spot in itself causes slight damage, but acts as a source of fungus spores which can cause stem cankers, and thus is important in the overall consideration of the disease.

A control for this disease is to use native poplars or resistant hybrids wherever possible. Wider spacing in plantations may also reduce the humidity, decreasing the amount of infection.

CYTOSPORA CANKER

Cytospora canker is caused by _Cytospora chrysosperma_. This fungus attacks more than 70 species of hardwood trees and shrubs as well as some conifers. Poplars and willows are among the most common hosts, and are attacked throughout their range. Trees used for shade or windbreak, and also cuttings in propagation beds are particularly susceptible to this canker. This disease is most severe on trees growing under stress, such as those growing on an unfavorable site, or injured by drought, frost, fire, or severe pruning. The fungus is a normal inhabitant of the bark and becomes parasitic only when the tree is weakened.

A canker begins as a gradual, circular killing of the bark of a limb or stem. This infected tissue soon appears as a brownish, sunken patch, around which the host often produces raised callus tissue. Small black fungal structures (pycnidia) appear as small pimples on the dead bark. During moist weather, thin threads of spores exude out from the pycnidia. The inner diseased bark becomes dark and odorous. The cambium is killed and the sapwood becomes watery and reddish brown as it becomes infected. Water spouts may form below the canker before the tree dies.

Since this disease is most severe on weakened trees, shade trees should be watered and fertilized to maintain healthy, vigorous growth. Wounding and severe pruning should be avoided. Cuttings should be selected from healthy planting material grown in disease free areas. If cuttings are to be stored for any length of time, they should be kept at a temperature below 35°. This low temperature will keep new infections from occurring, even if spores are present.

CHESTNUT BLIGHT

Chestnut blight is caused by the fungus _Endothia parasitica_. It can be found on American chestnut throughout its range where it has virtually eliminated this valuable species from eastern hardwood forests. The chestnut blight fungus is also parasitic on other hosts including common chinkapin, Spanish chestnut, and post oak. Japanese and Chinese chestnuts are resistant.

Stem cankers are either swollen or sunken and the sunken type may be grown over with bark. The fungus forms fruiting bodies some distance back from the advancing cankers, and the spores may exude from bark crevices as orange curl-like masses during moist weather. Young cankers are yellow-brown in sharp contrast to the normal olive-green color of the bark. The chestnut is a vigorous sprouting species but the fungus survives in previously killed stumps and later kills the new sprout growth.

No effective control has been developed for chestnut blight, even after several decades of intensive research. The most promising control involves the development of a blight-resistant species. Research is presently underway on this matter but results are inconclusive.

OAK WILT

This serious vascular wilt of oaks is caused by the fungus _Ceratocystis fagacearum_. At least fifty species and varieties of oak are susceptible. The disease has been most damaging in the Lake States but is also found in the eastern United States.

Oak wilt symptoms are most noticeable during late spring or early summer. Red oaks may be killed in as little as three weeks, the lower branches being affected last. In white oaks, symptoms are usually confined to a few branches each year and trees may live several years before death. Leaf symptoms are similar for both red and white oaks. Leaves turn yellow or brown and become dry progressively from the edge or tip to the midrib and base. Mature leaves may fall at any symptom stage from green to brown. Premature leaf shedding is the most outstanding symptom. A definite characteristic of the disease is the raising and cracking of the bark due to pressure of mats of the fungus growing between the bark and wood.

Oak wilt is spread over long distances by insects that pick up spores while crawling on the mats of infected trees. The disease may also spread from tree to tree via root graft. Short-distance spread is controlled by severing all roots of living trees around infected trees by use of a ditchdigger. Another control is to fell all trees in a 50-foot radius of infected trees; felling and burning of all parts of infected trees is sometimes done to prevent overland spread.

DUTCH ELM DISEASE

Dutch elm disease, caused by _Ceratocystis ulmi_, is the most devastating disease of elm trees in the United States. This disease has been recorded in most states east of the Mississippi and as far as Idaho in the Northwest and Texas in the Southwest. All of the native elm species are susceptible, while many of the ornamental Asiatic species are highly resistant.

Trees suffering from Dutch elm disease may show a variety of symptoms. Leaves become yellow, wilt, and turn brown. Premature defoliation and death of branches usually occurs, causing the crown to appear thin and sparse. Internally, a brown discoloration appears in the outer sapwood. _C. ulmi_ is transmitted from diseased to healthy elms by elm bark beetles, mainly the small European elm bark beetle and the native elm bark beetle. These beetles make characteristic galleries under the bark of dead and dying elms. Adult beetles pick up the sticky fungus spores from under the bark and then feed on the young tender elm twigs of healthy trees, inadvertently inoculating the healthy tree with the fungus. The fungus may also spread from diseased elms to adjacent healthy elms through root grafts.

Controls to combat this disease generally involve sanitary measures aimed at the beetles. Dead and dying elms should be burned. This eliminates the elm wood which normally serves as a breeding place for elm bark beetles and thus reduces the beetle population. In areas where most native elms are infected, other tree species or resistant elm species, such as the Chinese or Siberian elm, should be planted rather them native elms.

ELM PHLOEM NECROSIS

Elm phloem necrosis is a disease of elm, caused by a virus or virus-like organism. The disease has occurred in the United States for many years, probably as early as 1882. The pathogen is transmitted from infected to healthy trees by the adult white-banded elm leafhopper, _Scaphoideus luteolus_, which feeds on the leaf veins. It is now present throughout most of the central, eastern, and southern portions of the United States. The disease is known to occur on American and winged elm, but all native elms are probably susceptible to attack by the pathogen.

The earliest symptoms of the disease appear in the top of the crown, at the outer tips of the branches. Here the elm leaves suddenly wilt, turn yellow, the margins curl upward and the leaves die. Leaf-fall causes the crown to appear sparse. In large trees, the foliage symptoms may initially appear on one branch or only a portion of the crown. However, the symptoms during the advanced stage of the disease are exhibited throughout the crown. The most reliable symptom appears as a yellow to butterscotch discoloration on the inner bark surface or phloem. This symptom initially appears under the bark of large roots, later spreading to the base of the main stem and finally to the larger branches. Phloem and cambial discoloration is often found in advance of the foliage symptoms. The moderately discolored phlomen has a slight odor of wintergreen. Thus far, all trees known to be infected with the pathogen have died. Acutely infected trees, while initially appearing healthy, may wilt and die in three to four weeks.

No effective controls are known. The “Christine Buisman” elm, which is highly resistant to Dutch elm disease, has demonstrated resistance to elm phloem necrosis.

MIMOSA WILT

A vascular wilt of the mimosa (silktree) is caused by the fungus _Fusarium oxysporum forma perniciosum_. The fungus is known only to attack the mimosa, a tree imported from eastern Asia and grown throughout the southeast as an ornamental. Since the discovery of the disease in North Carolina in 1935, the fungus has spread north to Maryland, south to Florida, and west to Texas.

The fungus causing mimosa wilt is soil-borne and gains entrance into the tree by attacking the roots. Once entrance is gained by the fungus, the pathogen enters the outer water-conductive system in the sapwood. As the fungus grows throughout the system, it hinders or completely inhibits the water movement from the roots to the aerial portion of the tree. The first outward symptom of disease is the wilting of leaflets, usually in the upper portion of the crown. The wilted leaflets turn yellow, then brown, and die. Often a branch or two will succumb at a time until the entire crown is dead. A second symptom of the disease is found in the outer sapwood of the tree. A brown discoloration, appearing as spots or a ring, is observed by cutting into the outer sapwood of the infected stem or branch.

Control of the fungus is very difficult, since it is soil-borne and enters through the roots. However, resistant varieties of mimosa trees, developed by the U.S. Department of Agriculture, are now available at most commercial nurseries.

VERTICILLIUM WILT

Verticillium wilt, caused by common soil-inhabiting fungi belonging to the genus _Verticillium_, is found on a number of hardwood hosts. In southern and eastern United States elms and maples are attacked throughout their natural ranges.

Infected trees may die within a few weeks after the first symptoms are observed, or they may survive for years. The first symptoms of the disease involve a wilting of the foliage, with the leaves turning yellow and finally brown. The early symptoms are often restricted to a single limb or portion of the crown. Vascular discoloration, which is brown in elms and green in maples, is present in the outer sapwood. In elms, this discoloration is similar to that produced in trees having Dutch elm disease.

This disease is not of serious consequence in forest stands. However, it is often extremely important in high value shade trees. Dead or dying limbs on lawn trees may be pruned out. This may not always save the tree, but may help keep it alive for years. Trees should be well watered and fertilized as necessary. If a lawn tree dies from Verticillium wilt, it should be replaced by a resistant species.

ARMILLARIA ROOT AND BUTT ROT

Armillaria root and butt rot is caused by the fungus _Armillaria mellea_. This disease is common in orchards, vineyards, gardens, parks, and forests throughout the world. Both coniferous and hardwood forest trees are attacked. The fungus is especially troublesome in plantations, particularly in stands recently thinned.

The fungus is spread by spores produced by honey-colored mushrooms, rhizomorphs, and root contacts between diseased trees or stumps and healthy trees. Rhizomorphs are visible strands of compacted mycelium (fungus material) that appear as black or reddish-brown “shoestrings.” They may be flattened when found between bark and wood or cylindrical when found in decayed wood or soil. Rhizomorphs increase in length at their tips and in this manner the fungus may move through the soil from infected trees to uninfected trees. The honey-colored mushrooms are produced annually and are fairly short lived; they are subject to desiccation, and are favored by small mammals. The size of the top varies from two to five inches in diameter. The top is usually flecked with dark brown scales. White to light yellow gills are borne underneath. Crown symptoms of the trees affected are similar to those caused by any malfunctioning of the roots. A reduction in size and production of leaves or needles, a general thinning of the crown, branch dying, and yellowing of foliage may precede death or trees may die rapidly with a rapid red discoloration of the foliage. Trees often die in groups, but single-tree kill is also common. Cankers bleeding resin, gum or other exudate at the tree base are common symptoms.

Control is not attempted for this disease under forest conditions. Losses may be reduced by following proper planting procedures, and by salvage cutting.

CYLINDROCLADIUM ROOT ROT

This nursery disease is caused by two fungi species. _Cylindrocladium scoparium_ and _C. floridanum_: Cylindrocladium root rot has been found on two hardwoods (yellow-poplar and black walnut) and two conifers (white pine and Fraser fir) in forest tree nurseries in six southern states. These include Virginia, West Virginia, North Carolina, Tennessee, Alabama, and Mississippi.

Root rot symptoms on hardwoods and conifers are quite different. The most characteristic root symptoms on yellow-poplar and black walnut are the blackened and longitudinally-cracked infected roots that are in sharp contrast with healthy white roots of these two species. Infected seedling leaves become yellow and later turn reddish-brown.

Root rot symptoms on conifers involve either rotting of the seed or seedling (pre-emergence damping off) before emergence from the soil or seedling root rot following emergence. These symptoms also involve a shrivelling and reddening of foliage, needle blight, and stem cankers. The most characteristic symptoms are the “patchy” irregularly scattered pattern of infection in conifer seedling beds and the loosening of the root epidermis on infected roots—making it very easy to pull off.

The most practical and effective control for root rot diseases of this type involves strict quarantine to either keep the disease out of the nursery or keep it confined to known infected areas by avoiding the transportation of root material, organic matter, and soil from infected to non-infected seed beds. Soil fumigation may control this disease if applied under favorable conditions.

LUCIDUS ROOT AND BUTT ROT

_Polyporus lucidus_ is the cause of root and basal stem decay of hardwood trees throughout the eastern, central, and southern portions of the United States. The fungus is known to attack and kill maple, hackberry, orange, lemon, ash, sweetgum, oak, locust, elm, tupelo, willow, and mimosa. Mimosa trees are very susceptible to attack by _P. lucidus_ throughout the South.

The disease is characterized by a rapid decline and death of the host. Examination of infected roots will reveal a soft spongy white rot with black spots scattered throughout. Fruiting bodies are formed at the base of infected trees or on the surface of exposed roots. The mature fruiting bodies are a reddish brown above and white below, with the tops and stems appearing glazed or varnished. The fungus is believed to gain entrance into the host through bark and root injuries and can spread from infected to healthy trees through root contacts and grafts.

No control is known for this disease. Avoidance of lawnmower and other mechanical injuries to the base of roots of susceptible shade trees will reduce the chance of infection by the fungus. The spread of the disease from infected to healthy trees can be reduced by planting at a wide enough spacing to avoid root contacts and grafts.

_If you find damage on your trees from insects or diseases you may consult with any of the following offices or your state forester._

FIELD OFFICES: _Alexandria Office_ U.S. Forest Service 2500 Shreveport Highway Pineville, Louisiana 71360 Phone A/C 318 445-6511 Ext. 311 FOR STATES OF: Alabama Arkansas Louisiana Mississippi Oklahoma Texas _Asheville Office_ U.S. Forest Service P.O. Box 5895 Asheville, North Carolina 28803 Phone A/C 704 254-0961 Ext. 625 FOR STATES OF: Florida Georgia Kentucky North Carolina South Carolina Tennessee Virginia _AREA OFFICE_ Group Leader Forest Pest Management Group U.S. Forest Service 1720 Peachtree Street, N.W. Atlanta, Georgia 30309 Phone A/C 404 526-3734

FOREST PEST MANAGEMENT GROUP

Southeastern Area S. and P. F.—7 1972

Transcriber’s Notes

—Silently corrected a few palpable typos.

—In the text versions, delimited italics or underlined text in _underscores_ (the HTML version reproduces the font form of the printed book.)