Part 11

Spiculosa canker is considered to be a canker-rot disease, a type of decay in which the causal organism incites not only heart-rot but also large irregular cankers. Infected trees have cankers that appear as rough circular swellings on the bole. The canker centers are depressed and old branch stubs are discernible. Fungus fruiting bodies, or conks, usually are not present on living trees but develop on snags or decayed logs. The conks grow flat under the bark and push it off to expose the brown fungus fruiting surface.

Control for Spiculosa canker is similar to other canker rots: salvage to remove undesirable cankered trees that may be later replaced by better quality trees.

IRPEX CANKER

Irpex canker, caused by Irpex mollis, is prevalent in bottomlands and on upland areas of the Southeast. In the bottoms, Nuttall, water, and willow oaks are affected. White, chestnut, southern red, and black oaks are the hosts of this disease on upland sites.

Irpex canker is also considered to be a canker-rot disease. Symptoms on infected trees frequently involve irregular cankers up to two feet in length. Cankers are usually found on trees eight to ten inches in diameter or larger, at a height of twenty feet above the ground. Branch stubs, signifying probable infection points, are usually present in the centers of cankers. The portion of the trunk affected is usually swollen but sometimes may be sunken. At the base of the sunken portion of the canker are small, creamy-white, toothed fruiting bodies or conks with a leathery texture. Conks also appear on hardwood logs. The wood behind cankers is characterized by a tough, spongy, white rot which extends as much as eight feet above and below the canker. The decay pattern may also extend downward into the roots. In cross-section, the rotted areas appear as finger-like projections radiating out toward the sapwood. Gradually the rot column tapers to a thin central core beyond which white flecks appear, and this early rot stage is concentrated along the rays of the oaks.

Control for Irpex canker is the same as for other canker-rots—salvage of undesirable cankered trees.

HISPIDUS CANKER

Polyporus hispidus is the cause of trunk cankers and localized decay of hardwoods throughout eastern, central, and southern United States. The fungus is also known to attack hardwoods in Oregon and California. Reported hosts are: hickory, ash, mulberry, willow, walnut, and oak. In the south, P. hispidus is common on oaks, including willow, water, black, white, Nuttall, and cherrybark.

The fungus usually enters the tree through dead branch stubs, from which it grows into the heartwood. After becoming established, the fungus begins penetrating the sapwood and attacks the living cambium. Callus folds are formed by the host around the dead cambial area, forming an elongated swollen canker, commonly referred to as a “hispidus canker”. The cankered area of the stem is bark-covered and sunken, usually containing a remnant of a branch stub or branch scar. During late summer, fall or early winter, the fungus produces conks (sporophores) on the surface of the cankers. The annual bracket-shaped conks are large, spongy, and yellowish-brown to rusty-brown on the upper surface. When fresh, the under surface is a light tan color. After a few months, the mature conks dry to a rigid black mass and fall from the canker. Old conks are commonly found at the base of cankered trees during the spring and early summer. Decay produced by the fungus appears spongy, light yellow, and is commonly separated from the sound wood by a black zone line. The rot is of the delignifying white rot type. On southern oaks, the rate of canker elongation has been estimated at one-half foot per year, with the internal rot usually extending about one foot above and below the cankered area.

No effective control in forest stands is known. Removal of diseased trees provides additional growing space for crop trees. Some degree of shade tree protection can be obtained by pruning of dead branches flush with the main stem of the tree.

BOTRYOSPHAERIA CANKER

Botryosphaeria ribis causes cankering and mortality of more than 50 woody plants. The fungus is widely distributed throughout the eastern one-half of the United States. The pathogen infects the following economically important hosts: sweetgum, redbud, willow, poplar, tupelo, pecan, and hickory.

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.