Part 12

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