Part 8

FUSIFORM RUST

Fusiform rust, caused by Cronartium fusiforme, is one of the most important diseases on southern pines. This rust is found from Maryland to Florida and west to Texas and southern Arkansas. The rust’s most important impact is in nurseries, seed orchards, and young plantations. Loblolly and slash pines are very susceptible to this rust. Pitch and pond pines are moderately susceptible, longleaf pine is fairly resistant, and shortleaf pine is highly resistant.

The most easily recognized symptom is the spindle-shaped canker on the pine branches or main stem. In early spring these swellings appear yellow to orange as the fungus produces powdery spores. As host tissue is killed, older stem cankers may become flat or sunken. Cankers often girdle trees and wind breakage at the canker is common. Fungus spores from the pine infect oak leaves. Brown hair-like structures, produced on the underside of the leaves in late spring, are the most conspicuous signs. These projections produce spores which in turn reinfect the pine trees, completing a “typical” rust cycle.

Silvicultural practices may lessen the incidence of infection in plantations. Avoid planting highly susceptible species such as slash and loblolly pines in areas of known high rust incidence. In these areas more resistant species such as longleaf or shortleaf pine should be planted. Pruning infected branches will prevent stem infection in young plantations. Rust-resistant pines should be readily available from the nurseries in the near future. Culling out seedlings with obvious galls before outplanting will reduce the disease incidence in new plantations.

WHITE PINE BLISTER RUST

White pine blister rust, caused by Cronartium ribicola, was introduced to North America on nursery stock about 1900. It is the most important disease on white pine in the United States. In the South, the disease is found on eastern white pine in the Appalachian mountains.

The disease is caused by a fungus that attacks both white pine and wild and cultivated currant and gooseberry bushes, called Ribes. Both hosts must be present if the fungus is to complete its life cycle. Attack by the disease is followed by the development of cankers on the main stem or branches. Infected pines die when a canker completely girdles the main stem or when many of the branches are killed by girdling. The most conspicuous symptoms of the disease are the dying branches or crowns (“flags”) above the girdling cankers, and the cankers themselves.

Initially, a narrow band of yellow-orange bark marks the edges of the canker. Inside this band are small irregular dark brown scars. As the canker grows, the margin and bank of dark scars expand and the portion formerly occupied by the dark scars is now the area where the spores that infect Ribes are produced. During the months of April through June white sacs or blisters containing orange-yellow spores (called aeciospores) push through the diseased bark. The blisters soon rupture and the orange-yellow spores are wind-dispersed for great distances. Generally, there is some tissue swelling associated with the canker, which results in a spindle-shaped swelling around the infected portion of the stem.

Loss of white pines from blister rust can be prevented by destroying the wild and cultivated Ribes bushes. Bushes may be removed by uprooting by hand, grubbing with a hand tool, or with herbicides. Pruning infected branches on young trees will prevent stem infections and probably tree mortality.

COMANDRA BLISTER RUST

Comandra blister rust, caused by the fungus Cronartium comandrae, is a canker disease of hard pines. The disease presently occurs in widely scattered areas throughout the western, central, and southern United States. In the South, the primary hosts are loblolly, shortleaf, pond, and Virginia pine. Herbaceous plants of the genus Comandra, commonly known as false toadflax or comandra, are also attacked.

The fungus infects pines through the needles and grows from the needle into the branch or main stem where it forms a gall or canker. Dark orange-colored spores which are produced on the surface of the gall in the spring are wind-blown and infect the leaves or stems of the comandra plants. Two to three weeks after infection, urediospores are produced on the underside of the comandra leaf. These are wind-blown and can only infect other comandra plants. Eventually hair-like structures known as telia are produced on the comandra leaves and stems. The telia produce spores which are wind-blown and infect the pine host through the needle. The necessary combination of a susceptible pine host, the alternate host, and the pathogen is presently known to occur only in northern Arkansas, eastern Tennessee, and northern Alabama.

No effective method of controlling the disease in forest stands is presently known. Silvicultural or forest management practices which reduce the abundance of the alternate host offer promise of long term control. Maintenance of dense stands and heavy ground cover as a means of shading out the intermediate host plants, may be helpful in reducing rust damage in many areas.

EASTERN GALL RUST

Eastern gall rust, caused by the fungus Cronartium cerebrum, attacks many species of eastern hard pines. The disease ranges eastward from the Great Plains and is most severe in the South on Virginia and shortleaf pines. Like most rusts this fungus requires an alternate host in addition to its pine host. In this case oaks, especially the red oak group (black, red, scarlet and pin) are the alternate hosts. Damage to the oaks is generally not of economic importance as only the leaves are affected.

On pines the fungus causes the formation of globose to sub-globose galls. Canker formation occurs occasionally but mortality generally results from wind breakage at the gall rather than by canker formation as literally hundreds of galls may appear on a single tree. They are not lethal to the tree, but may ruin tree form and on a large stem they can lead to an open decayed wound, as decay fungi are often secondary invaders of rust infections. The shape of galls and the arrangement of the spore sacs filled with red-orange spores present a cerebroid (brain-like) appearance. During the spring the bright orange galls are very striking.

This disease is sometimes a problem in nurseries where seedlings are attacked and killed. This is where control efforts are concentrated. Fungicide protectants are applied to the seedling foliage to prevent infection from spores produced on oaks. Contact your local forestry extension agent or the nearest Division of Forest Pest Management for the latest recommendations. Under forest conditions, control is not economically feasible. Trees of poor form should be removed during thinning operations.

SOUTHERN CONE RUST

Southern cone rust is caused by the fungus Cronartium strobilinum. It has been reported to completely destroy slash and longleaf pine cone crops in Georgia and along the Gulf Coast from Florida to Texas.

Like most other rusts, the fungus requires oaks and pines to complete its life cycle. Although infection of oak leaves occurs annually, no significant economic damage is done to the oaks. Fungus spores produced on oak leaves infect the mature female pine flowers about the time of pollination (January-February). The fungus grows through the developing conelet causing it to swell abnormally. By early April or late May the infected cones are three to four times larger than the normal first-year cones and even exceed the maturing second-year cones in size. The swollen cone scales are reddish in color. Cavities in the cone filled with orange-yellow spores burst and the cones become orange-yellow. The swollen orange-yellow cones in the tree crowns can be easily distinguished from normal cones by an observer on the ground. By late summer most of the diseased cones have died and fallen.

Control at present is confined to seed orchards. Hydraulic spraying of the flowers with fungicides gives a significant reduction in infections. Consult your local forester, county extension agent or the nearest Forest Pest Management Office for current control recommendations.

PITCH CANKER

Pitch canker, caused by the fungus Fusarium lateritium forma pini, is rapidly becoming widespread throughout the South. The disease apparently is most serious on Virginia, slash and south Florida slash pine. The fungus also attacks shortleaf, pitch, and table-mountain pine.

Pitch canker may cause tree mortality. On Virginia pines the fungus reportedly enters through small insect wounds in the twigs or mechanical wounds in the bole. Shoots may be girdled and killed within a few weeks, but it takes a period of years for the fungus to girdle the bole of larger trees. On slash pine the disease apparently attacks plantations in wave years. During years of heavy attack the fungus can cause rapid crown deterioration in addition to causing bole canker infections. Cankers on leaders in the crown can result in death of two-thirds or more of the crown by mid-summer in a tree that appeared healthy in the spring. In the majority of tree infections only the leader and one or two laterals will be infected. The tree recovers in a few years with a crook in the bole as the only evidence of attack. Pitch cankers usually retain the bark and old cankers on the hole may be sunken. The most diagnostic characteristic of the disease, and the one that definitely separates it from similar disease, is the heavy pitch soak of the wood beneath the canker. Pitch cankers are often so soaked with pitch that heavy flow of pitch is observed flowing down the bole.

At the present time, no known method of control exists. Observations in slash pine plantations indicate that some trees are resistant while others range in their degree of susceptibility.

WOOD DECAY

Wood decay of southern forest trees is responsible for nearly 80 percent of all loss attributed to disease. This decay is caused by fungi which mainly attack heartwood in the central portion of stems, branches, and roots. Wood-rotting fungi gain entrance into the tree through broken branches, wounds, and damaged or exposed roots. Spores, which land at these damaged areas, germinate and produce a microscopic mycelium which attacks and spreads throughout the heartwood. The decay is caused by the action of the mycelium, which penetrate the cell walls and dissolve or alter the wood in various ways. Fungus development within the tree may continue for many years without any apparent effect on the growth of the host. Eventually the mycelium will aggregate and break through the bark to form the reproductive stage, either before or after the death of the host. The fruiting body (sporophore, conk) produces vast amounts of spores which are capable of spreading the fungus to other trees.

Heartrots may be separated into broad classes on the basis of the host portion attacked, such as root rots, root and butt rots, stem rots, and top rots. Decay fungi may be further separated into two broad classes based on their effect on wood. The first class causes white rots, decomposing all components of the wood and reducing it to a spongy mass with white pockets or streaks separated by firm wood. The second class, causing brown rots, utilize the cellulose, leaving the lignin more or less unaffected. This usually results in a rot which appears as some shade of brown.