Insects and Diseases of Trees in the South

by Anonymous

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 infection in terminal branch and main stem of 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.

The separation of wood decay fungi on the basis of their host range, the portion of the host attacked, and the type of rot produced are useful aids to a pathologist in determining a tentative identification of the fungus responsible for a particular type of rot. However, there are numerous fungi which cause decay, many of which are exceptions to the various methods of classification. This forces the pathologist to use microscopic examination and various artificial keys to arrive at the proper identification of a given rot-producing fungus.