The Pecan and its Culture

by H. Harold Hume

FUNGOUS AND OTHER DISEASES OF THE PECAN.

The fungous diseases attacking the pecan have not been thoroughly investigated. They have not, however, become so numerous or common as to cause serious damage except in a few instances. The true fungous diseases are usually propagated and disseminated by means of spores, and the most effectual method of control usually consists in spraying with Bordeaux mixture or some other fungicide. For all fungous diseases of the pecan which may be controlled by spraying no substance will give better results than Bordeaux mixture, and directions for preparing it are given at the end of this chapter. Paris green, at the rate of four ounces to each fifty gallons of liquid, may be added to the mixture for the destruction of biting insects. For effectual work in spraying large trees, a platform should be erected on the wagon-bed to make it possible to reach the tops with the spray.

Pecan Leaf Blight (Cercospora Halstedii): This disease of pecan leaves causes them to turn brown, wither up and drop prematurely. At first, small brown spots are noted. These become larger, and at length the whole leaf is destroyed. When attacked by this disease the tree makes no progress. An examination of the discolored areas, under a microscope, shows the presence of tuft-like growths of spores upon short conidiophores. As they become matured the spores are scattered by the rain or wind and so the disease is spread. It probably lives over from one season to another on the diseased leaves.

The most effective remedy is to spray thoroughly three times with Bordeaux mixture. The first application should be given just when the young leaves are expanding, followed by two others at intervals of two or three weeks. The fallen leaves should, if feasible, be gathered and burned.

Pecan Scab (Fusicladium effusum): This disease attacks the fruit, leaves and twigs. The husks of the diseased nuts become covered with dark spots or specks. They become hardened and crack open in places. As a result of the attack, growth is stopped, the fruit does not fill out and mature, but drops prematurely or, in some cases, remains attached to the trees long after the leaves have fallen. Round, black spots form on the leaves when attacked by the fungous. These become dead and brown and in most cases the whole leaf is destroyed. When attacked, the trees are usually so badly injured that they make little progress. Not all varieties are subject to the disease in the same degree and some appear to be entirely exempt.

Photo by H. A. Gossard.
Fig. 36.
Spraying Pecan Trees.

Those varieties which are not attacked should be given preference in propagating work. The disease may be further controlled by spraying with Bordeaux mixture, as directed under leaf-blight.

Pecan Rosette: [L]"The earliest symptoms are a peculiar crimping of the leaves at the ends of the branches. These leaves are smaller with crimped margin, and when held to the light show light green or yellow streaks between the veins. The leaf tissue in these light-colored areas is thin and undeveloped and often breaks away leaving angular holes in the leaves. A tree usually shows the disease over the whole top at once, though sometimes only a single branch is affected at first. As the disease progresses, the foliage assumes a bunched appearance, due to the formation of tufts of leaves at the ends of the branches. This characteristic has led us to use the term "Rosette" as a name for the malady.

"The next stage of the disease which is observed the second year or later, is a dying-back of the branches from the tips. This is followed by the development of numerous small, lateral branches from adventitious buds. These are short, producing thick clusters of small, unhealthy leaves, sometimes reduced to mere skeletons, so that the rosetted appearance of the tree is intensified. This goes on from year to year. The growth of the tree is checked and these abnormal branches are formed only to die back each year. Trees in the earliest stages of rosette have been observed to have light crops of nuts, but, when badly diseased, are barren and unsightly or worse. Rosette has been found in all ages, from nursery stock to trees forty feet high.