The diseases which affect the pecan are of four different types: namely fungus, bacterial, virus and physiological. The fungus diseases, the most numerous and widespread, are caused by small microscopic molds. Approximately 12 different fungus organisms cause harmful diseases of the pecan.
The bacterial disease organisms, unlike the disease producing fungi, are single celled and can be seen only under a microscope. Bacterial diseases are fewer and of less economic importance than fungus diseases.
Virus diseases are caused by extremely small agents which can be seen only under special ultra-microscopes such as the electron microscope. Plant viruses are protein substances, but their exact nature is unknown.
Physiological disorders (sometimes called physiological diseases) are caused by a variety of environmental conditions. A physiological disorder in a pecan tree may result from infertile soil, excessive moisture, or the absence or degree of available nutritional mineral elements to the growing tree. These various environmental factors have special adverse effects, manifested by specific symptoms caused by insufficient levels of a given nutritional mineral element or elements, which are easily corrected by supplying the tree the necessary mineral elements either through soil application or foliage sprays.
SCAB
Pecan scab, caused by the fungus Cladosporium effusum (Wint.) Demaree, is the most destructive disease of pecans in Texas. The fungus invades the young rapidly growing shoots and leaves and later the developing nuts. Severely infected nuts on highly scab-susceptible varieties fall or fail to develop, resulting in a total nut crop loss. Early season defoliation often occurs in seasons of frequent rains and high humidity which facilitate the rapid development and spread of the scab fungus.
The scab fungus overwinters in infected shoots and in old shucks and leaves in the trees. In the spring when temperature and moisture conditions become favorable, the fungus begins to grow in the shoot lesions, old leaves and shucks, and within a few days produces great numbers of spores. These spores are spread by wind and rain to newly developed leaves where they germinate and invade the tender tissues, initiating primary infection. The fungus produces a great abundance of spores on the surface of these primary infection sites and spreads throughout the tree and infects young shoots, leaves and nuts.
On the leaves, primary infection lesions occur on the lower leaf surfaces and are characteristically olive brown, somewhat elongated in shape and vary in size from a barely discernible dot to lesions one-fourth inch or more in diameter. Frequently, adjacent lesions coalesce, forming large very dark lesions. Primary scab lesions commonly occur on or along the leaflet veins but often may be found between the veins on the underleaf surface. On the nuts, scab lesions appear as small black dots, which are elevated or sunken in older infections. Adjacent lesions on the nuts may coalesce forming large sunken black lesions, [Figure 1]. When infection is severe, the entire nut surface is black in appearance, development is arrested and the nuts drop prematurely.
Figure 1. Scab lesions on leaves and nuts of Delmas variety. Note concave lesions and overall scabby appearance of severely infected nuts.