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.
Pecan varieties vary in their susceptibility to scab disease. Among the highly susceptible varieties are: Burkett, Delmas, Schley, Moore, Halbert and most western varieties. Moneymaker, Success and Curtis are moderately resistant. Mahan, Stuart and Desirable varieties are highly resistant to the scab fungus. However, this character of resistance varies, depending on the area of the state, local environmental conditions and the particular strain of the scab fungus present.
Scab disease development is favored by rainy periods and cloudy days when the humidity remains high and leaf surfaces are wet. Under these conditions, spores of the fungus in contact with the wet leaf surface of a pecan leaflet or nut germinate rapidly, invade the tender tissues and initiate infection within 6 hours. Lesions resulting from these infection sites, become visible to the naked eye within 7 to 14 days, depending on environmental conditions. A period of warm dry weather after infection occurs may retard lesion development.
Control.—The control of pecan scab disease depends primarily on the protection of tender leaf, nut and shoot surfaces with proper application of an effective fungicide. A protective film of fungicide chemical prevents scab fungus infections by killing the spores immediately after their germination, thereby preventing invasion of susceptible tissues. Unfortunately, once the fungus has invaded the tissues it becomes protected from chemical attack and produces spores in great abundance. Therefore, thorough coverage of leaf, nut and shoot surfaces with a fungicide chemical must be maintained to prevent secondary infections, ([6], [10], [11]).
Sanitation measures, such as removal of old attached shucks and leaf stems in trees and plowing or disk harrowing under fallen leaves and shucks help reduce primary infections. See spray schedule, [page 4], for scab disease control.