Nut and Foliage Diseases

Pecan Scab

The pecan scab fungus, Cladosporium caryigenum, 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 facilitates 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 resumes its growth in the old lesions, and within a few days, produces great numbers of spores.

Based on work done in Georgia, 70°F appears to be the minimum temperature at which spores are formed. These spores are spread by wind and rain to newly developed leaves where they germinate and invade the tender tissues, initiating a primary infection. It is to control this primary infection that the bud break and pre-pollination sprays are applied. The fungus produces a great abundance of spores on the surface of these primary infection sites and then spreads throughout the trees infecting young shoots, leaves, and nuts.

When infection is severe, the entire nut surface is black, kernel development is stopped, and the nut drops prematurely. Pecan varieties vary in their susceptibility to scab disease. Refer to Table 1 for varieties and their disease reactions. Scab disease development is favored by rainy periods and cloudy days when the leaf surfaces are wet. Under these conditions, spores of the fungus in contact with the wet leaf surface of a pecan leaflet 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 9 days, depending on environmental conditions.

Control of pecan scab disease depends primarily on the protection of tender leaves, nuts, and shoot surfaces with the application of an effective fungicide. A thin film of the fungicide prevents the scab fungus from developing, by killing the spores before they can invade susceptible tissues. Unfortunately, once the fungus has invaded the tissues, it becomes protected from most fungicides and can continue to produce spores. Therefore, thorough coverage of leaf, nut, and shoot surfaces with a fungicide must be maintained during the season to prevent secondary infections.

Sanitation measures, such as plowing or disk harrowing under fallen leaves and shucks, help reduce primary infections. Pruning to open up the tree for better air circulation will help reduce scab occurrence by reducing the number of infection periods occurring during the year.

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