Newspaper Columns

Columns written by Greg Grant and a Smith Co. Master Gardener which have appeared in the Tyler Morning Telegraph, are posted here.

Scab fungus to do major damage to a pecan crop

Published on Wednesday, 16 August 2017 19:50 – Written by GREG GRANT, Texas A&M AgriLife Extension Service 

Every year we get questions about black spots on pecans.
They are caused by our most common pecan disease, scab.  Scab fungus (Cladosporium caryigenum) invades young, rapidly growing shoots and leaves, and later the developing nuts. Severely infected nuts on highly susceptible varieties fall or fail to develop, resulting in a total 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 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.

These spores are spread by wind and rain to newly developed leaves where they germinate and invade tender tissues, initiating infection. Unfortunately, this is the only time of the year that fungicides labeled for pecan scab can be effectively applied. The fungus produces a great abundance of spores on the surface of these primary infection sites and becomes visible to the naked eye within seven to nine days and then spreads throughout the trees, infecting young shoots, leaves and nuts. On the leaves, primary infection lesions occur on the lower surfaces and are characteristically olive brown, somewhat elongated in shape and variously sized from a barely discernible dot to lesions a quarter-inch or more in diameter. Frequently, adjacent lesions coalesce, forming large, brown lesions. Primary scab lesions commonly occur on or along the leaflet veins, but may be found between the veins.

The disease develops faster during rainy periods and cloudy days, when the leaf surfaces are wet. Under these conditions, spores of the fungus in contact with the wet surface of a pecan leaflet germinate rapidly, invade the tender tissues and initiate infection within six hours. Lesions resulting from these infection sites become visible to the naked eye within seven to nine days, depending on environmental conditions. Control of scab disease depends primarily on protection of tender leaf, nut and shoot surfaces with application of an effective fungicide. A thin film of fungicide prevents the fungus from developing by killing spores before they invade susceptible tissues. Unfortunately, once the fungus has invaded the tissues, it becomes protected from most fungicides and continues to produce spores. Therefore, thorough fungicide coverage of leaf, nut and shoot surfaces must be maintained during the season to prevent secondary infections following rainy periods sufficient to allow germination and penetration.

Unfortunately, most homeowners can’t afford to have their trees sprayed repeatedly, and even if they could, it would cost more than purchasing shelled pecans. Therefore, there’s nothing you can do but hope for a drier year next year.

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