Pecan trees (Carya illinoinensis) are North American natives commonly grown for their edible nuts as well as for shade. The state tree of Texas grows well in U.S. Department of Agriculture plant hardiness zones 5 through 9, but only produces nuts in regions with long, hot growing seasons. Plant diseases occasionally affect nut production, with pecan scab being one of the most damaging to pecan trees planted in home landscapes.
Pecan scab is a fungal disease that attacks newly developing leaves, shoots and nuts early in the growing season. The pathogens (Cladosporium caryigenum, Fusicladium effusum) thrive in overly wet or humid weather conditions, needing just two hours of standing moisture to become established. Pecan scab fungi overwinter in small, matted substances called stromas, which form on previously infected nut shucks and foliage. When spring rolls around, the rising temperatures and rain showers prompt the stromas to produce fungal spores that spread to new pecan trees via splashing water and wind gusts.
Lesions appear on pecan leaves within 7 to 10 days of infection. The lesions can range in size from tiny dots to brown or black circles about 1/4 inch in diameter. Several small lesions might fuse to form large, necrotic areas. Large lesions often interfere with the photosynthetic process and cause premature defoliation. Infected tissue sometimes falls out of the leaves, giving the pecan tree a shot-hole appearance. Scab infections that spread to the pecan fruit cause sunken lesions to form on the shucks. Severe infections might turn the nuts black or make them fall from the tree before they are fully ripened.
Horticulturists at the Louisiana State University Agricultural Center report that it's difficult for homeowners to chemically treat pecan scab, so it's best to prevent the disease from occurring in the first place. Crowded growing conditions promote scab development by increasing the humidity levels around trees. Plant your pecan at least 60 feet away from other trees or buildings to improve air circulation and allow more sunlight to reach the leaves. Removing low-hanging branches also increases air flow beneath the canopy, which helps reduce the amount of standing moisture on the foliage. Prevent the scab pathogens from spreading to other pecan trees by raking up and disposing of all fallen plant litter at the end of the growing season.
Pecan Tree Care
Taking proper care of young pecan trees helps keep them vigorous enough to fend off scab infections. Pecans prefer moist to wet, loamy or sandy soils in fully sunny to partially shady locations. These trees need 10 to 15 gallons of water each week for the first two to three years. Water your tree with a garden hose when your area doesn't receive enough rain. Mulching your tree with 2 to 3 inches of organic material such as leaves, sawdust or pine straw helps retain soil moisture. Feeding young pecan trees with a 10-10-10 fertilizer helps keep them healthy by giving them a little nutrient boost. Give your tree 1 pound of fertilizer in March and another pound in June. Keeping the fertilizer about 1 foot away from the trunk, sprinkle the product directly on the soil in a 25-foot radius around your tree. Water your tree immediately after application to get the nutrients down into the soil.