I have black spots on my leaves and nuts. What is causing it and how can I prevent it?

Randy S. Sanderlin, Bollich, Patricia A.  |  8/4/2006 7:00:12 PM

Pecan scab lesions on nuts.

The spots may be pecan scab disease, which is caused by a fungal pathogen. Severe infection of leaves can cause the tissue to die and result in leaf loss. Damage from infection of nuts can vary from complete crop loss to a minor loss of nut size depending on how severe the infection is and at what stage of nut development infection occurs. Pecan scab disease can be controlled by application of fungicides that prevent infection from occurring. Infections occur during rainfall periods, and generally four to eight fungicide applications are needed each year from spring through the summer months to prevent infection. The number of applications needed will depend on several factors, including the frequency and amount of rainfall and the susceptibility of the pecan cultivar to infection by the scab pathogen. To obtain adequate scab disease control, it is necessary to completely cover a tree with fungicide. Unfortunately, this requires an air-blast orchard sprayer for trees over 10-15 feet in height, and it is not practical to try to apply fungicide to a large tree growing in a yard.

Some level of disease control can be obtained by growing pecan cultivars with some resistance to infection. Cultivars with useful levels of resistance that are currently recommended for small plantings include Candy, Elliott, Melrose, Sumner, Jackson and Caddo. Caddo and Melrose are not recommended for the southern half of Louisiana. A description of these cultivars can be found at the USDA Pecan Breeding Program Web site.

More information can be found in the Fungicide Application Recommendations for Pecan Disease Control and in the Pecan Disease Synopsis on the Plant Pathology page of the Pecan Station Web site.

Question answered by Dr. Randy Sanderlin, Pecan Research-Extension Station plant pathologist.

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