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What’s That Disease: Pecan Scab

Robyn Stewart

Did you know that Georgia is the #1 state for pecan production in the United States? Typically, pecan harvest occurs between October and November for commercial orchards, but homeowners can often harvest year-round. Pecans are a good source of protein, vitamins, and minerals, as well as fiber—they make a good addition to any diet (so long as you aren’t allergic!)        

I have had several questions lately in the Extension office about a common disease on pecan trees called Pecan Scab. Tree owners will mention black lesions on leaves, nuts, and twigs that worsen with time. Pecan Scab can also cause small, poor quality nuts, early nut drop, and issues with shelling the nuts. This disease is the most common disease in pecan trees and can be very challenging to manage for homeowners and small operations.

Pecan Scab is a fungal disease caused by Fusicladium effusum. The disease cycle is as follows: Fusicladium effusum spores from infected leaves, twigs, and nuts are spread by wind and rain in early spring. The spores land on new growth and nearby trees where they germinate and start breaking down tissue. At this point the tree owner will see the characteristic black lesions on their tree while the fungus continues to spread spores to reinfect the current season’s growth. Part of what makes this disease so challenging is that fungal spores will overwinter in infected tissue from the previous season. Pecan Scab grows and spreads most effectively in hot, wet, sunny weather—so owners should keep a close eye on their trees during and after those conditions.

Pecan Scab infestation can have a significant impact on pecan yield and quality. The disease causes early nut drop, reduction in nut size, and reduction in nut quality. Yield losses for an infected tree range from 50-100%. In addition to the active nut loss, the reduction of photosynthesis due to lesions on the leaves can increase alternate bearing (where the tree produces heavily one season followed by one or more years of little to no production).

There are several things that can be done to help prevent and manage Pecan Scab. The first and most important if you are establishing a new tree is to choose a resistant cultivar. The most scab-resistant varieties we know of are Jubilee and Melrose, so if you are planting new trees, consider one of those. In addition to selecting a cultivar, be sure to allow adequate tree spacing and follow appropriate pruning techniques. Exposure to sunlight and good air flow is essential for decreasing the hot, moist environment that Pecan Scab prefers. Removing leaf material and debris is also essential for preventing Pecan Scab.  For management of existing trees, the use of a fungicide is typically the most effective route but can often be very expensive. Preventive fungicides are the only effective chemical control for Pecan Scab. Fungicides must be used starting early in spring when new leaf growth is ~1 inch in length and must be used every few weeks (following label instructions) throughout the growing season in order to be effective—trees may require up to 12 treatments per growing season.  Homeowners should be wary, however, that using a fungicide requires commercial air-blast sprayers to ensure thorough coverage of foliage and fruits. Fungicides should not be used if they cannot get adequate coverage or continuous treatment as this will contribute to resistant forms of scab. UGA Extension highly recommends that you contact a professional tree service to ensure that the appropriate fungicide is used at the correct rate and spray coverage throughout the growing season.

Pecan Scab can be a significant issue for commercial and homeowner pecan growers alike. Appropriate management and use of fungicides can be effective tools for preventing and managing yield loss. If you suspect you have a Pecan Scab issue and need further assistance, please reach out to us at uge3181@uga.edu or 706-359-3233.