Test Your Knowledge - September

Pecan scab
Pecan scab
H.C. Ellis, University of Georgia, Bugwood.org

Yes! This is pecan scab.

Scab is the most common and damaging pecan disease. It is caused by the fungus, Cladosporium caryigenum. The fungus forms small, circular, olive-green to black spots on leaves, leaf petioles and nut shuck tissue. In severe cases, the scab fungus can also attack new twigs and even catkins (male flowers). Scab lesions often run together, causing the terminals to die and the catkins to drop. Pecan tissue is most susceptible when it is young and actively growing. When scab attacks young expanding leaves and nuts, it stunts and deforms them. By the time the leaf is mature, it is no longer susceptible to scab. The greatest scab damage occurs from nut infection. Early-season infection can significantly reduce yield and crop quality. Fruit, infected early in the season, often drop or crack where scab lesions run together and serve as a point of entry for other pathogenic fungi.

Pink mold (Trichothecium roseum) often enters these wounds and causes further decay of the shuck and nut.

The scab fungus survives the winter on plant parts infected the year before. Most spores are released in mid-April, just after budbreak. Spores are spread locally by dew and splashing rain and over longer distances by wind. Scab spores need free moisture to germinate, usually supplied in the form of dew. Spores also require moderate temperatures to germinate, between 65 and 85 °F. Very little infection occurs during dry, hot weather, which is the major reason scab is less important in the more arid Western states.

Prevention & Treatment: The best way to control scab is to plant scab-resistant varieties, such as Cape Fear, Elliott, Gloria Grande and Owens. Complete removal and destruction of leaves and shucks during the winter can reduce carry-over of scab. A very effective - but for homeowners not very feasible - means of controlling scab is a preventive fungicide spray program. It is critical to begin fungicide applications at budbreak to prevent early scab infection. Two to three fungicide sprays should be applied at 10- to 14-day intervals during this critical period. Subsequent fungicide applications can be stretched to 21-day intervals once leaves begin to mature and the nutset stage has passed. A fungicide that homeowners can use for control is thiophanate-methyl, the active ingredient in Ferti-lome Halt Systemic Fungicide and Thiomyl Systemic Fungicide. They can be mixed at 2-1/2 teaspoons per gallon of water to spray pecan trees. Follow the directions on the label.

For more information on pecan diseases, see HGIC 2211, Pecan Diseases.

Millie Davenport
Home & Garden Information Center

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This information is supplied with the understanding that no discrimination is intended and no endorsement by the Clemson University Cooperative Extension Service is implied. All recommendations are for South Carolina conditions and may not apply to other areas. Use pesticides only according to the directions on the label. All recommendations for pesticide use are for South Carolina only and were legal at the time of publication, but the status of registration and use patterns are subject to change by action of state and federal regulatory agencies. Follow all directions, precautions and restrictions that are listed.