Some Common Pecan Diseases and Their Control
in North Carolina
Ornamental Disease Information Note 3
R.K. Jones, Extension Plant Pathologist
D.F. Ritchie, Extension Plant Pathologist

[General Information] [Scab] [Powdery Mildew] [Blotch] [Wood Rots]
[Rosette and Bunch] [Internal Breakdown] [Crown Gall] [Spanish Moss]
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General Information

Pecan trees are widely grown in North Carolina both for shade and nuts. Pecan trees have several disease problems which reduce both the shade value and the nut crop but seldom kill a tree. These diseases are difficult for the homeowner to control with chemicals because of tree height. Therefore, spraying pecan trees usually is not feasible for homeowners.

Scab

Scab is the most common and damaging pecan disease. It is caused by a fungus that affects rapidly growing leaves, shoots, and nuts. Symptoms on all plant parts are similar. Velvety olive-brown to black spots occur on the husks. Several spots may develop to form black blotches or may blacken the entire surface of the husks. Severely affected nuts of susceptible varieties may drop prematurely or they may stop growing, die and remain attached to the shoot. Leaf symptoms first appear on the underside as tiny olive-brown lesions on the veins. Later, leaf symptoms appear on the upper surface as small olive-brown to black spots. Severely infected leaves may be shed prematurely and weaken the tree, thus reducing the crop next year.

The most practical method of scab control is to plant varieties having some degree of resistance to scab such as: Desirable, Cape Fear, Elliott, Chickasaw, Gloria Grande, and to a lesser extent, Stuart. Varieties severely affected by scab are Schley, Mahan, Success, Van Deman, and Mohawk. Use of "resistant" varieties does not necessarily assure complete absence of the disease as no variety is totally resistant. If possible, removing and destroying all leaves and husks may help reduce the amount of scab the following year.

A spray schedule is available for growers who have spray equipment adequate to cover the trees, or for growers who hire custom spray applicators. The following fungicides are registered on pecans: benomyl (Benlate 50WP), propiconazole (Orbit 3.6EC), fenbuconazole (Enable 2F), and thiophanate methyl (Topsin-M 70WSB). Spray applications should be made every 2-3 weeks beginning when leaves first emerge until shell hardening (early August). Spray at 2-week intervals when the tree is growing rapidly or during wet periods.  Check fungicide labels for restrictions on grazing in sprayed orchards and the need to use tractors with enclosed cabs while spraying.

In other states the scab fungus has become resistant to some of the fungicides. This may not be true in orchards that have rarely, if ever, been sprayed with fungicides.

Powdery mildew

Powdery mildew is occasionally a problem on highly susceptible seedling trees and can be identified by the white powdery growth on the surface of nuts and young shoots. Powdery mildew usually does not cause serious damage to most pecan varieties. Scab sprays usually provide adequate control of powdery mildew.

Blotch

Blotch and several other leaf diseases can occasionally cause defoliation in late summer. Generally blotch occurs only on trees low in vigor or deficient in zinc. Nursery trees are especially susceptible. These diseases appear as circular black spots or large, irregular yellow blotches on the leaves. Removing or plowing under old leaves helps reduce the infestation from one year to the next. Scab sprays usually will control leaf spots.

Wood or Heart Rots

Wood or heart rots can cause extensive wood decay and thus weaken the branches or trunk. These fungus diseases occur following mechanical damage, commonly caused by ice, wind, improper pruning, or mechanical damage during construction. Damaged trees should be properly pruned to ensure normal healing and treated with asphalt-base wood dressing.

Rosette and Bunch

Rosette occasionally occurs on pecans in North Carolina. This problem is caused by zinc deficiency or by certain soil types where zinc is unavailable to pecan trees. Initial symptoms occur mostly on the branches in the top of the tree. Leaves are yellowish and mottled. In advanced stages, leaflets become narrowed and crinkled on lower branches. New shoot growth is inhibited. Eventually twigs and branches die back from the tips. Zinc deficiency can be corrected by spraying the leaves twice with zinc sulfate alone or added to other sprays at the rate of 2 pounds per 100 gallons of water (1 tbsp per gallon). The zinc also may be applied to the soil around trees by spreading the material from near the trunk outward to the drip line. A soil test is desirable to determine the amount of zinc required but in general can be applied at the rate of 1/2 pound for each inch of trunk diameter. A disease called bunch disease can produce similar symptoms but differs from rosette in two ways: leaflets of bunch diseased trees neither become yellow between the veins nor extremely crinkled like those affected with rosette. Bunch disease is thought to be caused by a phytoplasma. There is no known effective control for bunch disease, however the following practices will reduce spread. Use only graft wood from bunch disease-free trees for propagation and do not top-work affected trees. On mildly affected trees, prune the affected shoots several feet below the region of symptoms. Candy, Lewis, Caspiana, and Georgia seem to be highly resistant.

Internal Breakdown

Internal breakdown of almost mature nuts in late summer occurs each year. The inside part of the nut becomes soft and watery. This is a physiological disorder the cause of which is not known. Its severity varies from year to year, but appears to be most prevalent on certain varieties such as Moneymaker and Mahan.

Crown Gall

Crown gall is a disease that results in round to irregular swollen tumors or galls, usually found at or near the soil line on the trunk or roots. Infected trees show a lack of vigor, foliage lacks normal green color, and occasionally the tree may die. To prevent crown gall, plant disease-free trees. For individuals with a small pecan nursery, it is important not to locate a nursery in an area where crown gall has been observed previously. As a preventative control measure, it would be advisable to treat the seed with Galltrol-A before planting.

Spanish Moss

In southeastern and coastal areas, Spanish moss can occasionally become thick enough in pecan trees to increase ice and wind damage. A small amount of Spanish Moss does little or no damage to trees. Where it is a problem, spray the moss with copper sulfate at the rate of 10 pounds per 100 gallons (6 tbsp per gallon) during the dormant season.

For assistance with a specific problem, contact your local North Carolina Cooperative Extension Service Office.

Outside North Carolina, look for your state extension service partners.

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Recommendations of specific chemicals are based upon information on the manufacturer's label and performance in a limited number of trials. Because environmental conditions and methods of application by growers may vary widely, performance of the chemical will not always conform to the safety and pest control standards indicated by experimental data. All recommendations for pesticide use were legal at the time of publication, but the status of registration and use patterns are subject to change by actions of state and federal regulatory agencies.

Published by the North Carolina Cooperative Extension Service Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University at Raleigh, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.

Last update to information: January 1999
Last checked by author: January 1999
Web page last updated Nov. 2000 by A.V. Lemay.