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By: Amy Grant
Downy spot of pecans is a fungal disease caused by the pathogen Mycosphaerella caryigena. While this fungus only attacks foliage, severe infection can result in premature defoliation which affects the overall vigor of the tree, thus pecan downy spot control is integral to the health of the pecan tree. How do you treat pecan downy spot? The following article contains information on pecan downy spot symptoms and tips for treating a pecan tree with downy spot.
Downy spot of pecans symptoms usually manifest in late June to early July. Primary infection of new spring leaves stems from spores that have overwintered in old, dead leaves. The actual sign of a pecan tree with downy spot occurs near bud break in the spring.
Downy spots appear in the late summer on the underside of new leaves. This downiness is caused by innumerable spores on the surface of the lesion. The spores are then spread by wind and rain to nearby leaves. Once the spores have distributed, the lesions turn a greenish-yellow. Later in the season, these downy spots become brown due to cell death in the diseased lesion. They then take on a frosty appearance and the infected leaves often drop prematurely.
All pecan cultivars are somewhat susceptible to downy spot, but Stuart, Pawnee, and Moneymaker are by far the most vulnerable. The fungus survives the winter in infected leaves from the previous season and is fostered by cool, cloudy days with frequent rains.
Pecan downy spot control depends on preventative fungicide sprays applied at budbreak. Even the application of fungicidal sprays may not entirely control pecan downy spot, but it should reduce the primary infection.
Remove and destroy any fallen leaves from the previous year well before budbreak. Also, plant resistant or tolerant cultivars like Schley, Success, Mahan, and Western. Unfortunately, you may be swapping one problem for another since Schley and Western are vulnerable to pecan scab while Success and Western are susceptible to shuck dieback.
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Organic food production is one of the fastest-growing sectors of the American food marketplace and is driven largely by personal health preferences and environmental ethics. Organic food sales in the U.S. rose from $13 billion in 2005 to $35 billion in 2014. Organic farmers are required to follow an ecological soil management program and are restricted in their use of chemicals. In order for a crop to be marketed as organic, it must obtain organic certification and maintain records of the production practices in use on the farm. The U.S. Department of Agriculture’s organic certification information is available at https://www.usda.gov/topics/organic. A three-year transition period is required before a crop can be sold as “organic” with the USDA- certified organic seal. Pecan production generates unique challenges to organic production methods in the humid Southeastern U.S. because it is an environment conducive to heavy pressure from insects, diseases, and weeds. Therefore, the foundation of any organic pecan production program in the Southeast will be based on selecting pest-resistant cultivars.
Figure 3. pecan scab (Venturia effusa ) (G. Winter) Rossman & W.C. Allen 2016 by Clemson University – USDA Cooperative Extension Slide Series, Bugwood.org (CC BY 3.0)
Pecan scab is a common fungal disease of pecans. Symptoms include black lesions and tissue death on twigs, leaves, and nuts from early spring until late summer (Fig. 3).
Wind and rain spread spores of the disease organism. Because rain and high humidity increase scab infection, pecan scab is more severe in Central and East Texas.
Control: Planting pecan varieties with resistance to pecan scab can reduce disease incidence in Central and East Texas. Other actions to minimize pecan scab are proper pruning and wide spacing between the trees, which can improve air circulation and speed leaf drying.
Fungicides applied as a foliar spray on young trees can prevent pecan scab. To maintain control, they must be applied early and then reapplied. The fungicides labeled for control of pecan scab on bearing trees are not available in small quantities marketed for backyard trees.
Several other diseases can infest pecans, including bacterial leaf scorch, downy spot, powdery mildew, shuck dieback, and stem-end blight. Soil-borne diseases include cotton root rot, crown gall, and root-knot nematodes.
Hypoxylon canker of pecan is caused by a fungus (Hypoxylon atropunctatum). The fungus commonly occurs on oaks, it has recently been found on pecans in Oklahoma. The disease is characterized by sloughing off of bark and a reddish powdery substance formed on the wood under the bark. The disease is characterized by sloughing off of bark and a reddish powdery substance formed on the wood under the bark. This should not be confused with the normal red powdery material formed on the underside of the bark itself. A chocolate brown to black crusty substance will eventually form on infected wood. Infected limbs and trunks will eventually die. Stressed trees, particularly those that are water stressed, will very likely become infected. There is no control of the fungus once it infects a tree. The problem can be prevented by maintaining a proper balance of water in the root zone, painting over wounds, preventing compaction of soil around trees, avoiding cultivation around trees, and maintaining a satisfactory rate of growth by fertilization.
A twig dieback of pecan trees is associated with a fungus (Botryosphaeria berengeriana). It has been found in the southeastern states, Arizona and many areas of Texas. Though it has not been reported in Oklahoma, it may occur here. Some limbs may die back as much as 24 to 30 inches. It is most often found on limbs that are under stress. The stress may be caused by over-production, defoliation by scab and other foliar pathogens, drought, or excessive shading. Twig death is usually general over a tree except under shady conditions, where it will occur only in the shaded areas. To control dieback, prune out dead wood and destroy it. Determine the cause of stress and, if possible, correct it.
Crown Gall (caused by the bacterium Agrobacterium tumefasciens)
The disease is characterized by large proliferating galls on the roots and bases of the trunk of infected trees (Figure 8). Infected nursery trees should be burned. Do not plant infected trees or plant trees in an area known to be infested, as this practice can spread the causal bacteria. This pathogen infects a wide range of plants.
Figure 8. Galls on roots of a pecan tree caused by the crown gall bacterium.
Pathogenic root-knot nematodes (Meloidogyne spp.) invade small roots of the pecan tree and cause small galls to form on the roots. Pecan roots are also susceptible to infection by a ring nematode (Criconomella rusium). Infected nursery trees should not be planted in an orchard. They should be destroyed. Infected trees sometimes show stunting and resetting of the foliage, which applications of zinc compounds and fertilizers will not correct. Use the same control measures as described for the crown gall disease described above. These nematodes have a wide host range and should not be introduced and spread.
Department Head, Entomology and Plant Pathology