Overcoming disease and pest challenges
- Pecan scab
- Stem end blight (Water stage nut drop)
- Shuck dieback (Physiological, nut drop, variety reaction)
- Downy spot (Foliage loss, Stuart)
- Brown leaf spot (Late season, defoliation, weak trees)
- Fungal leaf scotch (Defoliation, susceptible varieties, leaf cutting)
- Powdery mildew (White powdery fungus, high temperatures, dry conditions)
- Vein spots (Leaf veins, brown lesions, defoliation)
- Leaf blotch (Weak trees, old foliage)
- Articularia leaf mold (Minor leaf disease, white tufts, weak trees)
- Pink mold (Secondary diseases, behind scab lurks pink fungus)
- Bunch disease (Mycoplasma, sucker growth)
- Fungal twig dieback (Black pustules, drought)
- Kernel discoloration (Several fungi, insects, drying)
- Cotton root rot (Soilborne disease, rapid death, fungus)
- Root knot on pecans (Slow decline, zinc deficiency, nematodes)
- Crown gall (Bacteria, weakened tree)
- Regional disease pressure
- Honeydew sooty mold
The southern pecan tree (Carya illinoinensis) has a history of being an important shade tree as well as a nut-producing tree for growers. However, pests and disease can affect the appearance of the stately pecan and reduce the shade value and nut production. These factors seldom destroy the tree, but can change its appearance. It can be difficult to spray fungicides or pesticides on pecan trees, which can grow to 100-foot heights.
According to a report by Texas A&M University, diseases that affect pecan trees are due to bacteria, fungi, nematodes, mycoplasma and physiological problems. The report states that viruses have not been shown to occur on pecans, but over time low nut production may be traced to viral infections.
Growers depend on a variety of practices to control diseases and pests, including planting resistant varieties, spacing, proper pruning, sanitation and spraying when other methods fail.
Plant resistant varieties
To reduce insect and disease problems associated with growing pecans, look to resistant varieties. With resistant varieties, you can reduce the amount of pesticides needed to produce a healthy crop, saving money on product. This also decreases labor costs and exposure to chemicals. By using a resistant variety, growers can postpone the development of scab during periods of rainy, windy weather when spraying is not feasible. The cultivar ‘Pawnee’ is less prone to aphids, while ‘Cheyenne’ is highly susceptible to aphids.
Like other plants, pecan trees need room to breathe. Lacking the proper space, disease can overtake the trees. The closer the trees are planted, the more likely disease epidemics will take place. During heavy rainfall, foliage becomes wet and stays moist when there is poor circulation, delaying leaf drying and increasing the opportunity for disease to appear within an orchard.
When planting a pecan grove, know the direction of the prevailing wind and plant rows accordingly. Why is this important? If the rows are planted perpendicular to the prevailing wind, the rows on the outside of the grove will block airflow to the interior part of the orchard. The closer together trees are, the greater the problem. When orchards need thinning, dense shade creates a welcoming environment for the black pecan aphid to multiply.
Without proper control of disease and pests, pecans appear like these in the photo.
Pruning helps reduce disease
One of the first signs of neglect in a pecan orchard is a lack of proper pruning. Pruning removes low-hanging limbs, which can create problems when managing grass growth between rows. Pruning also increases air circulation around and under the trees and exposes trees to more sunlight, making it easier for foliage to dry following rains. This will help reduce black pecan aphid problems.
Create a clean environment
Good orchard sanitation is important for high yields. Do you know what lurks on the ground beneath your pecan trees? Unless the ground in your grove is cleaned of debris, you’re likely to have an environment that sustains disease and insects. Especially in mild winters, these organisms overwinter in old shucks, leaves and small twigs. A light disking or raking of the area under and around the base of the tree will provide for good crop sanitation. Proper hygiene reduces the chance that fungal pathogens and insects will take over next year’s crop.
When other methods fail
There are many factors that determine a successful pecan crop. Weather, including high wind, hail and freezing temperature at the wrong time can determine your outcome. Inclement weather is a factor over which you have no control.
However, using a fungicide can reduce and often prevent significant losses to disease. To achieve effective coverage of a tree that may reach heights up to 100 feet is difficult. Check to make sure foliage is covered with a moist film. If you’re not receiving the necessary results, check the tractor speed, pump pressure, nozzle size and nozzle arrangement. Having the proper equipment makes the task easier and more successful.
For additional help, check with your agricultural agent or a university that supports research on disease control of pecan trees. Check Internet sources or Google your topic for more information.
The common pecan disease
Clemson University Cooperative Extension Service has conducted extensive research on pecan diseases. Their work focuses on South Carolina and may not apply to other areas. It is noted that the recommended pesticides were legal for the state at the time of publication, but state and federal regulatory agencies may have changed. Follow the directions, precautions and restrictions listed on the pesticide container.
According to Clemson University Cooper-ative Extension Service, scab is probably the most common and damaging of the pecan diseases. Caused by a fungus, it forms small, circular, olive green to black spots on leaves, leaf petioles and nut shuck tissue. Scab is most damaging when the pecan tissue is young and still growing. If this happens early in the season when nuts are forming, the yield and crop quality is significantly reduced.
To avoid scab disease in your orchard, plant resistant varieties, such as Cape Fear, Elliott, Gloria Grande and Owens. Orchard sanitation is essential to avoiding this disease. A preventive spray fungicide program at regular intervals helps control scab. When applying any chemicals, follow directions explicitly.
Knowing and understanding pests
The appearance of various pecan pests is closely related to seasonal development of the nut. However, the severity of pests cannot be predicted on an annual basis; growers should be aware of the time when management problems can be anticipated. Specific insects are associated with the various developmental stages of the pecan. Both arthropods and pathogens attack the pecan. The arthropods can occur throughout the growing season. However, many are not a threat to the yield, as natural enemies, adverse weather, and defensive or repair mechanisms of the pecan itself are control measures.
Regular pesticide application is usually not recommended. Several reasons:
- Pests will develop a resistance to the pesticide
- Natural enemies may be destroyed
- Problems may develop from nontarget pests
- Unnecessary risk of pesticides to the grower and the environment
Pesticides should be used only when other methods fail and pests appear in damaging numbers. Growers are encouraged to recognize these pests and their natural enemies, which will aid in making management decisions.
An excellent Internet resource was compiled by Bill Kee (2003), “A Partial List of Damaging Insects Attacking Pecans in the United States.” Kee organized the pests into three major groups: fruit; foliage; trunks, branches and twigs. The list, which includes the family, scientific name and common name, can be found at: http://pecankernel.tamu.edu/insect_list/index_print.html.
Webworm caterpillars defoliate pecan leaves. Usually, this will not kill the tree but affects the appearance and destroys the nut.
Dealing with fall webworms
One common pest is the fall webworm caterpillar, which builds large webs in pecan trees. Feeding on pecan leaves, each web may contain 100 or more caterpillars. When large webs cover a tree, the plant experiences severe defoliation. Larvae are deposited by moths that lay eggs on the underside of pecan and other tree leaves. Two to four generations per year appear, with the fall generation being the most destructive.
Natural enemies include insect parasites and predators. Insecticides may help reduce the webworms, but must penetrate the web to be effective. Spot spraying may be an option. If a large tree is healthy, it will survive.
Pecan disease reduces the yield in nut production.
Pecans – a growing business
According to Texas A&M University, worldwide production of pecans exceeds 250 million pounds annually. Foreign counties, including Israel, South Africa, Brazil and Australia, are now growing pecans. Georgia, a state that had no pecans a few hundred years ago, is now the number one pecan producer in the U.S. Texas comes in second, producing about 35 million pounds per year. Pecan growing gained in popularity when a transition was made from the native to improved pecan production. In 1919, Texas did not grow the improved varieties, but by 1990 the improved pecans made up about 35 percent of the Texas crop.
With growing interest in pecans as a crop, the U.S. and other counties must continue to research diseases and insects that affect this cash product.
Larvae feed on leaves of the pecan trees.
The author writes from Jackson, Tenn.