Common Diseases of Pecan Trees

A deciduous tree native to the fertile and well-drained Mississippi floodplains of North America, the pecan (Carya illinoinensis) belongs to the same family of fruit trees as hickory and walnuts.

Pecans are large trees that often grow taller than 70 feet, and have a trunk diameter of a minimum of 6 feet. They can grow in plant hardiness zones 5 through 10. 

If you happen to be in an area south of zone 7, you can safely place your bets on healthy pecan production. 

History of the Pecan (Carya Illinoinensis) in the United States

For thousands of years, these forest trees were a food source for Native Americans who disseminated the pecan nut eastward across the rest of the modern-day United States. 

Today, people grow pecan trees in greater numbers than ever before. Not just for shade, but also for commercial cultivation of the delectable nuts that are high in health benefits. 

Substantial cultivation happens in the midwestern and southwestern United States, where the climate is arid or semi-arid during the growing season.

But, pecan trees often fall prey to diseases caused by fungi, bacteria, viruses, and nematodes. Cultivators need to identify these conditions early and determine the appropriate course of action. 

Let’s dive into some of the most common afflictions of pecan trees.

Pecan Scab

Pecan scab is a fungal disease caused by the spores of the Cladosporium caryigenum. Wind and rain spread the spores, which can cause multiple infection cycles in a single growing season. 

The fungus needs wet conditions to thrive and is particularly dangerous for young pecan leaves. 

You can expect an onslaught of scab in your orchard or garden if you’ve had a wet spring. 

When the disease is in its early stages, small, dark patches appear on the leaves, twigs, and shucks. With time, the lesions expand and merge to produce prominent black patches.

Scab is the most serious disease to affect pecan nuts, bringing heavy financial implications for growers. It can also cause a significant reduction in the number of healthy pecans harvested. Scabs occurring on the shucks of developing nuts can affect their size and kernel quality.

While there is no known cure yet for this disease, you can take measures to ensure scab resistance. Some methods that have proven to be useful include:

• Regularly clearing the orchard floor or the trunk base of debris; studies have found that the pathogen grows on leaf litter left behind from the previous season.

• Thinning and pruning the trees to ensure improved airflow throughout your orchard.

• In an orchard with several cultivars, it is best to implement a broad spectrum fungicide spray, which will also help combat fungal diseases besides scab.

If you’re looking to start a new orchard or plant a tree in your garden, the soundest long-term disease management strategy is to select a variety among resistant pecan cultivars like Choctaw, Gormley, Graking, Mohawk, etc.

Vein Spot

Caused by the Gnomonia nerviseda fungus, vein spot is a common disease of pecan tree leaves. It seems to be more common among pecan trees that grow in areas where the soil is less fertile and has a lower percentage of zinc content.

The most prominent symptom is the appearance of lesions on the leaves, much like the ones caused by the scab disease. In this case, the lesions are more linear than round, occurring only on the leaf veins and edges, and are generally dark brown or black. 

While it is not a severe condition and not known to cause financial losses, affected leaves fall off, leading to premature defoliation. It also hampers overall tree growth and the quality of harvested pecans.

The fungus spores are spread by winds and rain splash. They grow on leaves shed from the tree and thrive amid the debris. So it is important to regularly clean the orchard floor and the area around the tree. 

Fungicides used to treat scab are also useful in dealing with vein spots.

Liver Spot

A foliage disease of the pecan tree caused by the fungus Gnomonia caryae var. pecanae, the liver spot is characterized by the appearance of reddish-brown, liver-colored dots on the underside of the leaves, along the midrib. 

The first of these spots appear around May and June. As the disease progresses, the dots enlarge to a radius of about 1cm, reaching their peak in late summer.

The disease mostly attacks young trees with low vitality, particularly ones growing in crowded orchards. But, it has also been found to affect the occasional vibrant, mature tree and cause untimely defoliation. 

Liver spots are more common in pecan trees in regions with higher temperatures (above 75°F, or 24°C), low rainfall, and reduced soil moisture. 

Some cultivar pecan varieties like Schley and Georgia Giant are highly resistant to liver spots, while others like Stuart, Van Deman, and Pabst are susceptible to it.

While it is not a serious condition, liver spots can affect overall tree health. The best way to combat it is by watering and cleaning your orchard or garden regularly and getting rid of tree debris. Periodic spraying of fungicides in measured doses is also helpful in this regard.

Twig Dieback

Twig dieback of pecan trees is a fungal disease caused by the Botryosphaeria berengeriana. 

Wilting branches on the nut trees is the earliest symptom of this affliction. A closer look will reveal small, raised pustules or cankers with black centers on the branches.

The fungi grow in dead plant tissue and then eat into the plant through wounds – including those caused by pruning and insect damage, or even through growth cracks. 

The fungus can cause branches to die back by as much as 24 to 30 inches. 

No fungicide is adequately effective at treating this disease. So, if you find signs of twig dieback on any Carya illinoinensis, the only cure is to prune the infected and dead branches up to the healthy portions. You can also try to identify the source and cut it out.

Remember to disinfect your pruning tools with alcohol periodically to arrest the spread of the bacteria to healthy tissue through contact. 

You must also destroy all the dead and infected wood. It’s best to burn it in a controlled manner, to release minimal air pollution.

The Botryosphaeria berengeriana is an opportunistic fungus, meaning it only affects trees that are already stressed or weakened by other factors. The best way to deal with it is to prevent it altogether by choosing resistant cultivars when you plant trees.

Crown Gall

A bacterial disease caused by the soil-borne Agrobacterium tumefaciens, crown gall affects the roots and entire root system of over 40 plant families, including the pecan tree. 

Its most common symptom is the occurrence of wart-like growths or tumors on roots or at the base of the trunk.

The disease progresses slowly, over many years. The infected trees exhibit poor health and dieback, fail to grow new leaves, experience reduced nut production, and eventually die. 

The bacteria enter tree tissue through wounds caused during the cultivation process, or by insects. Microscopic plant parasites known as nematodes can also cause wounds, leading to crown gall infection. 

The disease can also spread to healthy plants through pruning or grafting tools used to work on infected trees. Thus, it’s important to disinfect pruning tools with rubbing alcohol and avoid laying them on the ground.

There is no effective management for a pecan tree affected with crown gall. As a grower, you must inspect every tree that arrives in the nursery carefully, and reject the ones that show any suspicious growths or knots.

Growing a Pecan Tree the Right Way

Managing diseases in adult pecan trees can be challenging. 

The alternative option is to exercise caution right from the start of the cultivation process to maintain nut quality and grow healthy trees. This can be achieved by keeping the following pointers in mind:

• Pecans thrive in soils with moderate moisture retention capacities. It is best to plant them in slightly sloping areas to ensure proper drainage.

• The ground should be between 4-6 feet deep to allow the taproot to penetrate. 

• Pecans have a high annual growth rate of about 3-5 feet. The area around should be free from obstructions so that the pecan tree can spread and grow.

• The site should also be devoid of overhead obstructions like power lines.

• Pecans require optimum exposure to the sun for overall health and good crops.

You can read our insightful blog for more pointers on growing healthy pecans that are resistant to disease and provide large quantities of delectable nuts.

Photo from Pixabay by glynn424