How To Treat Pecan Tree Diseases?

Pecans are one of the few commercially farmed nuts in North America. They are a native tree to the US and are considered to be a valuable commodity. Pecan trees suffer from pests and diseases that can negatively affect the nut quality. Home growers should also be aware of this and know how to diagnose and treat pecan tree diseases.

Types of Pecan Tree Disease

There are several diseases that affect pecan trees. Here, we will take a look at the most common.

Pecan Scab

Small black spots on the leaves, twigs, and shucks of the nut are the first signs you are likely to notice for pecan scab. These small lesions soon spread and start to meld together as the disease progresses.

It is caused by a fungus called Venturia effusa, and the spores are spread by rain and wind several times through the growing season. This means it tends to spread in cycles allowing it to infect more trees.

The fungus thrives in wet conditions and must be moist for an infection to take hold. If the spring is very wet, it’s vital to take precautions against pecan scab.

The disease reduces the tree’s ability to photosynthesize and will cause leaves to die and drop off. This weakens the tree. When it infects the fruit, the shucks will be smaller, and the nut will not fill correctly. 

The total damage caused will depend on the severity of the infection. 

Prevention

The best way to help prevent the disease is to take proactive steps:

• Remove all leaf litter and dead twigs and branches from around the trees, as this harbors the fungus.

• Practice good pruning, using sterile tools, so trees are maintained with an open form to allow the maximum amount of airflow through them. This allows them to dry more rapidly after rain.

• Don’t plant trees too close together, as this helps spread infection and also restricts airflow and light.

• When planting new pecan trees, don’t place them in low-lying areas. This is because moisture persists longer in these places. Instead, plant them in an open environment, such as on a gentle hillside.

• Plant cultivars that are resistant to scab, such as Lakota or Kanza.

• Use a fungicide spraying program to help prevent scab and other fungal diseases affecting pecan trees. This needs to start in early spring before pollination has occurred. Additional treatments will be required throughout the season.

Treatment

There is no cure for scab. However, work has been done by scientists to understand the disease better and is producing promising results which may help growers control it in the future.

In this video, you will see how to identify and control problems with pecan trees:

https://www.youtube.com/watch?v=yytdKOHHP9Y

Brown Spot

The fungus called Cercospora fusca is responsible for brown spot. It doesn’t appear until early to mid-summer and is more often associated with neglected trees and orchards where rainfall or humidity is high.

The first signs of the disease are small dots that appear on the lower part of the leaves. As it progresses, these get larger and take on a reddish-brown color with a gray hugh. 

If no steps are taken to control the disease, it will often cause complete defoliation by early fall.

Prevention

Keeping the trees well-tended, pruned, and healthy is the best defense against brown spot. 

To do this, ensure that the trees are watered and fertilized adequately throughout their growing cycle.

Removing leaf litter and dead twigs and branches from around the trees is helpful too.

Treatment

Fungicides used to control scab, are also effective against brown spot. No cure is possible once the tree has become infected, but management may be beneficial and can be done by removing all damaged areas as soon as they are seen. 

Downy Spot

Caused by another fungus – Mycosphaerella caryigena, downy spot is often responsible for leaf loss early in the season. 

Some cultivars are more susceptible to downy spot, such as Moneymaker, Pawnee, and Stuart. 

The first signs of downy spot appear in late spring when you will see spots on the lower surface of young leaves. As the disease progresses, the fungus produces spores, and the spots will turn white before going brown and also appearing on the upper side of the leaves.

If heavily infected, leaf drop will happen earlier than for healthy leaves. Repeated premature defoliation from a heavy infection of downy spot will damage the vigor of the tree, and only a few nuts will be produced.

As with other fungi, the fallen leaves will harbor the pathogen and start infecting other trees in spring before bud break. 

Prevention

Always ensure your trees are well cared for with a sufficient amount of water and nutrients. A fertilizing regimen suited to your climate and soil type will go a long way to help.

Plant varieties of pecan that are resistant to downy spot, such as Success, Schley, Western, and Mahan, but be aware that Western and Schley suffer from pecan scab, and Western and Success often get problems with shuck dieback.

Remove all fallen leaves, twigs, and branches regularly and burn them if possible. Some states have laws that prohibit this, so always check fire regulations first.

Use a fungicidal spray program to help prevent infection. 

Treatment

Once infected, subsequent treatment is ineffective.

Gnomonia Leaf Spot

Caused by the Gnomonia dispora fungus, Gnomonia leaf spot is often confused with brown spot.

It attacks trees that are deficient in zinc and causes spots to appear on the leaves. As the disease progresses, the spots expand but stay contained within the lateral veins.

Symptoms first appear in early summer, shortly after the infection has taken place, and the lesions get larger and more elongated over time. 

Fallen leaves are again how the fungus survives through the winter and re-infects trees the following spring.

Prevention 

Removing all leaf litter, twigs, and branches from around the trees is essential.

Correct fertilization programs with a fertilizer that contains the right amount of zinc for pecan trees will be very beneficial in prevention.

The application of fungicides, the same ones used for scab, will also help prevent an outbreak.

Treatment

A cure is not possible, but control by removing infected areas may help the spread.

Zonate Leaf Spot

Caused by a fungus called Cristulariella moricola this disease is more transmissible during wet summers. 

The fungus causes extreme defoliation of the trees during mid-summer months, usually if there is frequent rainfall.

Grayish-brown spots with concentric rings appear on the upper surface of the leaves. On the underside, the spots are pale brown in the center, darkening to a deeper brown towards the edge.

The spores form a crystalline film over the spots. If there are many lesions, the leaves dry out and will curl up until they die and fall off the tree.

The fungus can survive the winter by forming a sclerotium. This is a hard, compact mass of fungal mycelium. Inside the mass, there are food reserves that help the fungus survive harsh environmental conditions.

For the fungus to develop, it requires moisture, and growth is most rapid on wet leaves.

Prevention

No cultivar of pecans is resistant to this fungus, and it is a problem for areas with high humidity and soil moisture. 

Good pruning practices that keep the tree’s canopy open to allow good airflow can be beneficial.

Treatment

At the first signs of this disease, using copper hydroxide or thiophanate-methyl can help to contain the outbreak. It is important to follow the directions for these products as laid out by the manufacturer. 

Liver Spot

Liver spot is caused by the fungus Gnomonia carvae. In late spring and early summer, the appearance of dark brown, circular spots on the underside of the leaves along the midrib may be a sign of liver spot. The spots turn a paler cinnamon or liver brown by late summer.

Extensive defoliation is a significant problem with liver spot, and the disease flourishes during times of prolonged rain or high humidity.

Prevention

Healthy trees are far better at resisting liver spot, so the best prevention is good pruning practices and ensuring trees receive the right amount of water and fertilizer.

Treatment

If a tree has been newly infected, this can be controlled with the use of the same fungicides that are used to prevent scab. 

Powdery Mildew

Powdery mildew disease is caused by another fungus called Microsphaera alni. You will notice a characteristic white powdery substance on both leaves and nuts. 

Although the leaves are not generally seriously damaged, it can be more problematic for the nuts, and the extent of this damage will depend on what developmental stage the nuts are at. 

For nuts affected while still young, the chances are they won’t develop kernels correctly and will be generally small.

Almost no problems are encountered for nuts infected when they are mature.

Prevention

Cultivars such as Schley and Stuart are highly susceptible to powdery mildew, and both prevention and cure can be dealt with by using fungicides that control scab. 

Treatment

Use fungicides suitable for scab.

Anthracnose

Another fungal disease, this time caused by a fungus called Colletrotrichum. 

The ailment can be diagnosed by the presence of black-brown lesions on the leaves and shucks. They have a sunken appearance.

You may also find salmon or cream-colored spores that form concentric rings on the shucks. 

Heavy, frequent rain seems to be a causal factor in this fungus spreading and infecting trees.

Spores are broadcast during the rains in spring and early summer. The more frequent the rain, the more trees contract the disease.

Prevention

There are some varieties of pecan that have a natural resistance to Anthracnose.

Treatments

Only commercial growers can access fungicides strong enough to have any effect on the disease. The only other thing to do is spot it early and cut off and burn any parts of the tree which are affected.

Crown Gall

Rather than a fungus, Crown Gall is caused by the bacteria Agrobacterium tumefaciens. 

This bacteria can change healthy plant cells into tumerous ones. These growths are easily confused with callouses, but as they progress, they become rounded with a rough texture and dark color.

The bacteria live in the soil and can do so for several years. They are introduced into the pecan tree through the roots or low down on the trunk by entering through any small wounds, such as those made by insects, at graft sites, or by any other means.

The growths are known as galls, and they inhibit the flow of nutrients and water in the tree’s vascular tissues. 

Due to a lack of water, the exterior part of the galls fall off the tree, and this enables the bacteria to be reintroduced into the soil. 

Prevention

Great care must be taken when planting new trees to ensure that the roots and trunk are not damaged. Trees must be disease-free and healthy if they are to flourish. 

If you know there is a problem with crown gall in your area, you can take the extra precaution of dipping the tree’s roots in a solution that contains an antagonistic bacterium called A. radiobacter (strain K84) prior to planting. This is not harmful to the tree but helps to prevent the invasive bacterium from striking.

Treatment

There is no treatment for crown gall once the tree has become infected.

Stem End Blight And Shuck Dieback

The first signs of Stem End Blight are brown or black spots that appear on the shuck of the nut near the base.

This will gradually spread until the dark coloration envelopes the entire nut. Once this occurs, the nut will often fall from the stem. 

Shuck Dieback becomes noticeable when the shuck turns black. The tip of the nut will also die. 

The reasons for these two blights are not properly understood. The fungus Phomopsis has a close association with the diseases.

The shuck is killed, and the quality of the nut is reduced with both ailments. 

Some cultivars of pecans seem to be more susceptible than others, “Success” is one of these, but it is not the only one.

Stress is the main cause for the tree to become infected by either disease, and this can come from almost any cause, water, drought, nutrition, etc.

Prevention

The most effective way to prevent trees from contracting either of these diseases is through good tree management and preventing stress.

Ensure they are correctly irrigated, thin the tree to allow good light and air circulation, and ensure the trees have the right soil pH and nutrient values. 

Treatment

Unfortunately, fungicides are ineffective for these problems. 

Why Is It Important To Treat Tree Fungus?

There are important reasons why you shouldn’t just ignore tree fungus once you spot it. Keep in mind also that prevention is always better and easier than cure. 

1. It only takes a small wound for fungus to infect a tree. This could be from careless pruning, an injury from an animal or lawn mower, or wind damage, to name a few.

2. The fungus quickly infects the wound causing discoloration around the area. In some cases, when the conditions are right, the fungus enters the tree roots without any damage being present.

3. Fungus produces enzymes that decay the wood. The decaying process allows nutrients to be released that the fungus feeds from. This has two effects, it weakens the tree by destroying the wood fibers and allows the fungus to spread.

4. The trunk and limbs of the tree lose strength, and branches and crown begin to die. If the fungus is in the roots, then the whole tree’s structure and integrity are significantly reduced. You will start to see litter from the dying branches on the ground.

5. A mushroom or conk will be formed on the tree, either on the trunk, limbs, butt, or roots. This is where spores are formed. 

6. As the fungus progresses, larger parts of the tree die and can fall off, causing damage to people, pets, or property. If there’s a strong wind, the entire tree may be destroyed.

What To Look For

The very first signs of trouble include slow growth, dieback of branches, and the leaves being smaller than usual. You may also notice the autumn leaves start to brown faster than for other trees. 

It is challenging to eliminate the fungus from a tree once infected. Often, it’s better to remove the tree so that the fungus cannot spread to others and, if possible, burn it to prevent spores from being distributed.

What Fungicide To Use On Pecan Trees

The most effective fungicide for leaf scab is Phosphite. The application should be made in accordance with the product label. Usually, 100 gallons of water is mixed with two to three quarts of product and then sprayed onto the tree.

Don’t forget to ALWAYS wear protective clothing, masks, and eye shields before using any chemicals.

Is Vinegar A Good Fungicide?

Because vinegar has antifungal properties, it could seem wise to apply it to your trees as a natural fungicide. 

Unfortunately, this is not as good an idea as you may think. Vinegar, when sprayed onto leaves, will burn them, and indeed, enough vinegar at the roots of the tree will dry them out and can cause the tree to die. So perhaps it’s best to keep the vinegar for your salad dressings!

If you do want to use a natural fungicide on your trees, then Potassium Bicarbonate can be a good choice. Commercial products are available from reputable garden store outlets.

It is considered safer for the environment than other commercially available natural fungicides, which often contain copper or sulfur. 

It kills mature fungi and spores, so it will treat a fungal outbreak and also help to prevent future infections.

Be aware that long-term use will raise soil pH, so it is advisable to test regularly and amend as necessary.

Can A Tree With Fungus Be Saved?

It depends on how much damage has already been done by the fungus, where it’s situated, and what type of fungus it is.

If the tree roots are infected, then it is extremely difficult to treat effectively and will generally spread, eventually killing the tree.

If the fungus is restricted only to the lesser branches, then, with careful treatment and pruning, you may be able to save the tree.

Be aware, however, that once fungus spores are in the environment, they are very tricky to eradicate, and re-infection is common. 

How Much Zinc Do You Put On Pecan Trees?

One of the most essential nutrients for pecan trees is zinc. It’s needed to help the tree grow and develop and should be applied at the start of every growing season. 

Zinc can be purchased from gardening stores as zinc sulfate, usually in the form of granules. These will simply need to be spread around the base of the tree at the appropriate volumes.

It’s important always to follow the directions given on the label of the product you choose.

The amount of zinc to apply depends on several factors. The size and age of the tree and the type of soil you have. Soils, where minerals are quickly eroded, will need more zinc added than those that retain amendments well. 

As a rough guide, the zinc sulfate granules are broadcast beneath the tree’s canopy at a rate of between one and four pounds per tree. Sandy soils will generally need more than loamy ones.

Apply in early spring and avoid treating the tree after the new buds have broken.

Conclusion

Prevention is far more effective than cure when it comes to dealing with pecan tree diseases. 

Many fungal diseases have no cure once the tree has been struck down. Fungal spores are generally very transmissible from one tree to another, particularly when they are being grown close together.

Preventative measures include good tree management, including pruning practices that maintain an open canopy allowing air circulation and light to penetrate right into the tree. Also vital is keeping the tree correctly fed and watered throughout the year.

The removal of all leaf litter and dead branches from around the trees is critical to prevent mold spores from overwintering there.

Moist conditions are the worst enemy of the pecan tree, as it is in damp, humid conditions where most of the fungal diseases are easily transmitted.

To learn more about pecan trees and how to grow and care for them, you can find many more articles on our website.