10 Willow Tree Diseases: Identification and Management

Willow trees are widely recognized in various environments for their visual charm, providing protection and serving as a habitat for wildlife. However, like any other plant, willows are susceptible to various diseases that, if left untreated, can cause significant damage.

In this article, we’ll discuss how to identify common willow tree diseases and outline effective strategies for managing them.

We’ll also provide helpful tips on preventing disease in the first place so you can keep your willow trees healthy and strong.

By following the advice given, you’ll be able to ensure that your willows remain beautiful and vibrant year after year.

1. Crown Gall

Crown gall is a common disease that affects willow trees. It is caused by the bacteria Agrobacterium tumefaciens, which infects plants through wounds and causes gall-like tumors on the tree’s bark or roots.

These tumors can be unsightly and lead to further damage if left unchecked as they can interfere with water and nutrient uptake in the tree, and the infection may spread to other parts of the tree over time.

Identification

Look for irregular, rough-textured growths resembling tumors on the branches and where the trunk meets the soil.

Management

There is no cure for crown gall, but following proper management practices may enable your tree to continue to survive.

Prevention:

  • Use clean tools when pruning the tree to prevent infection.
  • Avoid excess water and fertilizer, which can increase susceptibility to disease.
  • Plant trees in areas with good air circulation, and avoid overcrowded conditions.
  • Inspect trees for signs of damage or infection regularly, and take steps to treat any problems immediately before they become more severe.

Treatment:

  • Remove infected parts of the tree as soon as possible using clean tools to reduce further spread of disease.
  • Though there is no cure for crown gall, applying a copper-based fungicide directly on affected areas according to label directions may slow disease progression.

Long-Term Prognosis:

Young willow trees may succumb to the disease, especially if the galls encircle the entire trunk. Mature trees may continue to do well as long as infected areas are removed once they are noticed.

2. Black Canker

Black canker is the result of Glomerella miyabeana, a fungus that causes black lesions to form on the leaves. These lesions eventually lead to defoliation and will spread to twigs, causing cankers to form in the junctions of the twigs and branches.

Identification

The first symptoms of black canker are black spots or lesions on the leaves. Over time, the leaves will die and fall from the tree.

Black canker may spread to the twigs around the infected area, eventually resulting in growths, or cankers, forming where twigs meet branches.

Management

Taking action as soon as symptoms are noticed is your best bet at keeping your willow healthy.

Prevention:

  • Avoid splashing water onto foliage and branches when watering to avoid spreading spores.
  • Remove all fallen debris from under the tree routinely, and dispose of it properly.
  • Do not add debris from an infected tree to your compost pile.

Treatment:

  • Prune infected areas, sterilizing clippers after each cut.
  • A fungicide may slow disease progression and spread, but timing is critical, and it may best to have a professional apply the necessary treatments.
  • Avoid stressing the tree (don’t make any unnecessary wounds, keep moisture levels constant, etc.) as stress could increase the tree’s susceptibility to the disease.
  • Provide supportive care through fertilizer application, routine watering, adequate airflow, etc.

Long-Term Prognosis:

Black canker will not usually kill a tree unless the tree is very young or under a great deal of stress.

3. Blight

Willow blight is the simultaneous occurrence of two or more fungal diseases, specifically a canker and scab infection. Without professional treatment, the tree may not survive.

Identification

Symptoms of both scab and canker will be seen. Defoliation and twig and/or branch dieback are typical.

Management

Spotting early signs and enlisting the help of a professional are critical to the tree’s long-term outlook.

Prevention:

  • Avoid overwatering and fertilizing too frequently as this can increase the tree’s susceptibility to infection.
  • Ensure adequate space for airflow around the tree.
  • Inspect your trees regularly for signs of damage or disease before it becomes more severe.
  • Routinely remove fallen twigs, branches, and leaves from under the tree.
  • Consider applying fungicide as a preventative measure.

Treatment:

  • Prune all infected portions several inches below visible damage. Do not compost the pruned materials.
  • Only fertilize after a complete soil analysis.
  • Call a professional for specially timed fungicide applications.

Long-Term Prognosis:

Severely infected and young trees may not survive. Trees minimally infected may live with proper supportive care and the use of fungicide treatments.

4. Scab

Willow scab is caused by the fungus Venturia saliciperda, which infects new growth in the spring. Dormant spores overwinter in affected areas of the tree and spread when temperatures rise.

Identification

Look for brown or black spots or lesions on new leaves, particularly after wet periods in the spring.

Infected leaves will wither and die, but the infection likely has already spread to the twigs. Infected twigs may remain attached to the tree, but they will appear blackened and diseased.

Management

Prompt identification and treatment are important in controlling this disease. Prevention is always best, but it may not always be possible as this fungus is widespread and affects many willow species.

Prevention:

  • Keep your tree healthy by providing plenty of airflow, regular watering, and routine fertilization.
  • Inspect trees on a regular basis, especially after prolonged rainy weather in early spring.

Treatment:

  • Remove damaged areas immediately, sterilizing pruning tools after each cut and disposing of debris properly.
  • Apply a fungicide at the first sign of infection after removing affected portions.
  • Enlist the help of a professional if the disease appears to be spreading rapidly.

Long-Term Prognosis:

Healthy trees will not likely be severely impacted by mild cases of scab. Young or weakened trees are more susceptible, especially if cankers are present as well, resulting in willow blight.

5. Powdery Mildew

According to Cornell University, powdery mildew on willow trees can be caused by two separate fungi: Erysiphe adunca and Phyllactinia guttata, though others may cause the disease as well.

Identification

Powdery mildew presents as a fuzzy or powdery substance on leaves. Tiny, brown spores or fruiting bodies may be seen within the white coating as the disease progresses.

Management

Prevention practices and prompt treatment will impact the tree’s ability to remain vigorous.

Prevention:

  • Avoid overhead watering.
  • Prune to improve air space between branches and reduce humidity.

Treatment:

  • Cut off infected areas several inches below the damage.
  • Treat with a fungicide after removing all infected areas.
  • Avoid fertilizing until improvement is seen.

Long-Term Prognosis:

Powdery mildew is not likely to cause tree death, but managing the disease helps to stop the infection from spreading to other branches and plants.

6. Cankers

A large, oozing canker on the trunk of a tree.

Botryosphaeria berengeriana, Valsa sordida, Dothiorella spp., Leucostoma spp., and Cytospora sp. are some of the fungi that can cause cankers on willow trees.

These fungi grow in wet soil and spread through the air or by rain splash onto branches and twigs. The fungus then infects the plant tissue and causes discoloration, wilting, or dieback of branches or leaves.

If left untreated, these fungi can cause serious damage to a willow tree’s health and growth.

Identification

Cankers on willow trees can be identified by the presence of dead, sunken areas of bark on the trunk, branches, or twigs. These areas may be discolored, cracked, or oozing sap.

Management

Cankers can be a serious issue and may even lead to death in weak or young willow trees.

Prevention:

  • Keep tree stress to a minimum. This means watering consistently, avoiding unnecessary pruning, and providing fertilizers when needed.

Treatment:

  • Prune the affected branches, and remove any dead or diseased wood, making sure to sterilize pruning tools between cuts.
  • Apply a fungicide to control the spread.
  • If the canker is too extensive or the tree is severely weakened, the tree may need to be removed to prevent the spread of the disease to other trees in the area.

Long-Term Prognosis:

If the trunk is girdled by cankers, the tree will most likely die eventually and should be preemptively removed. Healthy trees may form a callus around the canker and continue to thrive.

The tree’s survivability depends on its overall health, management practices, and the specific type of fungus causing the canker.

7. Rust

Rust is typically caused by Melampsora fungi, which need an alternate host, such as balsam or white fir or tamarack, depending on the fungus species, to complete their life cycle.

Identification

In late summer or early autumn, yellow-orange pustules appear on the surface of infected leaves. Affected areas may turn brown or black with time and may fall from the tree.

Management

Rust is a serious issue with willow trees and could cause total premature defoliation on heavily infected trees.

Prevention:

  • Keep the surrounding area free of leaf litter.
  • Ensure adequate spacing between trees.
  • Avoid overhead watering.
  • Remove alternate host trees from the vicinity.
  • Fungicides targeting rust may be applied as a preventative measure.

Treatment:

  • Remove any fallen leaves and other plant debris from around the tree.
  • Treat with a fungicide that targets rust.
  • Some people advise dusting infected areas with sulfur to minimize the spread of the disease.

Long-Term Prognosis:

Though unsightly, rust does not often lead to tree loss. Prompt removal of infected leaves will help to break the cycle. As long as the tree is healthy overall, it should make a full recovery.

A weeping willow tree growing on the bank of a small pond.

8. Cotton Root Rot

The cause of cotton root rot is Phymatotrichum omnivorum, a widespread fungus that infects many tree species. Recent soil disturbance or injury to the lower trunk provides an entry route for the pathogen.

Identification

To identify cotton root rot, look for wilting or yellowing leaves, which may initially occur on one branch or section of the tree. As the disease progresses, the leaves on the entire tree may wilt and turn brown, and the branches may die back.

If you dig up the roots of an affected willow, you may see black, sunken areas on the roots and/or wooly strands of the fungus on the root surface.

Management

Cotton root rot is a deadly disease and very difficult to control. The good news is that the disease is restricted to the Southwestern U.S.

Prevention:

  • Keeping the soil acidic by applying sulfur may help prevent the disease.
  • Plant resistant species in a border around the tree.
  • Till in organic soil amendments, such as chopped wheat or oat plants, in the spring.

Treatment:

  • By the time symptoms are noticed, the roots are usually too far gone to try to save.
  • Ammonium sulfate and high-nitrogen fertilizers may help in some cases, but tree death is likely.

Long-Term Prognosis:

The outlook is not good. Trees typically die rapidly.

9. Leaf Spot

In willow trees, leaf spot is commonly caused by the fungi Cercospora sp. and Gloeosporium sp. although various other fungi, such as Phyllosticta spp. and Venturia saliciperda, may be to blame as well.

However, leaf spot is not limited to solely fungal causes as bacteria such as Xanthomonas campestris can lead to the disease as well.

Identification

To identify leaf spot on willow trees, look for small, round, or irregular spots on the leaves. They may be brown, yellow, or black in color and can range in size from a few millimeters to several centimeters across.

The severity of the leaf spot can vary depending on the species of fungus, but in many cases, the leaves will wither and be shed from the tree.

Management

Although leaf spot is not typically fatal, proper prevention practices and managing symptoms can help mitigate the effects.

Prevention:

  • Prune the tree to increase light penetration and airflow.
  • Have your soil tested regularly to pinpoint deficiencies.
  • Fertilize routinely based on soil analysis.
  • Water deeply and consistently to minimize tree stress.
  • Avoid overhead watering.
  • Check tree frequently for overall health, and remove any injured or diseased branches.
  • Beginning in the spring, spray fungicide at the manufacturer’s recommended intervals to prevent the disease from taking hold.

Treatment:

  • Remove affected areas using a clean pruning tool, sterilizing the tool after each cut is made.
  • Spray with a fungicide if you can identify the disease as being fungal in origin.
  • Rake up and dispose of all dropped leaves. Do not add them to your compost.

Long-Term Prognosis:

In most cases, leaf spot is more of a cosmetic issue than a life-or-death situation. Provide optimal conditions for tree growth, and the tree should recover nicely.

The main issue is reinfection year after year. Because of the consequent early defoliation, the tree is not able to properly photosynthesize, leading to a lack of stored nutrients and a weakened tree.

10. Tar Spot

Tar spot is caused by Rhytisma fungi that overwinter in leaf litter to infect new leaves in the spring. Spots are quite unsightly, but this is not considered a fatal disease.

Identification

Tar spots look exactly as you would imagine — clusters of raised black spots on the surface of leaves.

Initially, the spots are often small and yellowish, but over time, they may grow to 3/4 of an inch.

Management

Tar spot is more prevalent in rural areas and commonly seen when temperatures are cooler and the weather has been damp for prolonged periods.

Thus, prevention can be difficult, and infection is sometimes unavoidable.

Prevention:

  • Be diligent in removing fallen leaves from beneath the tree.
  • Avoid overhead watering.
  • Prune to increase airflow within the canopy if necessary.

Treatment:

  • Remove and destroy infected leaves as they fall to break the cycle.
  • Spray with a general-purpose fungicide.

Long-Term Prognosis:

This disease may look awful, but the outlook for the tree is quite good.

Continue tending to the tree as needed, be diligent in hygiene practices (i.e., clearing away fallen leaves promptly and sterilizing tools between cuts), and your willow should be just fine.

Final Thoughts

The list of possible attackers to your willow trees may seem quite imposing, but remember that several of them are merely cosmetic issues, and the likelihood that your tree will develop multiple infections at once is not that high as long as you’re providing good overall care.