How to Identify & Treat Common Tree Diseases

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A Guide for Central Virginia Tree Care


As living, organic things, trees grow and thrive. But, like any other living thing, they can acquire diseases, grow weak, and die. They must be taken care of to ensure that they’ll live healthy and long. Tree diseases do not only affect their aesthetic beauty and their value to a home or commercial space; they also negate the many benefits they provide. (See our last blog, 4 Reasons to Plant a Tree for Earth Day, to learn about these benefits). 4 tree diseases to look out for in Central Virginia include Black Knot Fungus, Seiridium Canker, Hypoxylon Canker, and Cedar Apple Rust, all of which can be exacerbated by tree stress, undermining tree vigor.


1. Black Knot Fungus

As its name suggests, black knots (also known as galls) are small knotty growths on cherry and purple leaf plum trees’ branches, according to Virginia Tech’s Disease Advisories. When trees are first infected, galls appear small and green and blend in, hardly noticeable. They infect and spread spores during the spring, the growing season.


Rain and wind are the main methods of spreading, especially during humid, wet conditions. Spores spread to other branches throughout the growing season, even from pruned branches. If not treated, black knot causes branches to die off. There are varieties of plum trees that are resistant to Black Knot, but cherry trees are not.

Treatments include, pruning, excising knots, and using fungicides. Remove all galled branches from infected trees. The disease can continue to spread from these branches, so make sure to dispose of them by burning or burying them. If the branches are large, simply cutting off the knots, and cutting a few inches into the wood, should suffice. You may also use preventative fungicides. For more information on using fungicides, check out the University of Maine’s fungicides chart for Black Knot Fungus.

2. Seiridium Canker

Sometimes spelled Seiridium Cancer (but pronounced the same way), this disease affects a wide variety of trees, but particularly the Leyland Cypress tree (see video). When infected, reddish leaves and branches develop amidst healthy green ones. Resinous or seeping pustule-like ulcers form at the base of the affected branches, close to the trunk, as well as at the base of branch offshoots, and the spores mostly spread by splashing rain. Seiridium Canker often occurs on trees that have undergone stressful conditions, such as drought.

Fungicides are not effective for this kind of canker. Instead you should focus on minimizing stress to the tree.

Long periods of sub-freezing temperatures cause desiccation, or the loss of moisture. To counter this, water the tree well during dry periods, and provide barriers such as mulch and compost around the base of the tree. If the ground cracks during the winter, add more soil to the cracked areas. Another cultural practice for minimizing stress to smaller trees involves using burlap to provide windbreak. Furthermore, much like with Black Knot, infected branches should be removed and burned.

3. Hypoxylon Canker

Hypoxylon Canker (or Hypoxylon Cancer) most commonly affects white and red oaks, though it also affects other shade trees. It is prevalent in Virginia due to its forests’ high volume of oak trees as well as “cumulative environmental stress,” according to the Virginia Department of Forestry.

The disease usually remains dormant in a tree until the tree undergoes environmental stressors and becomes damaged. Stress takes the form of nutrient and moisture deficiency, overcrowding, damage to the roots, and defoliation, to name a few. Once damaged, the canker infects sapwood, the soft outer layers of new wood growth, particularly in periods of drought.

In order to identify this disease, look for:

  1. Thinning of the tree’s crown
  2. Dieback, or death of branches beginning at the tips
  3. Bark beginning to fall off

If cankers are visible, the tree is already dead, and they release spores into the air to infect other trees.

The maintenance of tree vigor remains the only known effective treatment.

4. Cedar Apple Rust

Cedar Apple Rust affects both cedar trees and apple trees (as the name implies), but it also affects other varieties of fruit trees. If not caught early, this disease is fatal.

According to Cornell’s Department of Agriculture and Life Sciences, signs of Cedar Apple Rust include gelatinous growths, yellowish-orange in color, which are more visible after moist conditions due to absorption and swelling. Moreover, the undersides of leaves begin to develop fruiting, spore-producing bodies. Both of these growths form from galls, the gelatinous ones growing in spring, and the spore-producing ones growing in mid-to-late summer. The galls themselves are tough enough to last through winter to begin the growth process anew come spring.

This tree disease is very difficult to manage once it first develops, since spores travel far and wide via wind. Thus, you must focus on prevention rather than treatment. Make sure trees are in good condition and minimize stress in similar ways to those listed above. If planting new trees, consider varieties that are resistant to these cankers. Additionally, fungicides might work, but read the instructions on the packaging carefully. Lastly, you may remove galls before they can spread; however, this method is less reliable, due to the spores’ far-reaching ability.

The quick spread of each of these diseases can get out of hand. Know what to look for and catch them early, or they may end up killing your trees. If you need help identifying or treating a tree disease, contact Woodie’s Tree Service today!