Trees and shrubs, especially in ornamental horticulture, frequently fall victim to brown leaf spots. These spots weaken plants by interrupting photosynthesis. The good news is that leaf spot diseases usually do not seriously harm your plants directly. They can, however, allow other issues to affect its health.
Leaf spot diseases should be taken seriously, especially if they result in leaf loss for more than two consecutive years.
There are many causes for leaf spot diseases, and they are often fungal in nature. And, because many common fungal diseases are similar, they can often be managed the same way. Often it just takes some planning before planting and routine yearly preventative maintenance to avoid fungal diseases.
What Are The Brown Spots on the Leaves?
Brown spots on leaves are caused by many diseases. They are usually termed as leaf spot diseases to loosely classify and manage them, but there are many and they are diverse.
When looking at a tree or shrub and seeing brown spots, the usual culprit is fungal. The identifying spots can range in color from black, brown, red, orange, yellow to white, and come in a variety of shapes and sizes.
Most often the infection will be noticed on lower or inner branches, and the shape of the spots can be varied from circular to angular. Occasionally, in the center of the spot, you might see signs of the pathogen that produces the spores.
Sometimes your plant can catch something that isn't fungal, but bacterial, possibly bacterial leaf spot. This is common on lilacs. The symptoms are small dark brown spots with a yellow circle surrounding it, which eventually will dry up to leave a hole in the leaf. These spots will eventually merge causing the entire leaf to turn dark black-brown.
Common Leaf Spot Diseases
Here are some of the most common leaf spot diseases.
Septoria leaf spots are round, with black margins. It will look like there is mold growing on the leaf. The leaves will eventually shrivel and blacken, then fall to the ground where the spores will overwinter.
With Venturia, brown and black spots form in early spring causing irregular leaves. Infected shoots turn black and weaken. The stems twist and turn and become contorted. Only young shoots and leaves are susceptible. As the spring and summer continue the tissue matures and becomes resistant.
Death of new growth reduces height and can deform small trees by causing bends in the stems. Untreated infections of Venturia can weaken trees and make them susceptible to attacks by other organisms.
This nasty fungus can make beautiful flowering ornamental trees, shrubs, and delicious apple and berry trees look like mutant trees from your nightmares.
Rust fungi spots appear on the leaves shortly after leaves bloom. Late in the summer, brownish clusters of threads appear beneath the spots or on fruits and twigs. The spores associated with the threads infect the leaves of deciduous trees and shrubs. You will also find it in twigs and needles of junipers during wet, warm weather.
Anthracnose is used to describe a group of related fungal diseases that usually cause dark spots on the leaves of various trees. The number of trees it infects is extensive, but there are more and more resistant varieties being offered in the nursery trade.
Symptoms of anthracnose include small beige, brown, or black spots. Dead areas on leaves can be more irregular on trees such as ash and maple while sycamore and oak anthracnose spots will show along the leaf veins.
What Causes Leaf Spot Diseases?
Knowing the causes makes it much easier to develop a treatment plan. The most common causes of brown spots on leaves are:
1. Garden waste piles
2. Overcrowded plants
3. Lack of sunlight and airflow
4. Wet conditions
Tips for Treating and Preventing Leaf Spot Diseases
If you have significant leaf spot problems on large trees, hiring an arborist is recommended. There are, however, many preventative steps and treatments that you can use yourself.
- Fungicides and fertilizer. These are protective measures and need to be applied before symptoms appear on the leaves. They are usually not needed unless a tree has lost leaves for several years. Be sure to also test the soil of a tree with leaf spot before fertilizing.
- Moisture. Apply water to the soil and root zone, don't wet the leaves, and allow the soil to dry before watering again. Using a soaker hose is ideal for watering trees and shrubs. An annual application of mulch is beneficial, but don't create a mulch volcano and pile it thick and high up against the tree trunk. Mulch volcanos can result in root rot, disease and decay.
- Spacing and pruning. Consider the size of your tree or shrub at maturity when planting to avoid issues with overcrowding. Pruning can also increase the amount of sunlight and airflow to inner branches.
- Removing Waste. Promptly rake up and destroy infected fallen leaves and broken and pruned branches. Dispose of infected leaf litter in accordance with local ordinances; don't add infected leaf litter to your compost pile.
“Leaf Spot Diseases of Trees and Shrubs.” University of Minnesota Extension Office. Umn.edu. N.p., n.d. Web.
"Septoria Leaf Spot.” Iastate.edu. N.p., n.d. Web
Guyon, John. “Venturia Leaf and Shoot Blight.” US Forest Service. Usda.gov. N.p., n.d. July 2010. Web.
Barnes, Ervin H. “Cedar Apple Rust.” Atlas and Manual of Plant Pathology. Boston, MA: Springer US, 1979. 269–277. Print. via US Forest Service Website.
Marsden, Christy. “Anthracnose.” Wisc.edu. N.p., n.d. Web