What Causes Brown Spots on Leaves?

Knowing why, and what to do, if your tree or shrub has brown spots.

Closeup of a maple with spots due to Rhytisma acerinum

 Getty Images

Trees and shrubs, especially in ornamental horticulture, fall victim to "brown leaf spots" all the time. 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, but they are often fungal in nature. and, since they are similar, they can often be managed the same way. Often it just takes a little easy planning before planting and some routine yearly preventative maintenance. In the end, you will be left without needing to deal with any spots at all.

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 classify and manage them loosely, but there are many and they are diverse.

When looking at a tree or shrub and seeing any spots on the plant, 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 may see signs of the pathogen that produces the spores.

Sometimes your plant may 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

Some of the most common leaf spot diseases are outlined below.

Septoria:

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.

Venturia

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.

Cedar-Apple Rust

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

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 These Leaf Spot Diseases?

The most common causes of brown spots on leaves are outlined below. Knowing the causes makes it much easier to develop a treatment plan.

1.   Waste piles
2.   Overcrowding
3.   Lack of Light 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 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. Water the roots, not the leaves and allow the soil to dry before rewatering. Using a soaker hose is ideal for this. An annual application of mulch is beneficial, but make sure you don't volcano mulch. This is when the mulch is piled high in layers up against the tree trunk, and it 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 light and airflow to inner branches.
  • Removal of Waste. Rake up and destroy fallen leaves or pruned branches promptly. Don't leave leaf litter over the winter. The waste should be burned or disposed of in accordance with local ordinances