In ornamental gardening, trees and shrubs often develop brown patches on their leaves. These blemishes weaken plants by interfering with the process of photosynthesis.
The positive side is that leaf spot problems rarely cause direct, lasting damage to plants. However, they can open the door for other conditions that may compromise overall plant health.
Leaf spots can stem from several sources, but fungi are the most frequent culprits. Since many fungal infections behave in similar ways, they are usually controlled with the same methods.
In most cases, preventing leaf spot diseases comes down to thoughtful planning before planting and following consistent yearly care practices that help keep fungal diseases at bay.
Identifying Leaf Spot Diseases
- Leaf spots appear in many different shapes, sizes, and colors.
- They usually show up first on inner and lower branches where shade and humidity are greater.
- Because wind, rainfall, or irrigation spreads the pathogens, spots often form randomly across the leaf surface.
- Depending on the specific disease, spots may appear on the upper side, underside, or both sides of the leaves.
- Spots can be angular or round, raised or depressed, with either smooth or jagged edges.
- Their color may vary from yellow or yellow-green to orange-red, light tan, brown, or even black.
- A single plant may display leaf spots of several different sizes at the same time.
- Smaller spots indicate newer infections, while larger ones are older.
- The center of mature spots may reveal fungal spores or tiny spore-producing bodies.
- In severe cases, some pathogens trigger premature leaf drop, leaving the tree or shrub nearly bare.
Managing Leaf Spot Diseases
Leaf spot problems usually don’t cause permanent harm, but taking the proper steps can significantly cut down the disease from season to season. One of the most important things to manage leaf spot diseases is to rake up and destroy fallen leaves before winter sets in. Left on the ground, they become a breeding ground for pathogens that return in spring.
Proper spacing also matters. Avoid crowding your plants and allow them enough room to reach full size without competing for light and airflow. Pruning trees and shrubs further helps by opening up the canopy, allowing more sunlight to enter, and enhancing air circulation. Since wet leaves encourage disease, always water at the base of your plants. A drip or soaker hose works far better than sprinklers, which tend to splash water onto foliage.
Keeping trees healthy makes them less vulnerable to infections. Deep watering during the growing season ensures that the top six to eight inches of soil stay moist, especially in dry periods. Let the soil dry out a bit between watering sessions to prevent stress. Mulching also plays an important role. Apply a three- to four-inch layer around the base of the tree, keeping it a couple of inches away from the trunk to allow for air movement. Refresh this mulch annually to maintain effectiveness.
Fertilizers should only be used if a soil test shows a nutrient deficiency. Adding unnecessary fertilizer won’t improve a tree’s resistance to leaf spot diseases. Fungicides are rarely required, except when a tree has lost all its leaves for several consecutive years. These treatments are preventive, not curative, and must be applied before symptoms show. Since timing varies depending on the type of disease, professional guidance is often needed.
For large trees, fungicide treatments require high-pressure equipment to effectively cover the entire canopy. In such cases, hiring a certified arborist is the safest and most effective way to manage leaf spot diseases.
Causes of Leaf Spots
- Most leaf spot diseases develop from fungal infections, though bacteria or other pathogens trigger some.
- Many of these disease-causing organisms are host-specific, meaning they only infect trees that belong to the same plant family.
- Nearly all types of trees and shrubs are vulnerable to at least one form of leaf spot disease.
- On maples, tar spot is a common example, appearing as dark, shiny patches often clustered together on leaves.
FAQs
Should you remove leaves with brown spots?
Yes—if the infection is mild, it’s best to take off any leaves showing brown spots. Be sure to separate the plant immediately to stop the fungus from spreading to nearby plants.
How can you determine if leaf spots are caused by overwatering?
Overwatering often leaves clear signs such as limp foliage, brown patches with yellow edges, persistently wet soil, wilting leaves that turn brown, and sometimes edema (swollen, blister-like spots).
Can leaf spots go away on their own?
No, once a leaf develops brown spots, it won’t heal. But if you address the underlying problem, the plant can recover and grow fresh, healthy leaves.
