- When did the symptoms first appear?
- Was it a wet spring/summer or a hot, dry growing season?
- How much of the tree is affected?
- How does the browning appear on the leaf?
The first possibility is anthracnose, which is a fungal infection that over-winters in plant tissues. Anthracnose is prevalent when the spring is cool and wet and will appear as browning of the tissue on the leaves along and around the veins. Symptoms will appear in the spring, starting in the lower crown, working its way up, but leaving the upper crown quite healthy by comparison.
Brown leaves can also be a symptom of drought stress, which occurs when the tree is not getting enough water to make up for what it is losing through evapotranspiration. When the spring/summer is very dry and trees are stressed, the leaves of the tree will start to turn brown and brittle on the edges quite soon after the drought begins. This occurs in the crown from the top down and the outside in. The browning of leaves caused by drought stress differs from that caused by anthracnose: it starts at the edges of the leaf and moves inwards and is often very uniform in appearance.
The final possibility is bacterial leaf scorch (BLS), which occurs when a tree has become infected with a bacterium (Xylella fastidiosa). The symptoms are very similar to those of drought stress, but a yellow band will appear between the healthy growth and brown tissue, tissue death will be more blotchy than uniform, and the brown tissue will have the tendency to curl inwards. This will occur in late summer to early fall and develops randomly across the crown.
Anthracnose and BLS are best dealt with by raking up leaves as soon as they fall and disposing of them away from your property; this prevents re-infection from occurring. If your tree has been infected for more than two consecutive years, it may be time to consult with a certified arborist. All of these conditions can be alleviated with proper tree care, including a good watering regime and mulch application. Just be careful to check the tree before you water – symptoms of over-watering resemble those of drought stress! To determine if your tree needs to be watered, feel the soil under the mulch. If it is warm and dry, your tree needs water, but if it is cool and moist, no watering is needed.