Phyllosticta Leaf Spot
Phyllosticta spp.
45 host plants · Fungal
Last updated
On maple leaves you'll see discrete, angular brown or black spots appearing by late spring or early summer, concentrated on older foliage first. These spots have well-defined margins and don't typically form the explosive merging pattern seen in some other leaf spots. Wet weather in spring and early summer accelerates symptom development, and overhead watering creates ideal conditions for the fungus to spread through splash and spore dispersal.
Phyllosticta leaf spot on maples is rarely life-threatening but causes premature defoliation that weakens the tree and reduces its vigor. The fungus overwinters on fallen leaves, so your most effective strategy is sanitation: rake and destroy all fallen leaves in autumn before spring spore release. Avoid overhead irrigation whenever possible, and if you must irrigate, water early in the morning so foliage dries quickly. Good air circulation through pruning helps tremendously in humid climates.
Quick Reference
Management
Early spring (BBCH 12-16: leaf expansion). Young, tender foliage most susceptible. Peak infection window: late March through May.
Spring; cool wet conditions (50-65°F) with 8+ hours continuous leaf wetness. Fungus (Phyllosticta spp.) requires moisture for spore germination and infection.
Cultural Controls
- Rake and destroy fallen leaves.
- Avoid overhead irrigation.
- If overhead irrigation is necessary, water when foliage can dry quickly.
Phyllosticta leaf spot on Japanese maple is nearly universal in the Puget Sound lowlands but cosmetic. Cool wet springs favor infection; summer drying naturally suppresses secondary spread. Treatment is rarely warranted on established trees.