Pest Alert: Bacterial Leaf Spot
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Bacterial leaf spot and fungal leaf spot diagnosis can sometimes be difficult. Bacterial leaf spot symptoms typically have a more angular appearance than fungal leaf spot, with the spots bounded between the veins. Fungal leaf spots will cross veins. This is because the bacteria is inside the plant and cannot cross vein easily. Fungal leaf spots grow on the surface and cross veins easily.
The hydrangea pictured below has bacterial leaf spot. The disease is common during wet years. This infection likely began when the weather was cooler and wetter.
Notice the older leaves are affected but the newer leaves are clean. This disease needs lots of wetness to live. Once the weather gets drier, the bacteria stops spreading.
Even if you wanted to treat the plants there is little you can do. Notice that the older leaves on both the hydrangeas pictured above and the black eyed Susans below are affected but the newer leaves are much less affected. There really is no need to take any action during the growing season.