This question was submitted by Brian. Researched and answered by Advanced Master Gardener Darlene.
I realize that my hydrangea needs fertilizer, but what are the brown spots? They are more prominent on the top side of the leaf and less so on the underside. Is this insect damage? I’m in Cumberland on the east side of Indianapolis.
The spots on your hydrangea leaves are called Cercospora leaf spot. This disease affects most hydrangeas. If leaves become severely spotted they can become yellow-green in color and fall to the ground. From the photo, it looks like you have just a mild case. Since it is late in the season, I would not treat the shrub for this. The disease is spread by the spores that are blown by the wind, overhead irrigation and fallen infected leaves. Be sure to clear up all fallen infected leaves this fall. Then next spring you can apply nitrogen containing fertilizer and spray with a fungicide when the first signs of leaf spots appear. Tennessee State University has an excellent bulletin about this.
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