A: This is a disease called cercospora leaf spot, and it’s an unsightly disease. Thankfully, it usually won’t kill a plant but it can weaken them by thinning out and damaging foliage. There’s no single window of opportunity for this disease; it can hit throughout the year. The best treatment methods follow:
• Rake away fallen leaves. Remove diseased, dropped leaves to make it harder for the plant to get infected again after treatment.
• Try to not wet the plant’s leaves when irrigating. If your sprinkler heads wet the tops of your plants, consider running a soaker hose or drip line to irrigate these plants instead. Wet leaves make it easier for this disease to hit your plants.
• Treat with the proper fungicide. On azaleas, this disease is best treated with Bonide Fung-Onil (daconil) or Bonide Copper fungicides. If this disease has been a problem in the past, it’s perfectly acceptable to treat as a preventive measure during rainy months, as well.
If possible, make sure any future azaleas you plant get the early morning sunshine. Afternoon sun is harsh in our area for them, but morning sun will keep the plants in enough sunshine to grow vigorously, and it will also dry off morning dew or overnight irrigation quicker to reduce the chance of this problem.
Burton specializes in diagnosing and solving plant problems. If you have a question for Burton, please email him at email@example.com and include photos showing the problem.