When crepe myrtle leaves turn yellow, orange, and red, it means fall must be on the way. It also means it's time to watch for Cercospora (sir-CAH-spora) leaf spot, a fungal disease that appears on a variety of ornamentals and vegetables in the late summer and fall.
Why fall? There are several reasons that all relate to temperature.
Thomas Keever, a meteorologist at North Carolina State University, says that fall tends to bring more days when skies are fair.
"Fair skies promote cool nights, so temperatures get close to the dew point so dew can form," he says.
Because the sun rises later, morning temperatures remain cool and dew stays on plants longer. As little as five hours of dew promotes development of Cercospora leaf spot.
September weather in the Lowcountry is ideal for Cercospora. The fungus prefers temperatures below 86 degrees during the day and above 55 degrees at night. It also likes the alternating pattern of wet leaves at night and dry leaves during the day.
During the past few years, Cercospora leaf spot on crepe myrtle has become widespread in the southeastern United States. It appears as early as the beginning of August, even in years with normal rainfall.
You may not recognize Cercospora leaf spot as a disease on crepe myrtle because the disease causes leaves to turn color prematurely and drop, as they normally do later in the fall. If you examine diseased leaves closely, you will see brown spots caused by Cercospora among the yellows, oranges and reds.
Does Cercospora leaf spot harm crepe myrtles? The short answer is probably not.
Crepe myrtles are tough plants. (Some survive "crepe murder" each spring.) All three of my crepe myrtles get Cercospora leaf spot. Losing leaves a month early definitely has not impacted growth of the 'Natchez' crepe myrtle in my yard.
Is there anything to do about Cercospora leaf spot on crepe myrtle? Again, the short answer is probably not. Most crepe myrtles are too large to spray easily with a hose-end sprayer.
To prevent Cercospora leaf spot on a small crepe myrtle, a fungicide, such as Daconil, should be sprayed by Aug. 1.
Last year's fallen leaves are the main source of the problem. The Cercospora fungus survives in these old, diseased leaves. Fungus spores are blown by wind and splashed by rain from fallen leaves up into trees and from leaf to leaf.
Because crepe myrtles produce so many small leaves, it is impossible to clean up every one. Removing and disposing most of the diseased leaves from under trees, however, is a good sanitation practice.
Diseased leaves should not be put in home compost piles. Temperatures of 140 degrees are needed to kill fungi. Most home compost piles do not reach this temperature, so killing is not assured.
Other woody plants such as fig and hydrangea also get Cercospora leaf spot. The leaf spots are generally round, brown and may have target-like rings inside them. On camellia, spots are commonly seen as gray patches along the edges of leaves. The gray color makes it easy to find the characteristic black dots where spores form.
The fungus also affects a number of vegetable crops, including beet, carrot, watermelon, mustard greens and Chinese cabbage.
Beet is the vegetable crop most likely to be damaged. Organic beet seedlings with five leaves can have Cercospora leaf spot on all five leaves. The leaf spots on beet are perfectly round and pale yellow with a reddish ring.
On mustard and Chinese cabbage, the circular, light brown spots start on the older leaves. Because these two crops are closely related, Cercospora leaf spot can spread from one to another and to other related crops, like broccoli. Other vegetables and ornamentals have their own specific types of leaf spot that affect only one crop (or several closely related crops).
Conventional fungicides are relatively effective at controlling Cercospora leaf spot. Organic fungicides, however, do not have much effect on this disease. Copper or Bacillus products likely are the best bets among organic-approved fungicides. To be effective, they must be applied now or before Cercospora leaf spot starts.
Anthony Keinath is professor of plant pathology at the Clemson Coastal Research & Education Center in Charleston. His expertise is in diseases of vegetables. He is also an avid gardener. Contact him at tknth@ clemson.edu