Q. Our holly is getting black spots on the leaves. Then the leaves turn yellow and drop off. Can you tell us what causes this and what we can do to stop it?
A. The writer enclosed samples of the spotted and yellow leaves, as well as samples of the newer growth that showed no evidence of black spots or yellowing. The plant is an American holly (Ilex opaca). Although these plants are considered evergreen, they shed their older interior leaves annually in spring, just as needled evergreens do in the fall. This usually happens in conjunction with new growth at the tips of the branches that remains green and healthy. While the sight of a beloved plant suddenly shedding leaves is alarming to the owner, as long as they are the older interior leaves, there is no need for concern.
There are a number of leaf spot diseases that can affect American holly, but they usually affect the older leaves that will be shed anyway. Tar spots on older leaves are considered by many plant pathologists to be secondary invaders that take advantage of senescent foliage. They are not thought to damage overall plant health.There are steps you can take to maintain the health and vigor of American holly.
Rake up and destroy or dispose of infected leaves. This removes the causal organism from the site and can reduce the severity of spotting in the future. Also, test the soil around your holly and fertilize and adjust soil pH according to the results. American holly grows best in well-drained, slightly acid soils with a preferred pH of 5.0-6.0. Be sure to provide supplemental irrigation when we get into hot, dry summer weather. Irrigation is best applied at the base of plants rather than overhead irrigation that wets the foliage. Prune plants to allow for good air circulation and sun penetration into the interior. This allows the foliage to dry quickly after rain or heavy dews. Holly leaf spot diseases are more common during wet spring weather because tender new growth is more susceptible to infection than older foliage. Fungicide applications are rarely warranted in the landscape.
Soil test kits are available from your local Penn State Extension office. In Allegheny County, soil test kits are $12 for the first kit and $9 for additional kits ordered at the same time. They come with instructions for taking a good soil sample and understanding your results. Make checks payable to Penn State Extension and send to Soil Test Kit, Penn State Extension, 400 N. Lexington St., Third Floor, Pittsburgh, PA 15208.
Here are a couple of reminders for home gardeners to help keep your landscapes healthy:
• Bagworms have hatched and are actively feeding. This is the best time to control them while they are still small and susceptible to control with least toxic materials such as Bacillus thuringiensis or Bt (Dipel, Thuricide and others) or spinosad (Captain Jack’s Deadbug Brew). They are still quite small and unnoticeable unless you examine plants closely. If you had trouble with them last year, take a close look at affected plants. If you wait until you see large “pine cones” on plants that should not have them, it is generally too late to spray.
• Avoid pruning oaks while they are actively growing. Oaks should only be pruned during the dormant season (November-late March/early April). This is one of the most important ways to protect valuable oaks from oak wilt. This fungal disease is transmitted by bark and sap beetles that are attracted to fresh pruning wounds. It is almost always fatal, especially to species in the red oak group. Black oak, red oak, pin oak, scarlet oak, and shingle oak are some of the most common landscape oaks in our area. Prevention is the most important control for oak wilt; there is no chemical control once trees are infected.
Send questions to Sandy Feather by e-mail at email@example.com or by regular mail c/o Penn State Extension, 400 N. Lexington Ave., Pittsburgh 15208.