"April showers bring May flowers" and all kinds of leaf spot diseases. It has been a great spring for plant growth, and also, if you were a fungal organism, for infecting plant leaves. Many plant leaf diseases need mild weather and prolonged rainy spells or heavy dew for fungal spores to germinate. Often the symptoms don't appear until several weeks after the initial infection. Here are a few of the more common diseases we are seeing right now.
Oak Leaf Blister. A very common leaf spot disease this year is oak leaf blister. All varieties of oaks can get this disease, although water oak (called pin oak by some folks) seems to be the most susceptible. As the name implies, a blister-like spot is created on the leaves, often causing the leaf to distort. If the infection is severe, some of the leaves may fall off of the tree.
The main thing concerned folks want to know is whether leaf blister will hurt or kill the tree. Leaf spots alone will not kill oaks. Premature defoliation is stressful to trees, but if a tree is otherwise healthy, it will quickly grow back another set of leaves.
Leaf blister infection took place in March and early April. When leaf buds open in the spring, fungal spores present on the bud scales infect the expanding leaves. Infection is most severe in wet, cool spring weather. Because of the narrow window for leaf infection, the fungus seldom becomes severe enough to require treatment. Even in years when early infection is severe, it is normally limited to only the first few leaves that emerge. In areas where spring weather conditions frequently favor disease development, fungicides are applied at early bud swell and repeated in 10 days. Due to the limited injury done to the tree by this fungus, fungicide treatments are normally restricted to only those trees that are highly valuable and visible in the landscape.
Entomosporium Leaf Spot on Photinia and Indian Hawthorn. Red-tip photinias are very susceptible to this fungal disease, and plants will shed leaves when infection is severe. Repeated infections can result in dead red-tips. Indian hawthorns can also get this fungus. Mild temperature and prolonged leaf wetness are the requirements for infection. Dark spots with purple margins characterize for this fungus.
The disease will be worse where sprinklers are hitting the leaves, on crowded or dense groups of plants, plants in the shade (especially morning shade), and on plants that have had the disease in the past. Control of this disease begins with prevention. Not much you can do about the rain, but you can adjust sprinklers so they do not hit the leaves of photinias or Indian hawthorns. Rake up and destroy all of the leaves which have fallen off since they are a source for additional infections. Don't fertilize more than necessary to avoid promoting excess new growth which is more susceptible to the disease.
During wet, mild periods, protective fungicides may be necessary. For best control, alternate a contact fungicide, such as chlorothalonil (eg. Daconil), with a systemic fungicide like Funginex, Bayleton or Banner. No fungicides are needed during hot, dry weather. For fungicides to be effective, thoroughly cover the leaves with the spray. And don't wait until the disease is ravaging your plants. Start treatments when the disease first appears.
A recent study in Alabama screened Indian hawthorn varieties for resistance to Entomosporium leaf spot. Those varieties showing the most resistance (least amount of leaf spot symptoms) over a two year study included: 'Dwarf Yedda', 'Indian Princess', 'Jack Evans', 'Janice', 'Majestic Beauty', 'Olivia', 'Snow White', 'Clara', and 'Eleanor Tabor'. Some of these varieties may be difficult to find.
Powdery Mildew. This fungal disease is a bit different than the preceding two in that it doesn't produce dark spots on the leaves. Instead, leaves are covered with a white, powdery film that can causes leaves to cup or distort. Powdery mildew can affect many types of plants, such as squash and all curcurbits, phlox, roses, dogwoods, and crepe myrtles. Powdery mildew is also different from many fungal diseases because it does not require leaf wetness for infection to take place.
At this time of year, powdery mildew on roses and crepe myrtle get the most attention. Some rose varieties are more susceptible to powdery mildew than others. Once it gets hot, powdery mildew on roses usually disappears.
Many of the old fashioned crepe myrtle varieties are very susceptible. Put one in partial shade where the air is still, and powdery leaves will be likely. However, even plants in full sun can get this disease. The good news is that there are several powdery mildew resistant crape myrtle varieties. Many of them are named after Indian tribes, such as Natchez, Yuma, Zuni, Tonto, etc. These hybrid crepe myrtles are available in mature heights ranging from 8 feet to 25 feet tall, and a wide range of colors.
Control of powdery mildew can be difficult, but there are fungicides that can be used. Neem oil, triadimefon (Bayleton, Fung-away), triforine (Funginex), and chlorothalonil (Daconil) are some of the fungicides labeled for powdery mildew control.
Information given above is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement by Texas Cooperative Extension is implied.
Keith Hansen is Smith County Horticulturist with Texas Cooperative Extension. His web page is http://easttexasgardening.tamu.edu Educational programs of Texas Cooperative Extension are open to all individuals without regard to race, color, sex, disability, religion, age or national origin.
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