It has been a great spring for plant growth, and also, if you were a fungal organism, for infecting plant leaves.
Leaf spot diseases can vary in severity from year to year, from non-existent to severe – it all depends on the environment. Fireblight on ornamental pear (which has good resistance to this bacterial disease) is a good example – most years it is non-existent, but this year environmental conditions combined to cause an outbreak.
Many plant leaf diseases need mild weather and prolonged rainy spells or heavy dew, resulting in long periods of leaf wetness, for fungal spores to germinate. We had these conditions earlier this year. Usually the disease symptoms don’t appear for days or even 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 is oak leaf blister. All varieties of oaks can get this disease, although water oak (called pin oak by some folks), live oak, and southern red oak seem to be more susceptible. As the name implies, green, blister-like spots form on the leaves, often causing the leaf to distort. After a period of time, the spots turn brown. 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. The good news is that 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.
The infection process for Oak Leaf Blister took place back in March and early April. Infection is most severe during wet, cool, spring weather. Because of the narrow window of time for leaf infection, the fungus rarely becomes severe enough to require treatment. Even in years when early infection is severe, it is normally limited to only the first set of leaves that emerge. Due to the limited injury to the tree by this fungus, fungicide treatments are normally not needed, and restricted to only those trees that are highly valuable and visible in the landscape and which are affected annually. Fungicide treatments at this time will not help.
Black Spot on Roses. This leaf spot disease, when severe, causes rose leaves to fall off, resulting in a stressed, and ugly rose bush. Leaves start off with black spots, which later turn the leaves yellow, which then fall off. This disease can be problematic all growing season, and susceptible rose varieties should be protected with fungicides labeled for rose disease control starting early before the disease gets started. The Earth-Kind Rose program was developed to identify existing rose varieties with very good black spot resistance, thus need little or no fungicide sprays to keep the bushes alive and looking good. For more information on this program, visit the following web site: http://earthkindroses.tamu.edu/
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-tip plants. Indian hawthorns can also get this fungus. Mild temperature and prolonged leaf wetness are the requirements for infection. Dark spots with purple margins are the symptoms for this fungus.
The disease will be worse where sprinklers are hitting the leaves, on crowded or dense groups of plants, shaded plants (especially morning shade), and on plants that have had the disease in the past. So avoid those conditions when considering using these plants in the landscape.
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 these plants. 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. 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. Control can be difficult to achieve on severely affected plants.
Powdery Mildew. This fungal disease is a bit different than the preceding three in that it doesn’t produce dark spots on the leaves. Instead, leaves are covered with a white or grey, powdery film that can cause leaves to cup or distort. Powdery mildew can affect many types of plants, such as squash and other curcurbits, phlox, roses, dogwoods, deciduous magnolias and crepe myrtles. Powdery mildew is also different from many fungal diseases because it does not require leaf wetness for infection to take place, but rather high humidity, along with warm days and cool nights.
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 on its own.
Many of the old fashioned crepe myrtle varieties are very susceptible. Planted 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 crape myrtle varieties with excellent powdery mildew resistance. Most of them are named after Indian tribes, such as ‘Natchez’, ‘Yuma’, ‘Wichita’, ‘Zuni’, ‘Tonto’, etc. These hybrid crepe myrtles are available in mature heights ranging from 8 feet to 30 feet tall, and a wide range of colors.
Control of powdery mildew can be difficult, but there are fungicides that can be used. Always read and follow all label directions on pesticides, including making sure the plant(s) you need to treat are included on the label.