This summer has brought tremendous amounts of rainfall, and with it, perfect conditions for fungal leaf spot. Most leaf spots need high temperatures, high humidity and extended periods of leaf wetness to infect their host. Of course, this season the weather conditions have been optimal for fungal diseases. One tree species that is affected by this fungal disease is the maple tree.
Maple trees are popular in home landscapes all over Georgia. Some maple species are native to this part of the country; therefore, they naturally do well with limited maintenance. These include the red maple, sugar maple, silver maple and Japanese maple.
There are a few leaf spots known to affect maple trees. The first is called purple eye. The fungus Phyllosticta causes purple eye. Symptoms of purple eye are round or irregular spots on leaves. The spots are about ¼-inch in diameter and have purple margins with yellow or tan centers. A small, black fruiting body will appear in the middle of the diseased area. The black spot has the appearance of an eye in the purple border. The good news is that Phyllosticta leaf spots are mainly an aesthetic issue and rarely pose any long-term damage to maples. Phyllosticta leaf spot is a very common disease among Japanese maples, especially on the darker varieties, such as Bloodgood. On these varieties, the disease appears as round, bleached spots that are a stark contrast to the dark red of the leaves.
Another common leaf spot on maples is tar spot. The fungus Rhytisma acernium causes tar spot. We normally see this disease in spring and early summer. Tar spot first appears as light-green or yellow round areas. These spots then develop a black, shiny, tar-like dot on the upper side of the leaves. In many cases, leaves with several spots will prematurely drop to the ground. Fortunately, this is another example of a leaf spot that is more of an aesthetic problem.
The best defense against both of these fungal infections for maples and other trees is proper maintenance. Rake all of the leaves from around the maples in the fall and remove them from the property. The fungal spores survive the winter on the fallen leaves. Removing the leaf debris will decrease the possibility of future infection by the fungus. Pruning may also help control these diseases by allowing more air flow to dry out the leaves when rainfall is abundant. Chemical control is rarely needed and is most often not feasible because of the size of most maple trees. However, some homeowners choose to treat their Japanese maples because they are small enough to easily spray and are expensive enough to justify the cost. Propiconazole, a systemic fungicide, can be found in the brand names Bonide Infuse and Fertilome Liquid Systemic. Chlorothalonil, a contact fungicide, can also be used against Phyllosticta leaf spot. It can be found under the brand name Daconil. Always read and follow all label directions when applying pesticides.