Michelle Grabowski, University of Minnesota Extension Educator
For most areas of Minnesota, the spring of 2009 has been an unusually dry one. This cool dry weather has kept many of the spring leaf spot diseases of trees at bay. Diseases like anthracnose on oak, ash and maple have been absent up until the most recent wet weather. Anthracnose is now being reported, especially in areas that received significant recent rain like southern Minnesota. The fungi that cause anthracnose, however, may not be causing problems for long. Anthracnose fungi thrive in cool wet weather and with the recent onset of hot temperatures, the growth and spread of this disease is likely to slow down.
Photo 1: Oak anthracnose. Michelle Grabowski
The biology of these fungal leaf diseases explains why in some years anthracnose is a big problem and in other years only minor damage occurs. Anthracnose fungi survive Minnesota’s winter in infections on twigs, buds, and last year’s infected leaves. Spring rains splash spores from these infections onto newly developing leaves. These spores cause irregular brown leaf spots often centered on leaf veins. Since the infections occur on young developing leaves, it is common to see curling or distortion of the leaf tissue as parts of the leaf continue to grow, while others are halted by the anthracnose infection. If severe infection occurs, leaves may drop off the tree prematurely.
Photo 2: Ash Anthracnose. Michelle Grabowski
If cool wet weather occurs throughout spring months, the anthracnose fungi thrive and disease can be quite severe. This year, wet weather did not arrive until late spring. Some anthracnose is occurring on oak and ash trees, but the leaf spots are fairly minor and mostly restricted to a few lower shaded leaves. In contrast, more significant leaf browning has been noticed in maples.
Oak anthracnose is caused by the fungi, Discula quercina. Ash anthracnose is caused by the fungi Discula fraxinea. Both of these fungi primarily infect young developing leaves or wounded leaves. Once oak and ash leaves reach maturity, they are relatively resistant to the fungi. It is likely that less disease is being reported on these two trees because the wet weather that favors these fungi did not occur until the majority of the leaves were fully expanded and therefore somewhat resistant. For plant disease to occur, the pathogen, the susceptible host plant and the right weather conditions must all occur at the same time or disease will not develop.
Photo 3: Maple Anthracnose. Michelle Grabowski
There are several different fungi that cause anthracnose in maple including Discula umbrinella, Discula campestris and Aureobasidium apocryptum. Less is known about the biology of these fungi. They result in similar browning and curling of leaves and can even blight young shoots. Like oak anthracnose and ash anthracnose new infections often form in response to wet weather. Unlike anthracnose on oak anthracnose and ash anthracnose, the fungi that cause maple anthracnose have been reported starting new disease from late spring through late summer. Anthracnose on maple seems to be the most common of the three diseases this spring.
Although a tree heavily infected with anthracnose can appear ratty, this disease is not a serious threat to the health of the tree. Often leaf spots are limited to the lower shaded leaves. The majority of the tree’s canopy remains green and can provide energy to the tree through photosynthesis all summer long. Even in years when anthracnose is severe and leaf loss occurs, trees create a new flush of leaves early enough in the year to avoid serious harm.