Forest Insect and Disease Newsletter: 6/99

Cool + Wet + Fungi = ?

As we experience cool and wet weather for over two weeks this spring during leaf expansion, we satisfied the equation for many leaf and needle diseases.

Cool temperatures + wet weather + leaf-infecting fungi = Leaf and needle diseases.

Cool, wet weather early in the growing season encourages the development and spread of several foliage diseases on deciduous and evergreen trees.  Though appearing to be serious, these diseases are usually of minor consequence.  Dead spots or blotches and leaf or needle fall are typical symptoms, depending on the host tree and the fungus species involved. Young, newly emerging leaves and needles are more susceptible to infection than the older ones.  This first reports of the year follow.

Oak anthracnose in 13 southeast counties

The cool, wet weather is blamed for an outbreak of oak anthracnose, a non-lethal leaf disease that causes leaf spots and lesions on bur and  white oaks, across portions of southern Minnesota.  About 150,000 acres of bur oak are affected across 13 southeast Minnesota counties from the Wisconsin border to Mankato. Severely affected trees are light brown instead of green and may appear to have frost injury. Close inspection reveals injury to the leaves. The symptoms on the bur oaks are rapidly developing leaf spots, lesions on leaves, and browning and shriveling of smaller leaves on trees that are just beginning to grow leaves.

All of the affected trees will recover. People need only be patient and wait for the outbreak to subside and then for new foliage to develop. It will be mid-June now before much of the
affected bur oak in this Region will reach full leaf development.

Anthracnose leaf diseases occur in most years from the eastern provinces of Canada to the Gulf of Mexico, occurring to some extent in most years in Minnesota. Outbreaks can occur whenever weather conditions are good for disease development. Periods of continuous rains with moderate temperatures are ideal for leaf diseases to occur. Exactly like the weather patterns we have been experiencing in this Region for weeks.

The current outbreak may end or continue over the next week or two depending on the weather. 
As soon as the rains stop and the average daily temperatures continue to rise, the outbreak will subside.

This early season defoliation does not hurt the trees. They will be fine and soon develop new foliage free of injury. As individual tree defoliation reaches 50%, it will trigger a  refoliation response and the tree will begin to produce new foliage. Trees, and bur oak in particular, store a lot of energy reserves in the form of starch for situations just like this. In fact, a healthy bur oak tree can store five years? worth of energy reserves. Energy reserves are produced during the growing season from June to August. These trees would not experience any negative, long term effects unless we entered a period of extended stress like a prolonged and serious drought that rapidly depleted energy reserves. 

Ash anthracnose

During the week of May 17th , many ash trees in central Minnesota dropped leaflets in great numbers after more than two weeks of cool, rainy weather.  This disease is called ash anthracnose and it is caused by a fungus, Gnomoniella fraxini, one of the most common foliar disease of ashes in the United States.  

Round to irregular blotches appear along margins and midribs of leaflets. They are greenish-brown at first but turn brown with age.  Sometimes, infected leaflets curl and drop before the blotch symptoms are well developed.  Later during the season, numerous small, round lesions with gray centers and purple-brown margins may develop on leaves remaining on the tree.  Small anthracnose cankers may occur on twigs of trees severely defoliated for several years.  They may cause dieback.

Rake and burn or destroy fallen leaves and twigs. If chemical control seems necessary, apply benlate, thiophanate, or mancozeb.