NDSU Extension - Ramsey County


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May 24, 2010 Horticulture Column


The weather outside is gloomy, the rain is starting to bug me, I am not sure when it will quit but it’s time, its time oh its time to give us a break.  I can’t imagine how the rain seems to ruin things like it did for the eastern portion of the county on Saturday evening.  Amounts ranging up to 1.5 inches was recorded in the Edmore, Lawton, Brocket, Doyon and Southam country.  Deb and I worked in our garden, and made great progress.  I incorporated a pickup load of cow compost and hopefully will get one more load plus we also removed a rock area on the north side of the house, to make room for more hostas.  I have found four hostas, at our local nurseries, this weekend that I did not have.  Guess what, I now own four new plants and do have them in the ground.  All I know was that upon completion of a hard weekend of field work and gardening I was really glad to see the bed last night.  We first did have to make a graduation run to Munich late Sunday night. 


Conditions for development of ash anthracnose have been favorable because the weather pattern was cool with prolonged moisture. I have been receiving many calls and questions in regards to Green Ash trees losing their leaves. The seemingly healthy leaflets just drop off the trees. Ash leaves are "compound leaves", composed of (mostly) seven to nine leaflets arranged along a stalk or "rachis." In this condition, individual leaflets separate from the rachis and drop, but the leaflets appear a normal healthy green color and don't show browning or yellowing. Close examination may reveal a few tiny spots discolored brown or purple. These spots are especially noticeable on the rachis where leaflets have dropped.

This leaf drop condition is caused by a fungus disease, ash anthracnose, sometimes in combination with feeding by sucking insects. The cool, showery weather of the past several weeks favors infection by the anthracnose fungus. The cool temperatures also slow the development of the ash leaves, giving the fungus a longer time to infect. Once leaves start to fall, it is too late for any fungicide sprays to be effective because infection has already taken place.

If you see this happening to your tree, the first thing to remember is DON'T PANIC! While unsightly and worrisome, a single defoliation by anthracnose will not permanently damage the tree. When warmer weather comes, the tree will make new leaves to replace those lost. Ash anthracnose is not a new disease; it has been recognized in the midwestern states for more than 100 years. Ash anthracnose is one of many tree diseases know to occur in periodic "boom and bust" cycles -- years when it is severe followed by years when it is rare. During the early and mid 1980s, for example, anthracnose was very common in ND. During that time, several studies were done at NDSU to better understand the disease and how it develops.

What to do about the problem: As mentioned, a single year of anthracnose, even if it causes severe leaf drop, will likely not permanently harm a tree. For the current season, rake up and destroy (burn, bury or compost) leaves to reduce the disease carryover. Prune-out cankered branches and remove twiggy growth to promote air movement within the crown. Fertilize trees in spring to promote vigorous growth.

If your tree(s) see repeated anthracnose damage in successive years, a preventive fungicide treatment may help. These need to be carefully timed to the bud break of the tree to be effective. A fungicide containing the active ingredient chlorothalonil can be used.


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