nchris@iastate.edu
July 10, 2013
These are pictures of Summer Patch on Kentucky bluegrass at
the Horticulture Research Station. It is
caused by the fungi Magnaporthe poae. It typically shows up in early summer,
particularly in years like this that are very wet early followed by a quick drying
period and hot temperatures. This showed
up over the 4th of July (right on time).
The blighted areas with a green center that are surrounded
by a circle of dead grass are known as “frog-eyes” and are typical of a number
of patch diseases. It is believed that
the organism begins as a saprophyte (organism that feeds on dead plant
material) in the middle and moves outward in a circle without damaging living
grass. It only attacks living grass if
conditions are right and it reaches a certain level of virulence. The patches
here are 10 to 12 inches in diameter, which is common for this disease.
There are several systemic fungicides labeled for this
disease, but the trick is to get them down before the symptoms develop. Contact fungicides will not work. To treat now would do no good and the
symptoms will like last through the summer.
This is a disease for which good records are a must. On this area, I would need to apply a
systemic fungicide in late June next year before symptoms develop. Core aeration in the fall and irrigation
during the stress period of early summer can also help prevent its development.
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