Problem of the Polka Dot Lawns
With the grass starting to grow, many of us are gasping in surprise at the state of our lawns. What was green and uniform the previous fall is now covered in dry pink, grey or straw colored spots, some of them quite large. These are likely caused by snow mold, a fungus that thrives in cool wet turf.
Snow mold is caused by two different fungi, Microdochium nivale and Typhula sp. Both of these fungi grow very well at temperatures just around freezing and need high levels of moisture to thrive. Snow cover is not a requirement for growth for pink snow mold. This past year's winter was mild and wet for long periods of time, creating the perfect environment for snow mold in even the best cared for lawns.
The good news is that snow mold rarely causes permanent damage to home lawns. As days warm up and lawns dry out the fungus will go dormant and the grass will recover. To assist in this process, home owners should rake over the diseased patches to fluff up matted down grass and increase air circulation. A light fertilizer (1/2 - 2/3 lbs N/ 1000sq. ft) could be applied to help the lawn recover. Do not over apply N at this time of year, however, as it will cause excessive shoot growth at the expense of root growth.
Reducing thatch and compaction by dethatching and aeration in either spring or fall can help to reduce conditions that favor snow mold and create a healthier, happier lawn all around. Next fall rake up leaves and any other debris on the lawn. Continue mowing until the grass goes dormant. Long turf can become matted down and create humid pockets where snow mold thrives. Also avoid putting N fertilizer on your lawn in late September and early October. Fertilizing this time of year causes a flush of new leaf growth that will be very susceptible to snow mold in winter months.
Source: Grabowski, M., Yard & Garden Line News, Volume 8 Number 5, April 15, 2006, http://www.extension.umn.edu/yardandgarden/YGLNews/YGLNews-Apr1506.html