Dollar Spot and Leaf Spot

May 15, 2015

Over the last week, dollar spot and leaf spot have popped up across the state of Iowa.

Dollar spot is usually considered an early to mid-summer disease and seems to be about a month ahead of last year. It is caused by the fungal organism Sclerotinia homoeocarpa. Most of the damage I have seen this spring has occurred on bentgrass greens, tees, and fairways. The symptoms of dollar spot on close mown turf (such as bentgrass) are small, tan-colored spot equal to or smaller than 2.5 inches across. The name of Dollar Spot comes from the silver dollar-sized turf damage. Spots can be numerous and coalesce, making identification difficult. Dollar spot is most notable on nitrogen deficient turf. By increasing fertility, you can reduce future outbreaks. Dollar spot can spread by movement of mowers, equipment, water, wind, and people. It has a very wide range of activity. It is most active in temperatures between 60-90 degrees but can occur as long as nights are over 50 degrees (which has been consistent over the last week).  The wet weather/ and high humidity have created an ideal situation for early season dollar spot. 

Removing the dew and guttation water early in the morning through mowing and dragging will help manage dollar spot. Active dollar spot infections produce a cottony white mycelium mass that is often evident on the turf during the early morning hours, not to be confused with spider webs. Fungicides (DMIs) do very well against dollar spot. Chlorothalonil, propiconazole, fenarimol and others are labeled for dollar spot control. It is important to remember that several site-specific inhibitors have the potential for fungicide resistance. To prevent resistance use a rotation of chemicals and avoid repetitive use.  

 Pictures courtesy of Cody Freeman - Green King Turf

Leaf spot is an Ascomycete fungi caused by Bipolaris spp. and/or Dreschslera spp. Cool, wet weather favors the disease. Leaf spot affects all turfgrass species used in the state of Iowa. A majority of the damage recorded so far has been on Kentucky bluegrass lawns and golf course roughs. It can also hit bentgrass, fine and tall fescue, and ryegrass.  

The ecology of leaf spot is prolonged wet periods (10+ hours) alternating with drying. The conditions this spring have been perfect for leaf spot on Kentucky bluegrass. Off-color patches of the turf are the first sign of the disease. The turf on the area may also look like it is dry, even after rainfall. A closer look at the leaf tissue shows elliptical purple spots turning to straw-colored on bluegrass.

Leaf spot is most commonly observed in high N fertility situations, and poor surface and subsurface drainage areas. There are several resistant Kentucky bluegrass cultivars in blends or mixtures and this is one of the main reasons it only affects certain areas of the turf.Fungicides are a last resort and generally not recommended on Kentucky bluegrass lawns or low maintenance areas of golf courses. If control is necessary, thiophanates, most QoI (Strobilurins), and chloronitriles provide the best means of control. 

 Pictures courtesy of Cody Freeman - Green King Turf

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