Purple spots

Early spring brings focus to a major physiological event that occurs on creeping bentgrass greens: the appearance of reddish, purplish and bluish spots. Although often associated with old types of bentgrasses like Seaside, and “German bents”, the colorish spots also appear on the newly released creeping bentgrass cultivars. The symptoms tend to become more pronounced as the greens get older.

This physiological change is caused by sunny, warm days fallowed by cool nights, most often in late-fall. During mid-to-late fall its common to see relatively warm daytime temperatures (65 to 75 ^oF) and cool nighttime temperatures (32 to 55 ^oF). Creeping bentgrass is actively carrying on photosynthesis during these warm sunny days producing sugars in quite large amounts.

When the sun sets and a rapid drop in temperature occurs, these sugars get stuck in the leaves.  The excess sugar interacts with the plant pigment anthocyanin.  Anthocyanin found in leaves produces the reddish, purplish or bluish colored spots on the greens.   Anthocyanins are expressed as a stress response to protect the photosynthetic apparatus.  

The presence of the reddish, purplish, and bluish patches will persist through the winter and into spring. Often the patches are the first thing observed on creeping bentgrass in late-winter and early-spring.  What causes the patchy appearance is the differential anthocyanin expression in clonal types of creeping bentgrass.  In some situations however, the symptoms can be uniform across the green.  

The expression of anthocyanins can occur in late-winter and early-spring. The conditions required often is an early spring green-up of the creeping bentgrass followed by several nights of temperatures in the 20s or heavy frosts.  

Permanent turfgrass injury is not associated with the reddish to purplish-blue coloring. Once temperatures warm and turfgrass growth occurs, the coloring disappears. Thus, no control measures are necessary. You just have to wait for favorable growing conditions.

The colored spots are sometimes misdiagnosed as either phosphorus deficiency or melting-out/leaf spot diseases.  Phosphorus deficiency symptoms often appear as bluish areas in a creeping bentgrass green.  If you suspect phosphorus, you can check your soil test reports to see if adequate phosphorus levels are present - and if so - once the turf starts growing, symptoms should disappear. A second option is to make a test phosphorus application between 0.5 to 1.0 pounds per 1000 sq. ft. and see if you get a response.  

Leaf spot and melting-out are common diseases on many cool season turfgrasses.  Although creeping bentgrass can be attacked by Bipolaris, for example, I’ve not seen the melting-out phase on creeping bentgrass in the spring.  I would be skeptical to associate early reddish, purplish, or bluish patches with a “Helminthosporium” disease.  

In conclusion, don’t get too excited about purple patchy greens through late-fall into early-spring.  The colors are being expressed by a pigment that in some fashion has provided protection to a plant stress.