Why Do I Have Bluish-Purple Greens and Dollar Spot In January?

Posted on February 1, 2017

Peter H. Dernoeden, Ph. D.

Frosty nights in late autumn bring about pronounced physiological changes in plants. In creeping bentgrass and annual bluegrass, especially on golf greens, leaves may develop a purple, reddish or grayish-blue (wilt-like appearance) color. These colors persist all winter and into early spring.  The discoloration may be uniform in some cultivars (especially Crenshaw), but frequently the various shades of purple, red or blue-gray appear in circular patches. These patches constitute different clones (genetic variants) within the bentgrass polystand.  In annual bluegrass on greens during winter, some perennial biotypes can develop a purple color, while the annual types remain green. As individual plants grow, the more aggressive biotypes dominate in a natural selection process to produce circular patches (i.e., segregated clones). These patches are not unlike the circular areas of discolored turf associated with some diseases.  The purplish-blue discoloration can be is mis-diagnosed as red leaf spot disease.  Red leaf spot, however, is a warm weather disease of late spring and summer in colonial bentgrass. Red leaf spot may develop in some creeping bentgrass cultivars (e.g., Penn A-1), but it is uncommon.

If discoloration is due to frost, the underside of  leaves will retain their green color. The purpling and blue-gray color from frosts is especially prominent in Penncross greens, but similar color changes appear in older stands of most other bentgrass cultivars.  As previously noted, a similar purpling can occur in perennial biotypes of annual bluegrass. Segregated clones also can turn red or purplish when treated with plant growth regulators like Cutless (fluprimidol) and Trimmit (paclobutrazol) during autumn and spring when applications coincide with frost.  Trimmit can cause severe damage to areas sodded with bentgrass up to two years after installation.

Cold temperatures trigger the red, blue and purple color responses.  When nights are cold and frosty, sugars are not completely translocated out of leaves and accumulate.  There are several types of sugars (especially glucose), which chemically bind with anthocyanins. Anthocyanins are pigments and their function in plants is unclear.  The word "anthocyanin" is from Greek: anthos = "flower” and kyanos = "dark-blue".  Anthocyanins provide the red, purple and blue colors in flowers.  They also are expressed in the foliage of trees during cool to cold and bright weather to provide the spectacular colors in autumn leaves. Anthocyanins always are present in leaves, but normally are masked by chlorophyll. Creeping bentgrass and perennial biotypes of annual bluegrass experience a similar accumulation of anthocyanins during cold weather.  Frost denatures the green chlorophyll, thereby exposing the anthocyanin pigment.  These colors may intensify and persist throughout winter months and slowly disappear in mid-spring after turf begins active growth. Spoon-feeding during mild periods in early spring will speed growth and green-up.

Dollar spot is an inscrutable disease that can be active at almost any time of year. It normally appears in mid-to-late May in our region, generally is less prominent in the heat of summer, and then resumes activity in the fall. Dollar spot often appears following early frosts coinciding with “Indian Summer” weather from mid-November and into early December.  In 2017, the first 10 days of January were frigid, followed by a warming trend, and then an extended period of rainy and overcast weather. Unprotected greens experienced a sudden outbreak of dollar spot in late January. During chilly weather, dollar spot infection centers and/or the periphery of spots usually have a reddish color. These symptoms mimic Microdochium patch (aka pink snow mold and Fusarium patch). Greens with active dollar spot should be treated as soon as possible with a mix of a contact and systemic fungicide to prevent pitting, since weather conditions may remain mild enough from time to time for additional disease activity.