August 16, 2023: Think Tank – Turfgrass Science at Purdue University

August 16, 2023: Think Tank

Highlights: Final few weeks until the respite of September; consider seeding instead of treating lawns; yellow spots in bentgrass; too many trinkets in the spray tank?


Normal temperatures and a rainfall rebound over last 30 days.

Mild, seasonal temperatures have dominated the landscape over the past month. Indianapolis has experienced four days above 90 F in June, three in July, and one thus far in August (8 total). On average, Indy has 23 days above 90 F per annum. Cycling south, Evansville on average has 42 days above 90 F, and this season thus far has had 23 days. A little west, St. Louis has had 41 days (average – 44); east, Cincinnati has had 6 days (average – 24); and north, Fort Wayne has had 9 days (average – 17). Next week’s forecast indicates we should accumulate a few more + 90 F days through the end of August, which will be great to grow in a certain football field near this author. Most of the area, however, should still remain below normal.

The story over the last month has been the more than adequate rainfall. Indianapolis had just over 6 inches of rainfall in July or 1.64 inches above normal. In August tack on another nearly 3 inches of rainfall in Indy or double the amount normally observed mid- month. Supplemental irrigation should have been minimized over this period, and in my “around town” observations some of those areas that didn’t cut back have a fairly nice stand of nutsedge. Forecasts indicate a dry down over the next week. Particularly on over-irrigated lawns, high heat and low rainfall preceded by a rainy period can result in flash drought symptoms and rapid dormancy. Localized dry spot on sand based greens can also rear its head quickly in this scenario.

Summer forecast  to return along with a dry down next week.

Disease Damage in Lawns: Seed or Treat?

Despite the cooler temperatures, brown patch has been an issue on tall fescue lawns throughout the state, and dollar spot on Kentucky bluegrass has been submitted to the diagnostic lab on several occasions. As we head to September, consideration of whether to continue on with a fungicide treatment or devote the funds and effort in a few weeks towards aerification and overseeding would be a more fruitful effort. See this article in the Purdue Landscape Report for more information.

Scout for Gray Leaf Spot in Tall Fescue

Scout for Gray Leaf Spot on Tall Fescue

No observations in the state yet, but this is the time of year that gray leaf spot symptoms start to appear on tall fescue (and especially perennial ryegrass). This disease often takes over for brown patch and may not be controlled with the same fungicide regime. Stand symptoms of gray leaf spot start as smaller, more defined areas of leaf blight that appear similar to dollar spot on Kentucky bluegrass. The disease can progress quickly from spore production. Individual leaves will have well defined spots with a dark brown margin and grayish interior (hence the name). Affected leaves may also have a slight buggy whipping symptom where the blighted leaf tip curls down upon the lower leaf.If gray leaf spot is suspected, reach out to us and consider submitting a sample to confirm.

Yellow Spots on Putting Greens – Algae (sometimes)

Yellow spot on a bentgrass putting green

Several courses in the region have submitted areas of distinct yellow chlorosis on bentgrass putting green surfaces over the last few weeks. Chlorosis can be a symptom noticed in the early stages of several root diseases including Pythium root rot or summer patch or in the foliar phase of anthracnose infection. In some of these cases, considerable algae infestation by the genera Phormidium and/or Oscillatoria was observed in the leaf sheaths and upper canopy. These algae commonly proliferate on saturated greens and are associated with yellow spot and in some cases, black layer. Treatment with the phosphites, chlorothalonil, mancozeb and/or thiram should aid in control.

Think Tank

 Unrelated to algae infestion, chlorosis, etiolation and decline symptoms were apparent on several additional putting green sample submissions. The return of bacterial decline caused by Acidovorax spp. and other bacterial species was the concern and trepidation, but after slicing dozens of bentgrass stems no significant streaming of bacteria from vascular tissue was observed. No pathogens of significance were noted in these samples. In all of these cases, however, 9 – 13 products were applied just prior to and during a string of hot humid weather in late July that preceded the decline.

Heat stress interrupts several plant processes including photosynthesis and resultant carbohydrate production, which is limited by increased photorespiration in C3 cool season plants in high temperatures. Add on the “free stress” imparted by daily mowing at low heights, golf and equipment traffic, saturated soil conditions from rain, irrigation or lack of drainage, diseases, etc, etc… and the straw to break the camel’s back is quite thin.

During the summer heat, backing off on nitrogen rates and spoon feeding bentgrass putting greens is commonplace. Restricting sulfur applications and ammonium sulfate as a nitrogen source is a common recommendation to limit the potential formation of black layer. With the exception of the newer “cool” DMI fungicides, the previous recommendation of restricting application of the older DMI fungicide chemistries such as propiconazole, triadimefon, and others was suggested to reduce the effects of heat induced phytotoxicity from excessive growth regulation. Several herbicides, particularly some ester formulations, have high temperature limits that preclude their use and can potentially injure turfgrass during the summer heat. These recommendations are based on applications of a single product and don’t consider the potential adverse impact of chemical interactions or salt loads of massive tank-mix combinations.

When implementing a spray program with the complexity of double-digit ingredients in the same tank, can the question of safety to bentgrass putting greens in a high heat, stressful environment be adequately answered? If not, perhaps pulling some products out of the spray tank when the mercury rises, rather than putting more in, would be beneficial to putting green health and resilience during the summer stress period.  

Field Day & Lawn Diagnostic Training Event

Thank you to everyone that attended and participated in the Purdue Turfgrass and Landscape Field Day on July 18 and Lawn Diagnostic Training on August 1. Special thanks to vendors, sponsors, and the outstanding MRTF volunteers and executive secretary Ashley Breed!

Lee Miller
Extension Turfgrass Pathologist – Purdue University
Follow on Twitter:  @purdueturfpath



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