Yellow Spot or Bacterial Etiolation? That Is the Question.

— Written by Jim Kerns — Contributors: Lee Butler | July 30, 2024

With the rapid transition from hot and dry to hot and humid, yellow spots develop on cool-season grass putting greens. Some are worried that the dreaded bacterial etiolation is making a comeback. Yet, we have not officially observed bacterial etiolation in the Turfgrass Diagnostic Lab at NC State for quite some time. Its occurrence is quite rare. Remember that etiolation is a specific botanical term that is the process by which plants grow long, pale, and unhealthy due to the lack of light. So, when extended rainy periods occur, guess what else is limited – light! Thus, creeping bentgrass and annual bluegrass could elongate or etiolate without a bacterium. If your plants start to etiolate or elongate abnormally, it does not equal a bacterial issue in turfgrass or any plant. Moreover, if soils become anaerobic, this can also stimulate plants to elongate. Bottomline: small yellow spots with slightly elongated plants do not necessarily equal bacterial etiolation.

Close-up of short green grass blades and turf — Field inoculations of creeping bentgrass with Acidovorax avenae that resulted in etiolation.

Another option is a disease called yellow spot. On cool-season grass putting greens, spots about the size of dollar spot can develop. They are bright yellow, and strands of blackish-green algae can be observed growing up the leaves after incubation. We have not observed thinning in these spots, but the spots are reasonably evident, given the bright yellow color. A cyanobacterium of unknown species causes yellow spot.

Golf putting green with red spray-painted X marks and small red dots — Yellow spot symptoms in a fungicide trial on an 'A-1' creeping bentgrass putting green in Raleigh, NC, during the summer of 2014.

Regardless of the species, yellow spot can be effectively controlled with applications of a phosphonate fungicide tank mixed with a contact fungicide such as chlorothalonil. However, if warm, rainy conditions persist for extended periods, applications every seven days may be necessary.

Grass field with red grid of dots and corner markers and scattered brown patches — Yellow spot in the four control plots of a fungicide trial on ‘A-1’ creeping bentgrass. The treatments that provided excellent suppression of yellow spot are various mixtures of a phosphonate fungicide (Appear or Signature) and a contact fungicide (chlorothalonil or fluazinam).

In conclusion, do not freak out when small yellow spots are observed. Even if it is bacterial etiolation, we could NEVER cause turf thinning in field trials. I know someone will take offense to the last statement, but the turf thinned from other means, not the bacterium. The best thing to do is to submit a sample to a lab and wait for the diagnosis. In our experience, overreacting to a problem induces more turf thinning. As turf starts to decline, the KISS method is the best method to subscribe to.