Do growth regulators and biostimulants cause yellow etiolation of creeping bentgrass?

We've been seeing yellowing and etiolation of creeping bentgrass greens in North Carolina off and on since 2005. Some people are calling this 'bacterial wilt', but there are two problems with this name. First, no one has proven that this condition is caused by a bacteria. Second, and perhaps most obviously, wilt isn't one of the symptoms!

So, can we all agree to call it yellow etiolation instead of just slapping another erroneous name on it? I know that 'etiolation' is difficult to pronounce, but you can hear its correct pronunciation and practice along at thefreedictionary.com/etiolation.

Early on, I had a theory that biostimulants and/or growth regulators were involved in either causing this yellow etiolation or increasing its appearance. I wanted to share a couple of pictures recently sent to me by golf course superintendents that provide even more evidence of their role in this condition.


This first picture was taken at one of our research sites. Our plots were positioned in a ring around the cleanup pass of this green, and we asked them to avoid spraying our plots when they made certain applications to the greens. The area to the right that is declining severely received their sprays containing a biostimulant product and the growth regulator trinexapac-ethyl. As you can see very clearly, the areas NOT receiving these sprays are much more healthy than the areas that were sprayed.



This second picture was sent to me by a golf course superintendent in Pennsylvania. You are looking at a collar-height walkway between a green and a tee. The area to the left of the red dotted line received double applications of Primo, one along with the greens applications and another along with the tee applications. You can clearly see that the yellowing, etiolation, and thinning of the bentgrass is much more pronounced in the area receiving higher rates of Primo.

So what is going on here? To me, this is looking more and more like a physiological condition rather than a disease problem. We are still unable to consistently isolate any single pathogen, bacteria or otherwise, from turf exhibiting these symptoms. We are planning research to look at the influence of growth regulators and biostimulants on this problem. Until then, I suggest that superintendents who are battling this problem eliminate biostumulants from their programs and also avoid use of trinexapac-ethyl during periods of summer stress.

Update on Bacteria in Creeping Bentgrass Greens

In the last month, we've observed significant amounts of bacteria in 32 creeping bentgrass samples submitted to the Turf Diagnostics Lab. We isolated the bacteria in these samples and are identifying them by DNA sequencing and other techniques.

The good news first: the majority of samples contained bacteria that are known to be non-pathogenic to turf. We suspect that these bacterial infections occurred during aerification and topdressings performed after Memorial Day when it was very hot and humid. These non-pathogenic bacteria are not killing the plant directly but may be speeding up the dieback of leaves or tillers that were injured by these stressful practices.

We did isolate the plant pathogenic bacteria Acidovorax from 7 locations, most of which are in the Piedmont of North Carolina. The samples that contain Acidovorax are showing the symptoms of chlorosis, etiolation, and gradual thinning that I wrote about in previous posts. The caveat is that we're not isolating Acidovorax from all of the locations where these symptoms are present. As a result, we cannot be sure that this bacteria is the true cause of the symptoms.

Results from trial on L-93 in Raleigh. Rates are per 1000 sq ft.

We have ongoing trials at 2 locations where Acidovorax is present, and our preliminary data is presented here. First, a word on our rating scale. These ratings are on a 0 to 9 scale, where 0 equals no disease, 1 to 3 represents varying degrees of chlorosis, 4 to 6 represents chlorosis with varying degrees of thinning, and 7 to 9 represent severe thinning of the turf.

Results from trial on G-2 in Charlotte. Rates are per 1000 sq ft.

While we haven't found a silver bullet yet, we have noticed that Signature treatments seem to be helping. In the Raleigh trial, 8 oz of Signature applied 3 times has prevented the turf from thinning, although some chlorosis and etiolation is still evident. In the Charlotte location, plots treated with 8 oz of Signature 3 times have improved slightly, while all other plots have continued to thin.

Before you complain about the price of an 8 oz Signature spray: we don't set the prices, we're just trying to find something that works. We are continuing to evaluate other products and practices that may help to control this problem and will post regular updates on this blog as we learn more.

One important take-home message: just because a diagnostic lab sees bacteria in your bentgrass doesn't mean you have a bacterial disease. Most of the samples we've received have only contained non-pathogenic bacteria. The bacteria has to be isolated and identified in order to determine if it is a plant pathogen.

If it's yellow, just be mellow

Rumors have been swirling around the golf industry over the last week about 'bacterial wilt'. For the record, I want to make it clear that we have diagnosed NO cases of bacterial wilt in North Carolina or any other state.

I repeat, we have diagnosed NO cases of bacterial wilt in North Carolina or any other state.

Yellow spot of creeping bentgrass.

We have seen suspicious amounts of bacteria in bentgrass expressing symptoms of yellowing, etiolation, and gradual thinning. We don't know if these bacteria are the cause of the problem or if they are a secondary invader. As a result, we don't know if this is an actual disease or not. For more information, see our recent blog post.

A severe case of yellow spot on creeping bentgrass.

'Bacterial wilt' is caused by Xanthomonas translucens and is primarily a problem on Poa annua but also occurred on 'Toronto' creeping bentgrass back in the 1980s. Bacterial wilt causes rapid wilting and dieback of infected plants. Chlorosis, etiolation, and slow thinning of the turf doesn't fit the description of this disease. Therefore, what we are seeing now is NOT bacterial wilt.

We've had countless calls and samples over the last week from superintendents who fear they have bacterial wilt because they are seeing chlorotic or yellow turf. It's important to realize that many things can cause turf to turn yellow, and that if you aren't seeing the etiolation and gradual thinning then you probably don't have this suspected bacterial problem. For more information about some of the most common reasons for yellowing turf, please refer to this post on turfdiseases.org.

Several of the samples that came into our clinic this week turned out to be yellow spot. This disease causes chlorotic spots and patches on creeping bentgrass putting greens, but does not cause an abnormal growth habit and we've never seen it lead to thinning of the turf.

Yellow spot symptoms are especially prevalent during dry and hot weather. That certainly fits the conditions we're experiencing in North Carolina right now. For more information about yellow spot, please refer to this Golf Course Management article from the November 2006 issue.

Suspected Bacterial Disease of Creeping Bentgrass

Over the last two weeks, the Turf Diagnostics Lab at NC State received several samples of a suspected bacterial disease on bentgrass putting greens. All of the affected golf courses have been in the Piedmont region, from Raleigh to Greensboro to Charlotte.



Most locations are reporting yellowing and etiolation of the bentgrass followed by a slow thinning. The symptoms are relatively mild in these locations. Two courses experienced a more rapid and widespread decline. In all cases, the symptoms are limited to areas that were severely damaged last year by heat stress or other diseases.

We are working to isolate and identify the bacteria associated with these samples. Since there are bacteria in almost every turf sample we receive, we need to determine if the bacteria are pathogenic before we can be sure that they are the cause of the problem.

In the meantime, golf course superintendents who are seeing these symptoms are advised to avoid abrasive practices like aerification, verticutting, and topdressing. If the symptoms are localized to cleanup passes or certain greens, then dedicate a mower to those areas or wash the mower thoroughly and spray it with a 10% bleach solution before moving to other greens. Mowing affected areas last after the dew has dried or spraying with a hydrogen peroxide solution before mowing may also help to minimize the spread of bacteria.

In addition, anything that can be done to minimize stress on the turf is advised; for example raising mowing heights, switching to solid rollers, or skipping mowings on affected areas.

We are investing all of our effort and resources into determining the cause of this problem and developing a practical solution. We have initiated trials at 4 golf courses to evaluate practices that may help to control it. Be sure to follow our blog or sign up for email alerts to receive regular updates:

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