Fungicide PGR Effect

[oze]

Hey all!
Earlier this summer, I saw what appeared to be some small areas of as of then unidentified fungal infection in my lawn. Like the genius that I am, I ignored it, feeling that my healthy, vigorous lawn would fight off whatever it was. Nope. By the time I took samples and sent them off to Purdue University for analysis, the infection had become fairly widespread, including a couple of softball-sized dead areas. My money was on brown patch, and I wasn't 100% wrong, just wrong about the *major* culprit: leaf spot/melting out. Here is the pertinent part of the report:

Final Report
8-21-23
The sample was incubated to encourage fungal growth. Curvularia and Bipolaris or Drechslera grew from
dead and dying leaf blades which is suggestive of leaf spot/melting out. For more information on this disease
issue, please see the following link:
https://www.extension.purdue.edu/extmed ... -103-w.pdf
Rhizoctonia was also observed growing from sporadic leaves that have necrotic lesions. Given how little was
observed, it might be causing some disease, but is not the primary problem For more information on brown
patch, the disease caused by Rhizoctonia, please see the following link:
https://www.extension.purdue.edu/extmed ... -106-w.pdf

I had started treating for brown patch before submitting the turf samples, and luckily, the same fungicides are recommended for leaf spot/melting out. I started with Headway, which is a combination of Azoxystrobin (FRAC Group 11) and Propiconazole (FRAC Group 3). I applied 3 lbs/1000 according to the label, 2 weeks apart. Seeing no/limited results, I ordered and applied Cleary's 3336 DG lite, which is Thiophanate-methyl (FRAC Code 1) after another 2 weeks. I have lost some of the lawn, but seem to have stopped the spread of the disease.

All very interesting, but I also learned something that came as a big surprise to me. Since my wife doesn't want me to treat the back yard because of the dog, I didn't apply any fungicide to it. After the first application of Headway, and especially after the second, I saw that the lower-maintenance back yard grass was growing much faster than the front. I almost had a hard time keeping up with the mowing. I had no idea what was going on. I finally came across the article linked below, which discusses how Demethylation inhibitor fungicides (DMIs) have unwanted plant growth regulator effects on turfgrass. Holy moley! Propiconazole, one of the ingredients in Headway, is a DMI fungicide! It's been six weeks since my last application of Headway (kinda sounds like I'm in the confessional), and the front yard is now caught up with the growing pace of the back. Has anyone else ever seen this? Oh, and I didn't see some of the perceived positive effects, like a darkening of the turf, that I did a few years ago when I intentionally applied PGR. It was just...slow. Anyway, here's the link:
https://journals.ashs.org/hortsci/view/ ... -p1052.xml

[Green]

Hey oze. Great observation! I was actually just talking with an expert PGR user on this exact topic. He said that on short maintained turf, the growth suppression from Propiconazole (measured in clipping reduction) is likely about 18%, and the duration of that being around 400 to 600 growing degree days. And a total "effect duration" of 800 to 1200 GDD (the rebound phase taking up the latter half of that). Of course, with most PGRs, higher HOCs concentrate more active ingredient in leaf tissue and therefore experience greater durations; Propiconazole may be the same in that regard. As such, if you're following it with a conventional PGR, it probably makes sense to do so at around 400 GDD (C, base 0) to avoid the rebound. Generally, we want our reapplication to be at 1/4 or so of the way into the total effect duration (hich is halfway through the growth suppression phase or first half)...look up PGR growth rate curves online to get a feel for this. Propiconazole is definitely soil-active as a PGR once watered in...and I think I saw an effect from it foliarly, too, even though it's regarded as a class B PGR in its mode of action.

[Green]

...18% clipping reduction at standard preventative rate...greater reduction at normal curative rate, most likely.

[oze]

Thanks for the information, Green! Next year when, not if, unfortunately, I apply the first of the Headway apps, I'll know what to expect!

[oze]

Thanks for the information, Green! Next year when, not if, unfortunately, I apply the first of the Headway apps, I'll know what to expect!

Forgot to add that my cutting height was 3 1/2" when I first noticed some brown areas in early June, and increased to 4" in early July. When healthy, my lawn is thick and lush; I know that raising cutting height is one of the cultural recommendations to reduce the probability of leaf spot/melting out, but I wonder if maybe 4" could be *too high*; with the thick, tall grass slowing down the evaporation of dew and/or irrigation moisture.

[Green]

Velocity is another one you use that acts as a PGR (and fungicide). It's not a good quality PGR, but it definitely reduces growth significantly for three weeks or so. I've seen it classified as a Class C PGR. I made use of the effect this year. Also, no dollar spot appeared on the area treated with it.

[oze]

Oh yes, Velocity! My poa battle got sidetracked this year. I should've taken pictures before the fungus, but I am (was?) definitely winning that battle.