Following up on last week’s post, I received a couple of questions, both of which were good and worth writing more about.
Question 1: As you reduced N on greens, have you seen any more disease like dollar spot?
The short answer is-no, but more nuance is required.
My approach to fungicide on putting surfaces, much like everything else, has become more simple as the years have gone by. This year, I pushed the envelope more on fungicide more than past years.
This season, we didn’t make our first fungicide application until 25 July. Conditions had been warm, but very dry and my observations kept telling me there was little to no need for a fungicide. I track the Smith/Kerns dollar spot index each day, which can be seen in the chart below. For those unfamiliar, 20% is the point at which one can expect the dollar spot pathogen to be active.
Until 25 July, the index was bouncing up and down, only occasionally going above 20%, before dropping below again. This, plus observation is what I was basing my fungicide decisions on. I made a note that I saw some dollar spot on fairways on 28 June; I didn’t react immediately and the index moved back below 20%. As we moved toward the end of July, the index shot up and stayed above 20% for good. This is when I decided to make our first fungicide application of the season.
Over the next month, we rotated two fungicides back and forth over four applications. This took us to the beginning of September, when the index once again dropped close to and below 20%. We remained fungicide free until 29 September, when the course had been soaked by 6-plus inches of rain. Unusually hot and humid conditions sky-rocketed the index once again and we had an outbreak of leaf spot on the greens. This required the final fungicide application of the season.
To answer the question again–no–we did not see any increased incidence of dollar spot due to the low N rates. While we do apply fungicide during periods of high disease pressure, I’m more and more willing to push the boundaries due to our bentgrass populations and the comfort that comes from data collection and having observed the surfaces for many seasons. This brings us to the second question…
What is happening to the Poa?
In sports, the best players, coaches and managers are always working to exploit their opponents weakness. When I was at Northland Country Club, I wrote often about exploiting the advantages bentgrass had over Poa annua. At Northland, we were exploiting advantages to gain larger populations of bentgrass over time. At Hazeltine, the goal has been keeping the purity of bentgrass as high as possible.
The photo above was taken on 7 September 2017, near the end of a season in which I had used no PGRs and very low (less than 1lb/M) nitrogen. I came away from that season having learned a few things:
I didn’t think the ‘canopy architecture’1 was as good without PGRs. I liked surfaces treated with PGRs better than those without and would continue using them again in the future.
1lb/M of nitrogen for a golf season wasn’t enough to provide the kind of surface we desired. At least without PGRs.2 I didn’t find the density of the bentgrass to be acceptable. This observation was backed up by negative member comments.
The negative impact of low N on Poa was greater than anything I’d ever seen from PGRs. As seen in the photo above, it finished the season in an unhappy state.
Fast-forward to this season and we were once again on very low N; just 0.75lbs/M, even lower than in 2017. The differences are that 1) we’re once again using PGRs and 2) all of the decisions about N rates are tied to data, not just feel, as it was in 2017.
The photo below was taken on 6 November of this year. The Poa is in a very similar state to the 2017 version. While maybe not obvious from the photo, the density of the bentgrass was much more acceptable. This was backed up by positive member comments.
In the section above, I wrote about our approach to fungicide. There is pretty well known research from Rutgers that shows Poa annua is less impacted by anthracnose at certain N rates3 and when topdressing is applied regularly, in order to protect the crowns.
The cultural approach mentioned in this study is not the approach we are using on our bentgrass. It would stand to reason we can sometimes expect to see the small amounts of Poa in our surfaces suffering from anthracnose, especially when we are limiting fungicides to the threshold of bentgrass.
Our greens have been in play for 13 years and the Poa populations are not zero, but they are miniscule. At one time I felt PGRs like Trimmit were the biggest factor. As time has gone by, I’ve come to feel the biggest factor is nutrition. Nitrogen only, from ammonium sulfate only, in very small amounts, coupled with a fungicide program exploiting Poa’s weakness, should in theory, keep our greens with miniscule levels of Poa. If/when we are able to keep the populations miniscule, we can then employ a fungicide approach that may further drive Poa from our surfaces.
This is a term I’ve grown very fond of—the make up of a surface including density, upright growth, leaf texture, etc.
Research has shown turfgrass treated with PGRs requires less nitrogen. If you want more information—Google it!
The amount in the study was 0.1 lbs/M/week, which on bentgrass would be A LOT!
What's the thought on using only AmS as your only nitrogen source vs something like Urea?