LAWN: Have you observed gray leaf spot on tall fescue lawns before?

Yes - Treated for it
Yes - Didn't treat it
Not Sure

Update (8/10/2021)

See Spot Run

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Cool and Dry Pattern in Last 14 Days

  1. The current temperature spike is expected to last a few more days. – Source: NOAA
  2. After a rainy July, rainfall has disappeared over the last two weeks. – Source: HPRCC

Despite a few blips of scalding temperatures, July was 1.2 degrees below the long-term average in MO, and the first few days of August followed suit. High dew points, a trend over the past few decades, has resulted in more above average low temperatures in the last few weeks. Therefore while cool season turfgrass stress was down, it certainly was not out. These high dew points, a result of high humidity, also equals horribly uncomfortable "real feel" conditions when the summer heat does saunter in, like it will for the next few days. Precipitation in July was above average statewide at 5.12 inches, 1.41 inches above the long-term average. The rainfall has ground to a halt in Missouri, and much of the nation, in the last few weeks and the effects of drought on non-irrigated turfgrass areas is evident in most of the region.

A Warm Week, Followed by a Weekend Break

  1. Balmy temperatures this week are expected to moderate this weekend. – NOAA
  2. Droughty looking lawns look to persist through mid August. – NOAA

Quick Hits

Fairy Rings Abound

  1. Mushroom production in tall fescue research trial plots.
  2. Fairy rings are not a laughing matter for golf superintendents.

Fairy Ring - To go along with frequent reports, fairy rings have become increasingly apparent on our research greens and other turfgrass areas in the last few weeks. Fairy rings are caused by a number of different basidiomycete fungi that may form mushrooms or puffballs. Dark green rings and turf decline along ring margins can be caused by high ammonia release by these fungi degrading organic matter in the thatch. This organic matter degradation can also result in a coating of soil particles with hydrophobic organic acids, rendering the soil water repellant and causing turfgrass decline.

In lawns and higher cut turfgrass, fairy rings may be an indication of an old, buried tree stump, but they can also occur on their own. Control measures are very limited on lawns. Heavy watering along the fairy ring margins may reduce the effects of water repellancy and dilute the nitrogen. On putting greens, fungicides in the QoI (i.e. Heritage, Insignia, etc), SDHI (i.e. Velista, ProStar) and DMI (i.e. Torque) class may help with curative control. In a curative situation, these fungicides will need to be watered in with 0.125 - 0.25 in of post-application irrigation and tankmixed with a wetting agent.

Heat - As mentioned above, the significant heat and humidity this week will bring a round of late summer stress to cool season turfgrasses including tall fescue and creeping bentgrass. In the case of tall fescue lawns, make sure the mowing height is at 3.5 - 4 inches and if the grass isn't growing, you shouldn't be mowing. Many lawns now are drought dormant and mowing them only adds an added stress. On bentgrass greens, superintendents know the drill. Handwater and alternate mow/roll if possible. Raise the mowing height, use smooth rollers, and don't overfertilize. Next week should bring a bit of a cool down and a glimpse towards the end of the summer marathon. Another good piece of news is our daylength, or photoperiod from sunup to sundown, has dipped below 14 hours, a full hour less than during the summer solstice in late June. Just an hour less stress in our day.

Black Layer at Surface & Down Below
Core sampling may reveal some problems throughout the soil profile.

Black Layer on Putting Greens - In sampling last week, we noticed some black layer on putting greens. Be aware of this potential issue later in the season when summer stress on bentgrass has accrued on greens that don't drain well. Black layer is just another reason to sample greens regularly with a core sampler, particularly along low areas of the green or margins where the sand and native soil meet. Venting will help mediate this problem as far down as the tines go, but long-term, sustainable management may require remediation of drainage issues. Late in the summer, consider switching away from ammonium sulfate as a nitrogen source, which may increase black layer formation.

See Spot Run

Leaf Spot/Melting Out on Kentucky Bluegrass

  1. Leaf spots can affect a number of grass species, but are most severe on low mow Kentucky bluegrass.
  2. Significant conidia production leads to quick spread of this disease.

Leaf spot caused by Drechslera and Bipolaris spp. (formerly Helminthosporium diseases) have been especially prevalent over the past month. At least one of these species can affect all turfgrass hosts, and, along with Curvularia spp., can be found as secondary, opportunistic pathogens often. Kentucky bluegrass, however, has been especially afflicted by Drechslera poae, which can quickly blight areas and cause a melting out symptom that appears similar to Pythium blight. In several of these cases, the misidentification was misfortunate as mefenoxam, a Pythium fungicide, was applied repeatedly to no avail.

Leaf spot/melting out is more severe on younger turfgrass stands, and particularly on some low mow varieties that have been utilized on golf course fairways. In some cases, perennial ryegrass may also be part of the mix, in which case, another leaf spot pathogen or disease like brown patch or Pythium, may simply join in the fun. Several Kentucky bluegrass cultivars have been bred with good resistance to leaf spot, and should be at least part of a seed blend/mix to combat this disease. In a Kentucky bluegrass cultivar trial with experimental varieties, we saw this resistance quite plainly. See Kentucky bluegrass cultivar resistance at NTEP here.

Avoid excess nitrogen fertilization in midsummer and raise mowing heights when extended leaf wetness periods ensue. Numerous fungicides are available for leaf spot control on sites with a history of issues, and in a curative situation a tank mix of a contact fungicide such as fluazinam, fludioxonil, or chlorothalonil should be considered along with a systemic such as azoxystrobin, iprodione or penthiopyrad.

Scout Now: Gray Leaf Spot on Tall Fescue

  1. Not seen yet in 2021. Scout for these symptoms on tall fescue.
  2. Like other leaf spots, many spores = quick spread.

Gray Leaf Spot on Tall Fescue Warning

A different leaf spot, gray leaf spot on tall fescue, may be on the horizon soon and should be scouted for over the next few weeks. Gray leaf spot is a late summer/early fall disease in Missouri, particularly on irrigated tall fescue lawns. As the name implies, spots on the leaves can have a grayish hue, and are marked different than the tan, scalloped lesions of brown patch. Gray leaf spot is tied to long periods of leaf wetness, so proper irrigation and reducing shade can minimize disease occurrence. Tall fescue is moderately susceptible, but in severe cases fungicide application may be necessary. Thiophanate methyl has been demonstrated by the NC State turfgrass pathology program as effective, particularly tank-mixed with a QoI (or strobilurin) fungicide (i.e. Heritage or Insignia). Resistance to the QoI fungicides in populations of the gray leaf spot pathogen has been found, and reliance on this chemistry alone is not effective for control. This is in stark contrast to brown patch on tall fescue which is controlled most effectively with the QoI chemistry.

Lee Miller

Extension Turfgrass Pathologist
University of Missouri
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