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Update (8/22/2013)

Where's Spot? All Over

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August has been cool but is set to get hot

August was cool, (and very hospitable to cool season turfgrasses), but summer is set to arrive in blazing fashion over the next 6-10 days.  Average August temperatures are currently about 4 degrees below normal, and we have not seen a 90 degree day yet.  This is set to change, however, as yesterday and over the next 10 days it appears Missouri is set for above average temperatures, and perhaps the hottest sustained period of this 2013 summer.

A dichotomy of precipitation events has been experienced over Missouri this summer, with some areas in southern MO receiving 8-10” over average and northern MO regions entering into a moderate drought situation.  Northern MO will not get any relief as the only significant precipitation chance over the next 10 days may be tonight, and even those rain cells are slated to be spotty and localized.  Southern MO will enjoy the dry down, which should occur quickly  

This year we have experienced many cloudy days, which has kept short crop evapotranspiration rates 7” lower than those observed in 2012.  To close out August, however, a return to the blazingly sunny skies reminiscent of 2012 is forecasted to go along with this heat. Even with the shorter days of late August, supplemental irrigation will probably be necessary to keep turfgrasses from going dormant during this 10- day period.  For tall fescue/KBG areas, remember to water early in the morning to reduce the period of leaf wetness (see brown patch warning below), and water deeply and infrequently.  Potential evapotranspiration losses could be 1-1.5” a week, so plan on 2-3 ¼” morning waterings a week if no rain occurs.      

Quick Hits

    • *Brown Patch on Tall Fescue Warning* High temperatures over the next 10 days may prime us for a late season brown patch epidemic on industrial and residential lawns.  This will be especially true for those that jumped the gun by fertilizing during the coyingly cool early August period.  Brown patch will also be more prevalent in shaded areas and on lawns with irrigation systems.  It will be crucial to program irrigation systems to end the cycle as close to sunrise as possible.  Watering is more efficient in the early morning (less winds, evaporative losses) and can end the presence of guttation/dew and the duration of nightly leaf wetness events.  Also, simply rinsing off the leaves in the morning may disrupt the infection process of the brown patch pathogen.  As an aside, I see sprinklers running on commercial landscapes during lunchtime all the time in Columbia, and it drives me nuts.
Summer patch on KBG emerging in Missouri
      • Summer patch on Kentucky bluegrass – Numerous instances of summer patch on Kentucky bluegrass (KBG) have been reported from around the region this past week, and with the hotter temperatures I imagine more will pop up.  With the earlier cool temperatures, KBG could probably put up with some of these infections, but now that the stress is on, symptoms are occurring.  Summer patch is caused by a soilborne pathogen, making control and, more importantly, recovery tough.  Take note of where you have infections and on high amenity areas consider preventive spring applications of strobilurin and/or DMI fungicides starting when soil temperatures average 65F for 3-5 days straight (normally mid – late May for much of MO).  For curative applications, a small shot of ammonium sulfate (0.2 lbsN/1000 sq ft) along with the fungicide application may help in recovery.  All of these applications must be watered in.  Culturally, a switch to ammonium based fertilizers to acidify the rhizosphere and a spring application of manganese sulfate (2 lbs/A) may suppress the disease.  We do not have any instances in 2013 of this disease on creeping bentgrass putting greens, which was detailed in late July 2012.    
      • Fairy Ring/Anthracnose on Putting greens – Still seeing some infections throughout the area of anthracnose on bentgrass, and hearing more reports of late summer fairy ring breakthrough.  Click here to read the last report on these issues.  

Tell Spot to Sit

Dollar spot has been a constant problem throughout the 2013 season.
Copper spot is a rarely observed disease, but has been seen at the MU research farm.
Drechslera red leaf spot symptoms on 'Penn A4' research green.

The theme of the last few weeks at the MU research farm has been the various spot diseases, which led to reference of the children’s Spot series of books depicting that adorable yellow dog (Where’s Spot? was the first one published) .  In our case, however, spot diseases on bentgrass and other susceptible turfgrasses are far from cute, and have required significant attention this season. 

Due to our mild summer, dollar spot has been troublesome practically all season in creeping bentgrass and Kentucky bluegrass.  Considered the most economically important turfgrass pathogen due to its wide range of growing temperatures, large host range, and broad geographical diversity, the dollar spot pathosystem has been the focus of intense study by pathologists.  Recently at the American Phytopathological Society meeting in Austin, TX, several presentations focused on this disease, including characterization of fungicide resistance mechanisms (University of Massachusetts), and global population structure (North Carolina State University).  One of the more anticipated presentations from a group led by Rutgers University revealed a proposed new name for the dollar spot pathogen with the support of an intensive multilocus genetic analysis.  The proposed name must be first published before being utilized, but it is welcome news, as we know the dollar spot pathogen is not a Sclerotinia because it doesn’t produce sclerotia.

Copper spot is an odd disease that we notice on our bentgrass research plots, but is very rarely seen on golf course putting greens because it is controlled by so many commonly used fungicides.    Although there are reports the pathogen can cause diseases on other grasses (particularly sorghum), I’ve only seen copper spot on bentgrass in this region, and normally mixed in with more prevalent dollar spot.

Leaf spot and melting out caused by Drecshlera spp. are observed a bit more often on bentgrass and Kentucky bluegrass than copper spot, but less so than dollar spot.  This pathogen is closely related to Bipolaris & Exserohilum which also cause similar leaf spots, so much so that they used to all be grouped together as Helminthosporium.  Semantics aside, Drechslera caused significant melting out event on a low mow Kentucky bluegrass golf course last year, and has been noted at the MU turf farm and on one golf putting green in 2013.  Red leaf spot is the most common disease on bentgrass and occurs in warm, wet periods.  Spots are brownish-red in color, often coalesce, and can look very similar to anthracnose outbreaks. 

The common cultural control thread of all three of these foliar diseases is the need to reduce the period of leaf wetness as much as possible.  Mowing, rolling, irrigating, or any other dew removal practice in the morning will reduce the severity and amount of these diseases considerably.  These diseases also can be indicators of excess or deficient nitrogen.  Much like brown patch in tall fescue, leaf spots are much more severe in over-fertilized Kentucky bluegrass areas.  For this reason, it’s important on Kentucky bluegrass to go light on spring fertilization.  On the flip side, a significant dollar spot outbreak can be a sign of insufficient nitrogen.  So in Kentucky bluegrass at least, dollar spot severity in the fall may be mitigated by a nitrogen application. 

For bentgrass/nitrogen/spot disease, low dose N spoon-feeding of putting greens during the season takes precedent over disease control, and for this reason fungicides are often necessary.  At MU, we’ve conducted several trials on the lasting effects of early spring fungicide applications on disease control on putting greens.  Although it doesn’t necessarily mimic the programmatic approach necessary in most golf course situations, the trials do give us an idea of the impact of timing and which fungicides may be strongest on particular diseases.   Watered-in Emerald and Bayleton applications in early spring (April and/or May) have afforded the longest residual dollar spot control in our studies over the past 3 years, which corresponds with many other university findings. For Drechslera leaf spot, a single application of Honor at the 1.1 oz/M rate watered-in on 5/8/13 on our ‘Penn A4’ research green exhibited the longest residual control compared to single applications of Emerald (0.18 oz/M), Insignia SC (0.7 fl oz/M) or Bayleton FLO (1.9 fl oz/M). 

Thank you for your support of the program, and have a good weekend.

Lee Miller
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Extension Turfgrass Pathologist
University of Missouri