Summer stress temperatures arrive; Drought symptoms arrive in a flash; Leaf spot diseases prevalent in lawns this season;
Patch diseases and the role of rhizosphere pH in their suppression.
Weather
There summer blows! Like a sudden whale burst from the sea, the heat dome last week pushed summer into overdrive with the hottest temperatures of the season thus far in the region. After a mild start to June, high temperatures were 8 – 10+ degrees F over average for the span from 6/17 – 6/22 in Indianapolis. Highs in the 90s raise alarm bells for cool-season turfgrasses but lows in the mid 70s are the real kicker, not allowing soil temperatures (particularly in sand-based putting greens) to dip below the mid 80s. In addition, the water spigot turned off suddenly as only an inch or less of precipitation has fallen in middle to southern Indiana in June. Much of Indiana now rests in the abnormally dry category of the US drought monitor (https://droughtmonitor.unl.edu) extending into both eastern Illinois and much of Ohio. Many non-irrigated lawns have incurred a “flash drought” as noted below.
The bright side, or less bright side, is that the summer solstice has come and gone, and day length and summer turfgrass stress should start getting incremently shortly. Also fortunately, forecasts indicate a reprieve from the drought and the return of some fairly significant rainfall chances mid-week and scattered opportunities into the July 4 holiday. Indiana should also get a brief temperature dip for a few days in late June with a return to the 90s next week. I wish we could remove a few degrees and a lot of humidity today in St. Louis, which unfortunately are slated to endure 100+. Break out the iced towels my friends; survive and advance.
Lawns: Flash Drought
The sudden decrease of precipitation, which was sometimes excessively granted in the wetter than normal months of April and May, has caused a flash drought throughout much of the region. A number of factors play a role in flash drought development, including abnormally high temperatures such as last week’s heat dome, high winds and the longer photoperiod and more intense radiation in the summer months.
Another important aspect of flash drought development and survival is the plant, as a combination of species composition and the impact of growing conditions. Rough bluegrass (Poa trivialis) infestation is a conspicuous indicator of drought conditions, since its shallow rooted, stoloniferous growth habit results in rapid decline and frequent submissions to the diagnostic lab (several in the last two weeks). Conversely, turf type tall fescue survives drought with the most extensive and deep root system of cool-season turfgrasses. While maligned because it doesn’t stay as green for as long as tall fescue, Kentucky bluegrass also employs drought tolerance by turning brown and going truly drought dormant to protect the crown and recover when water returns. The age of the turfgrass stand matters, particularly if sodded, as newly sodded tall fescue can survive and withstand drought stress more than Kentucky bluegrass (see here for research). Species and cultivar composition also matter, as tall fescue and tall fescue:Kentucky bluegrass mixtures reduce irrigation needs along with using more improved drought resistant varieties. In this Purdue led research by Powlen et al, irrigating on a 70% green coverage threshold, using tall fescue alone or in combination with Kentucky bluegrass and planting improved drought resistant cultivars resulted in significant reduction in irrigation needs and provided good turfgrass quality compared to a routine weekly irrigation schedule. This study, as well as others, demonstrate allowing the lawn to go a little brown can conserve water without long lasting effects on lawn health.
Last but not least, total lawn health is confined to the soil underneath it. Heavy clay soils left behind after home construction have very little pore space, so will waterlog readily during spring rains and not allow roots to extend properly. When these soils do dry, they dry quickly and leave no water for the shallower roots to exploit and use for growth. Remember these lawns later this fall and target them with aerification and perhaps addition of quality topsoil or compost to reduce this underlying stress in the future.
Lawns: Thatch Accumulation
A few samples of Kentucky bluegrass lawns with considerable thatch accumulation have been submitted to the diagnostic lab in the last few weeks. Some of these lawns are also afflicted with summer patch (as noted below) but the long-term health fix lies in dethatching and getting rid of some of this organic matter. Since Kentucky bluegrass is rhizomatous, thatch accumulates more rapidly than a pure tall fescue, restricting water and nutrient passage to the soil and restricting deeper root penetration and establishment. In addition, thatch provides a great nest for aspiring pathogens. A verticut (i.e. dethatcher or “power-rake”) and/or core aerification on heavily thatched lawns is suggested in the fall.
See Spot Run
Various leaf spot diseases have been observed on both Kentucky bluegrass and tall fescue in the last two weeks with symptom severity ranging from taking advantage of older senescing leaves to completely melting out areas of the stand. These diseases are caused by the traditionally termed Helminthosporium pathogens, which have now been broken up into Bipolaris, Exserohilum, Curvularia, and Pyrenophora (or Drechslera) spp. Observations of severe damage due to this suite of diseases is normally on Kentucky bluegrass, particularly those mowed too low. This season we are also observing the disease on tall fescue mowed at 3 – 4 inches, which may be a testament to our very wet spring and quick snap into hot summer temperatures. Coincidentally, tar spot, the newly emerging disease that has caused problems in corn production, has also been observed earlier this year than in any previous.
This disease traditionally hasn’t warranted directed treatment. The most severe disease occurred right after the lawns went to seed, a time of metabolic stress with resources spent creating the seedhead. An application of either a slow release fertilizer at higher N rate or a small amount (0.25 lb N/1000 sq ft) of a fast release t like urea may encourage recovery. If a chemical application is needed, the QoI fungicides such as azoxystrobin and pyraclostrobin are effective and may double for control of brown patch on tall fescue.
Litmus Test – Patch Diseases & Stress
The first round of stressful summer temperatures provides the first litmus test for true spring development and health of cool-season turfgrasses. Drainage, localized dry spot and other soil problems on golf putting greens reveal themselves quickly. Also, the steady spring gnawing of soilborne pathogens such as take-all on creeping bentgrass fairways and summer patch on Kentucky bluegrass sports fields, roughs and lawns show their true colors and patterns when the heat and humidity ratchet up.
If prevention of these patch diseases is no longer an option, curative control relies on watering in a fungicide to stop the pathogen and adding a bit of nitrogen to encourage recovery. On native soils, adding a small amount of ammonium sulfate fertilizer (0.25 lb N/1000 sq ft) aids not only plant growth but also plays a role in further pathogen suppression. When ammonium is acquired by the roots, they kick out a hydrogen ion and in doing so change the rhizosphere pH acidic. In this more acidic environment, the plant is better able to acquire nutrients needed for plant defense and reduces the ability of patch disease pathogens to infect.
On the flip side, application of ammonium sulfate on golf putting greens during the summer is not recommended. In this system on a sand-base soil profile designed for quick drainage, the effect on rhizosphere pH from ammonium sulfate is likely more short-lived than on natural soils. Additionally, application of sulfur-based fertilizers can result in black layer formation caused by anaerobic sulfur reducing cyanobacteria. Therefore, ammonium sulfate on golf greens should be primarily applied in the spring and fall.
Purdue Turfgrass and Landscape Field Day – Tuesday, July 16th
The Purdue Turf and Landscape Field Day is an annual one-day event with the objective of providing professional turf and landscape managers exposure and educational opportunities with the latest research and technical resources. Field Day features research tours, talks on current topics, and a tradeshow with over 40 exhibitors displaying equipment, turf, and landscape products.
Lee Miller
Extension Turfgrass Pathologist – Purdue University
Follow on Twitter: @purdueturfpath
