Above Normal Temps in Fall… Again
Like the 80s Whitesnake song, September had a "Here I Go Again" feel to it. Average temperatures were 4-5 degrees above normal in the region, with a large spike in the middle of the month that felt much more like summer than fall. After a fall-like dip in late September that brought monthly temperatures back into reason, the first three days of October (and yesterday's 90 + highs) brought average temperatures 15+ degrees above normal. As discussed in the previous update, these warm falls are becoming quite a habit, and at some point we may need to just consider them as our routine climate. A cool front is descending today, and the forecast through the middle of the month shows a good chance that cooler fall temperatures may return soon.
Rainfall totals over the past 7 days have been meager, and most of Missouri and northern Arkansas have experienced a deficit of 1 – 2" of rain. This along with the warm temperatures have been tough on September seeding efforts and cool season turfgrass recovery. The shorter photoperiod while a grace, has not been a savior as typical summer time disease and cool season turfgrass stress was observed throughout much of September. Rainfall chances are very good over the next few days, with decent storms traveling through the southern parts of the region earlier today. Hopefully this cold front will bring sustained cooler temperatures and consistent moisture to mark the true end of a long summer (hottest May – July, and 3rd warmest May – August on record). As shown below, turfgrass problems pay attention to the thermometer and not the calendar.
The Seed Needs the Rain
Active Pythium on September 20
Gray Leaf Spot Observed in STL
Leaf Spot/Melting Out Contributes to Cool Season Decline
High Lance Nematode Counts in Bentgrass Putting Greens
Smith-Kerns Model for Dollar Spot Prediction*
According to the 20% model threshold, late September afforded a marked break in dollar spot pressure in the region like we haven't seen since early May.
*Model based on 5-day rolling averages of air temperature and relative humidity.
With all of the previous discussion of summer diseases that won't stop, our minds (and also likely our hearts) may be wondering when disease control can stop. Our focus tends toward the spring beginnings, and whether it's ignorance or pure fatigue, often forget that saving time and expense can also occur in knowing when the end is near or upon us. As intimated continually, fall in this region has not behaved like it for the last four years now, and relying on the same old calendar as a stop sign has led to incorrect management decisions.
The new Smith-Kerns model for dollar spot prediction might be a step in the right direction. Dollar spot has the widest seasonal infection window of our diseases, and is normally the first foliar disease to occur in the spring with verve and the last to leave the bar in fall. While of course crucial for predicting the first dollar spot wave, the model's use may be similarly critical in our region in fall, and may serve as a flickering of the lights to get patrons to leave.
Model results throughout the season from April to present are shown above. The 20% threshold shown is taken from estimates at the University of Wisconsin and may need to be adjusted here. However, if using this threshold, it's obvious that dollar spot pressure subsides rarely in this region, being driven primarily by our very high humidity. In fact, we may need to protect against dollar spot throughout the season, and reducing a fungicide application or two mid-season might not be an attainable goal. Instead, we might be able to slice an application off at the end using this model, or at very least be able to time a last strong dollar spot fungicide in fall to put greens happily to bed.
Soil Temperatures & Warm Season Turfgrass Disease Prevention
Late September cooled off enough that soil temperatures in most of the state reached the 70 F threshold for warm season disease prevention. Early October temps, however, snapped right back.
*Note the lack of variability in the STL readings. A layer of mulch was inadvertently placed over the site of the soil temperature probe.
On the flip side of this story, is when to start fall prevention of warm season turfgrass diseases? Recent results suggest the old calendar-based spray of September 15 & October 15 wasn't as effective as applications made later in the fall. Without a true model, perhaps the best threshold lies in monitoring the fall decline soil temperature. Like the dollar spot prediction model, prevention of soilborne diseases on cool season grasses focuses on the spring warmup of soil temperatures to key growth/infection periods of fairy ring, take-all patch, and summer patch pathogens. Fall applications for spring dead spot of bermudagrass and perhaps large patch of zoysiagrass may also be best timed with a 70 F soil temperature threshold in the fall.
The graphs above show the current status of soil temperatures in the region since August 1. A first dip in temperatures first occurred around September 12th, but was very short in nature. Particularly in Kansas City, a more sustained decline occurred later from September 24 -29, which despite the early October heat wave may been the appropriate first application timing for much of the region (with the second 21-28 days later). Note that down south in Springfield, particularly with the 5-day average being considered, a solid dip into the threshold range hasn't been reached yet.
As stated in the previous update, these aren't your Grandpa's falls any longer. With the extreme variation in fall temperatures, appropriate timing and maximization of disease control efforts is more difficult. Relying on the calendar might not be the best recourse to achieving results. Monitoring the weather and adapting to its changing story should be utilized in not only managing turfgrass pests, but also managing turfgrass health as a whole.
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Extension Turfgrass Pathologist
University of Missouri