GOLF: How do you determine when to stop spraying for foliar diseases like dollar spot on putting greens?

Stop getting clippings
When leaves fall off trees
calendar date (mid October - mid November
When high temperatures fall below 50-60 F
When soil temperatures fall below a certain range
After the first frost

Update (10/05/2018)

Summer Snarl Showing in September

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Above Normal Temps in Fall… Again

  1. September temps moderate late, but still above average. - Source: Missouri Climate Center
  2. After warm start to October, forecasted temps expected to drop mid month. - Source: NOAA

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

  1. Not much rain in the last 7 days to aid reseeding efforts. - Source: Missouri Climate Center
  2. Good precipitation chances in the next 6 - 10 days. - Source: NOAA

Quick Hits

Active Pythium on September 20

  1. Pythium blight observed on seedlings planted in early September.
  2. Cottony web mycelium observed in the early morning.

  • Pythium Blight in September?! – Seeding and sodding of cool season grasses is often suggested for early to mid-September to avoid the high heat stress of summer conditions and the associated disease problems associated with damping off diseases caused by Pythium and Rhizoctonia spp. When September keeps throwing out high 80 – 90 degree temperatures, however, the diseases obviously don't pay mind to the calendar. In the last two weeks, I have received samples from sod farms, athletic fields, and home lawns with considerable damage to newly planted tall fescue and Kentucky bluegrass from Pythium blight (on seedlings more aptly named damping off). This disease is a water mold, and the oft needed irrigation cycles to enable germination and establishment in the high heat are a perfect recipe for this disease. High rates of nitrogen to spur seedling growth also can spur these diseases, which is why we choose September over April or May for cool season turfgrass establishment in the first place. With temperatures set to decline, damping off diseases should abate and fungicide application may not be necessary. If seeding is necessary prior to warm temperatures, applications of a QoI (azoxystrobin, pyraclostrobin, fluoxastrobin, etc) and/or a Pythium fungicide (i.e. mefenoxam, propamocarb, etc) may be essential to grow seedlings disease free. For now, overseeding the diseased areas may be the best bet now that fall temperatures are a bit more in view.

Gray Leaf Spot Observed in STL

  1. Newly seeded areas of tall fescue are extremely susceptible to gray leaf spot.
  2. Symptoms include a distinct spot with a grayish interior (duh) and a "shepherd's hook" curling of the leaf tip.
  3. Numerous teardrop shaped spores of the pathogen allow rapid spread.

  • Gray Leaf Spot on Tall Fescue – Another disease that was warned about, did come to fruition last week as a sample of gray leaf spot on tall fescue was sent into the lab from a lawn in St Louis. This is another disease that prefers to dine on seedlings, but particularly on perennial ryegrass has no problem also taking out the adults. In this region, gray leaf spot can begin in early August but in summer-like falls like this one, pressure can extend well into September and early October. Again, this disease is reportedly exacerbated by excessive nitrogen application so the "normal" early – mid September fertilization practices can drive further damage from this disease. In areas with a history of the disease, preventive applications of thiophanate-methyl on 21 d or T-M plus a QoI on 28 d intervals starting in late July – early August may be necessary on susceptible tall fescue varieties. This disease is very spotty in its occurrence in this region and a vigilant scouting and early detection approach may be a more responsible course of action. Note that thiophanate methyl does not work on brown patch of tall fescue, and there is some QoI (i.e. azoxystrobin) resistance in the gray leaf spot pathogen. Therefore, gray leaf spot can slip through the cracks if azoxystrobin is solely used for disease control on tall fescue. Two varieties of tall fescue ('Coyote' and 'Coronado') have some resistance to gray leaf spot, and might serve well alone or in part of a blend to combat this disease.

Leaf Spot/Melting Out Contributes to Cool Season Decline

  1. This perennial ryegrass/Kentucky bluegrass field was impacted by leaf spot.
  2. Conidiophores of Drechslera spp. cover leaf blades and facilitate rapid pathogen spread.

  • Leaf Spot on Athletic Fields: On the western side of the state, yet another disease normally reserved for June – August severely impacted an athletic field. Leaf spot/melting out caused by Drechslera spp. were observed on a hodge podge field of Kentucky bluegrass, perennial ryegrass, and tall fescue. Of these three varieties, the Kentucky bluegrass and perennial ryegrass were particularly hammered with this disease. Again heavy nitrogen use (quick release urea) was used to recover from athlete traffic, and the field looked fine prior to that application. Other factors were also involved in this field (gray leaf spot on the perennial ryegrass, poor rooting and soil conditions), so assess the total field condition by examining the root structure in a few cores prior to dumping a load of nitrogen and expecting a perfectly normal turfgrass response in an abnormal growing environment. Last but not least, preventive fungicides curtailed in July or early August aren't going to protect turfgrass during a September summer.

High Lance Nematode Counts in Bentgrass Putting Greens

  1. Lance nematode are large, and feed both inside and outside bentgrass roots.
  2. Chlorosis & thinning of bentgrass putting green associated with a very high lance population.

  • Lance Nematodes – Several golf superintendents are sending in greens soil samples to assess nematode levels and the efficacy of their management programs. Within the last few weeks many of these have returned high, over threshold, counts of lance nematodes (Hoplolaimus galeatus). The most egregious of these, a sample from St Louis, had counts of 9,000 + nematodes/100 cc of soil! The bentgrass was chlorotic, thinned, and feeling the effects of these high counts. To put this in perspective, during my microscopic analysis of the roots, four lance nematodes were observed burrowing into one root section approximately 12 mm in length. As observed in the above photo, lance nematodes are proportionately large (adults are 1.5 mm in length) and the second biggest plant parasitic nematode. They feed both outside the root (ectoparasitic) and inside the root (endoparasitic) which makes control difficult. So difficult in fact, that turfgrass researchers don't have a highly effective recommendation on bentgrass greens for control. Fluopyram (Indemnify) doesn't work on them. Abamectin (Divanem) needs to touch them to kill them, and with a high organic matter adsorption characteristic and the nematode burrowing inside the root this is difficult. Last, results with fluensulfone (Nimitz G) have been varied, and in high temperatures may cause mild bentgrass phytotoxicity. The lance nematode may be a lasting problem, and management may need to rely on improving turfgrass root health through cultural practices and control of other root diseases to enable bentgrass to tolerate damage.

Where's the Fall Stop Sign?

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.

Where's the Fall Green Light?

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.


Lee Miller
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Extension Turfgrass Pathologist
University of Missouri