Fall is nearly complete, and bermudagrass sports fields are managed heading into dormancy. Here are some tips on how to extend bermudagrass color in the fall and get it back crisper in spring by prepping for winter dormancy.
Although there has been recent research suggesting that the honored dictum of late-season nitrogen fertilization of bermudagrass negatively affects cold tolerance, separate studies by Richardson at Arkansas, Munshaw at Mississippi State University, and Goatley and Ervin at Virginia Tech demonstrated that late-season nitrogen fertilization prolongs green color of bermudagrass before dormancy and promotes earlier green up in the spring.
Applications of 0.5 to 1 pound nitrogen per 1,000 square feet every two to four weeks prior to frost-causing dormancy are recommended. Be mindful to use sources such as urea, SCU, IBDU, ammonium sulfate or the methylene ureas. Microbial activity will be lower when temperatures decrease, and so will nitrogen release in fertilizers that require it. Also if you have a sand-based field, application rates should be lower to reduce the potential for nitrate leaching.
With all of this nitrogen flying around, a snap of high temperatures in the fall would also send bermudagrass growth flying. Studies from Richardson at Arkansas have also shown that there is nothing wrong with regulation. Trinexapac-ethyl (i.e., Primo MAXX) applications continued throughout the fall season until dormancy not only extended green color, but also increased the freeze tolerance of bermudagrass rhizomes at modest freezing temperatures (28 and 24 degrees).
So what about the phosphorus (P) and potassium (K)? Research suggests relying on soil tests to guide these applications. Potassium, in particular, has been billed as the ticket to increased bermudagrass cold tolerance and a corresponding reduction in spring dead spot development. There is a limit, however, and bermudagrass on soils with moderate to high potassium levels would not gain a winter hardiness advantage with a fall K application. A soil test in early fall is helpful in determining what the current levels are, and if any additional P or K is necessary.
Most troublesome weeds in turfgrass are attempting to store carbohydrates in the fall, rather than producing new leaf growth. Fall herbicide applications are therefore more effective than those applied at other times of the year, since more is transported down to the vital crown of the weed rather than up towards the leaf tips. Targeted herbicide applications, and even a weed-and-feed-type product, make more sense during fall than at any other time of the year. (Be careful if overseeding since many phenoxy herbicides require a three to four-week seeding interval).
When bermudagrass growth starts to slow, it would be wise to raise the mowing height up closer to 2 inches. This slight increase may allow leaf blades to penetrate through a potential ice event and allow oxygen to the crowns and rootzone more quickly.
Not everyone can stand the color brown or winter-long ruts from linemen, so overseeding with perennial ryegrass is a necessary evil. It is important, however, not to compound the detriment on bermudagrass with excessive seeding rates. A wise colleague from Chicago once said, “Sow one, seed one and leave one for the birds,” but this does not apply when overseeding. Seminal research conducted by Dr. Brad Fresenburg showed perennial ryegrass produced allelopathic compounds that inhibited bermudagrass growth. For this reason, it’s important to limit the spreader to not above 8 pounds of perennial ryegrass seed per 1,000 square feet, as higher seeding rates were detrimental to spring transition and bermudagrass stand. If overseeding isn’t working out well, consider experimenting with colorants.
Spring Dead Spot Prevention
Spring dead spot is the most important disease of bermudagrass that impacts its utility for sports turf use. The pathogen, Ophiosphaerella spp., infects bermudagrass roots, rhizomes and stolons, and doesn’t allow for suitable carbohydrate accumulation and proper dormancy. Because this infection period is during the fall, the fall is also the time preventive measures need to be taken.
It’s first important to assess the location and severity of spring dead spot in your fields. Since the pathogen is soilborne, it can also be quite localized. Therefore, mapping the affected areas during the spring and only targeting prevention to them in the fall can reduce management costs. Also, if the severity isn’t too bad in play areas, perhaps cultural methods such as the aforementioned fall fertilization practices and summer aerification can reduce incidence.
In areas that are heavily infested and require treatment, fungicide applications can be inconsistent and a multiyear approach may be required for complete symptom alleviation. Fenarimol (Rubigan) was the standard for spring dead spot control, and even at high rates and multiple applications symptoms were reduced only 60 to 80 percent in the first year. The DMI fungicides (Eagle, Banner Maxx) and some QoI fungicides (Heritage, Disarm) are currently recommended for control, but require two high-rate applications in the fall, followed by another potential curative application in the spring. At this time, another fungicide currently labeled for golf, tebuconazole (Torque), is not labeled for sports fields (particularly school fields).
This difficulty in controlling spring dead spot has been the impetus for numerous research projects at the University of Missouri. Derek Cottrill, master’s student in turfgrass pathology, has been working on the evaluation of cultural and chemical management practices for over two years and will present his findings at numerous conferences over the winter months. In addition, we have collaborated on a project through the National Turfgrass Evaluation Program (NTEP) to assess spring dead spot resistance in 35 cultivars (17 seeded, 18 vegetative) of bermudagrass. Host resistance would be a key management tool for spring dead spot, and would reduce or eliminate the headaches involved with inconsistent control. In the case of spring dead spot, the adage has been that if the cultivar has increased cold tolerance and doesn’t winterkill, then it also has increased spring dead spot resistance. We look to put this theory to the test by inoculating each plot with the spring dead spot pathogen and determining if the cultivar can resist infection and disease damage.
Although fall may not seem to be the time to focus on bermudagrass, paying attention to the final details before winter can pay big dividends the following spring.
Dr. Lee Miller is extension turfgrass pathologist at the University of Missouri. Dr. Brad Fresenburg is assistant extension professor at the University of Missouri. This article originally appeared in The Gateway Sports Field Manager fall issue, and is reprinted with permission.