Spring dead spot on a non-treated control plot during a trial at Nutters Crossing Golf Course in Salisbury, Md. Photo courtesy of PBI-Gordon
Editor’s note: The following article was supplied by PBI-Gordon. All product claims, research cited and other information is directly from the company.
Though summer 2018 was hot, humid and wet throughout the Southeast and Mid-Atlantic states, turf diseases such as Pythium root rot, brown patch and mini-ring decrease as temperatures drop and we head into winter. But for superintendents managing common and hybrid bermudagrass (Cynodon dactylon; C. dactylon × transvaalensis), fall is a crucial time of year in which they must start monitoring soil temperatures and begin timely, preventive fungicide applications for spring dead spot (SDS).
The what, how and why of spring dead spot
Spring dead spot is a disease caused by three ascomycete pathogens: Ophiosphaerella korrae, O. herpotricha and O. namari. In the United States, the most commonly found species in the Southeast and Mid-Atlantic is O. korrae, while O. herpotricha is most often found in the Midwest.
The primary differences between the three species are morphological, and for the purposes of this article, we will focus more on the ramifications of damage from these pathogens and on control options than on fungal nomenclature and species identification. Nonetheless, it is critical to understand the biology of the pathogen that causes a disease in order to know how to effectively control it.
The pathogens that cause SDS do not act in the traditional manner of many common diseases, such as Pythium blight caused by Pythium species. With Pythium blight, the species of Pythium actively attacks and kills the tissue to procure nutrients, resulting in rapid death of the plant tissue. This is not the case with SDS. Instead, the pathogen causing SDS weakens the root system and crown tissue, which reduces winter hardiness and results in loss of turf upon spring green-up.
When soil temperatures in fall drop to approximately 70 F, pigmented mycelium of Ophiosphaerella species begin to colonize the outside of the roots, rhizomes and stolons of bermudagrass host plants, and they continue to do so until soil temperatures drop to about 50 F. These fungi are the same type observed with the “patch” diseases and are formally termed “ectotrophic root-infecting fungi.”
The pathogen stays dormant underground during adverse conditions and will cause disease in the same spots year after year. The ectotrophic (or runner) hyphae — which “run” along the surfaces of the aforementioned subsurface plant structures — procure nutrients from the host tissue but do not kill the host plant. Therefore, if the plant is weakened and not equipped to survive the low temperatures of winter, the affected tissues will die.
Understanding how and when the fungus attacks the turfgrass plant is only half the battle. Vigilance is required in monitoring soil temperatures, particularly at the 2-inch depth. Because the pathogen is particularly active after soil temperatures drop to approximately 70 F at the 2-inch depth for a period of five to seven days, most university researchers have found that fall is an ideal time for a fungicide application.
“In Maryland, we recommend initiating fungicide applications once soil temperatures reach 70 F at a 2-inch depth for five days. These recommendations are based on previous research performed at the University of Maryland by Crahay and Dernoeden,” says Joseph Roberts, Ph.D., assistant professor of turfgrass pathology at the University of Maryland.
Research has also found that a single application of a fungicide is typically not an effective means of control. A follow-up (or second) application should be made while the fungus is still active before soil temperatures drop below 60 F at the 2-inch depth for a period of five to seven days. This often coincides with about a three- to four-week calendar period, but not always, especially when an unseasonably warm fall hangs on for an extended period of time.
According to Jim Kerns, Ph.D., associate professor and extension specialist in turfgrass pathology at North Carolina State University, reapplication is key to control. “Usually 28-day intervals are very effective, but reapplication intervals should be based on soil temperatures,” Kerns says. “The window for infection is between 55 F and 70 F. Therefore, a 21-day reapplication may be necessary if a dramatic cool down occurs after the initial application.”
Fungicide application for spring dead spot
Because SDS is not caused by a foliar pathogen, an understanding of how to properly apply the fungicides needed to prevent it is important. Ophiosphaerella species colonize the roots, rhizomes and stolons, so it is imperative to get the fungicide down to where the pathogen is located.
Carrier volumes of 2 to 4 gallons/1,000 square feet are recommended, with at least 1/8-inch to 1/4-inch irrigation immediately following application. You may be thinking, “That’s way too much water — it’ll move past the root zone!” But this may not be the case according to recent work being done at North Carolina State University.
“Based on our research at North Carolina State, the optimal amount of post-application irrigation is 1/8-inch when conducted immediately after the fungicide application,” says Kerns. “If turf managers do this, then they can spray in a carrier volume of 2 gallons per acre. Our research clearly shows that fungicides do not readily infiltrate through soil; therefore, leaching them past the pathogen is highly unlikely. It is more likely that the fungicide could move laterally when too much water (less than 1/4-inch) is applied.”
Fungicide options to control SDS are a pretty exclusive group. There is currently only one class of fungicides that offers reliable control: succinate dehydrogenase inhibitors (SDHIs).
SDHI fungicides fall under FRAC Group 7 and are also widely used for control of dollar spot. Currently, there are two SDHI chemistries that are used as stand-alone compounds to provide control of SDS: isofetamid and penthiopyrad. There is also a third SDHI chemistry, fluxapyroxad, which is used conjunction with a QoI fungicide, pyraclostrobin.
Isofetamid and penthiopyrad have been evaluated in numerous university fungicide evaluations and have both shown superior control compared with a frequently used industry standard, tebuconazole (a DMI fungicide). Isofetamid is used at rates much higher than those used for dollar spot control, but the results observed with it over a three-year period have been consistent across research trials at various universities, particularly with split applications at 1.6 fluid ounces per 1,000 square feet, followed up to 21 to 28 days later with another application at 1.6 fluid ounces per 1,000 square feet.
Kerns has seen these results firsthand. He notes, “SDHIs such as penthiopyrad and fluxapyroxad + pyraclostrobin have become standards for spring dead spot control. They have performed quite well in our trials. Isofetamid has performed exceptionally well in the past two years we have worked with it. It was the only treatment in our trials that provided complete suppression of spring dead spot.”
In his trials in Maryland, Roberts has seen the same trend. “Over the past few years in the Mid-Atlantic, we have observed positive results with SDHIs applied to control SDS. Isofetamid and penthiopyrad have provided the most consistent control in the trials we have tested when applied per label recommendations,” he says.
Although Rubigan (fenarimol) is no longer available, there are new, highly effective active ingredients representing FRAC Group 7 that offer consistent control of SDS and that far exceed the control observed with tebuconazole. While they do not compete in price with tebuconazole, they vastly outcompete tebuconazole in results.
But focusing on which chemistry to use should not be your sole concern. When to use that chemistry, how to apply that chemistry, and understanding the organism to which you are applying that chemistry are all integral parts of a spring dead spot management strategy to combat this frustrating disease as temperatures drop during fall.
Brian Aynardi, Ph.D., is the Northeast manager of university and contract research for PBI-Gordon.