By Brad Fresenburg, PhD, Lee Miller, PhD & Daniel Earlywine
Spring Dead Spot (SDS) is a problematic disease of bermudagrass sports turf managers have dealt with for many years. SDS is the most severe pest issue on bermudagrass in Missouri, and other regions where bermudagrass goes into dormancy. SDS is caused by a soilborne pathogen (Ophiosphaerella spp.) that infects roots, rhizomes and stolons as the plant goes into dormancy in the fall. During spring green-up, infected patches are dead and simply appear to remain dormant. Recovery can occur from the uninfected areas around those patches by shooting out rhizomes and stolons and oftentimes weed emergence in the affected areas occurs before complete recovery. Since the pathogen is soilborne, control is difficult, particularly in a curative situation. Current control recommendations rely almost exclusively on fungicide use. The effectiveness of these fall preventive applications can be sporadic, and recovery from an existing epidemic may be a multi-year process.
Integrating cultural practices with fungicide use may be a method to reduce disease. Observations indicated that newly established bermudagrass would not show signs of SDS until usually the third or fourth season. Additionally, high use areas, such as between the hash marks on football fields, often showed less spring dead spot severity than low traffic areas. This indicated thatch was perhaps a factor in SDS development so recommendations included thatch management by means of vertical mowing and aeration. The common practice for those who had the disease visible during spring green-up was to rake out the spots and fertilize to promote regrowth and recovery of the bermudagrass.
Several research trials have indicated hollow-tine aerification or other cultivation methods may reduce SDS severity and increase fungicide efficacy. Recently, a new intense surface cultivation method termed “fraze mowing” gained popularity as a method of thatch removal and playing surface renovation in sports fields.
The key to fungicide efficacy for SDS is to get the fungicide where it is needed. Therefore, the objective of this research is to determine the impact of fraze mowing on SDS severity, and determine how this practice can be implemented in an integrated pest management plan for this disease.
Trial 1 was initiated on 22 July 2014 at the MU Turfgrass Research Farm on a ‘Riviera’ bermudagrass block severely infested naturally with SDS caused by O. herpotricha. Plots were 5 x 10 feet and arranged in a randomized complete block with four replications. Treatments were arranged in a split plot design with fraze mowing as the main plot and nitrogen source as the subplot. Before treatments were applied, an initial disease rating was conducted on 24 June 2014 to assess treatment effects. Fraze mowing was conducted on 22 July 2014 at 0.16 and 0.32 inches with a KORO Field Topmaker or not cultivated. To encourage bermudagrass regrowth, ammonium sulfate or urea was applied weekly at 0.5 lb N/1000 ft2 for 6 weeks after fraze mowing. SDS severity and green cover were evaluated every 14 days by visual estimation of percent disease area and digital image analysis, respectively. Area under the disease progress curve (AUDPC) was calculated with the trapezoidal rule. All data were subjected to analysis of variance, and where applicable, means were separated with Fisher’s Protected LSD (0.05).
These same plots were fraze mowed again on 30 June 2015 and evaluated for a second season.
Trial 2 was initiated on 30 June 2015 at the MU Turfgrass Research Farm on a block of ‘Patriot’ bermudagrass. The entire 11,000 square foot block was inoculated with a four isolate mix of Ophiosphaerella herpotricha on 13 September 2013. Initial disease symptoms from inoculation occurred in late May 2015. Plots were 5 x 10 feet and arranged in a randomized complete block design with four replications. Treatments were arranged in a split plot design with fraze mowing as the main plot and nitrogen source, manganese, and fungicide application as subplots. Fraze mowing was conducted on 30 June 2015 at 0.16 and 0.32 inches with a KORO Field Topmaker or not cultivated. Dissolved ammonium sulfate or calcium nitrate was applied weekly at 0.5 lb N/1000 ft2 for 6 weeks after fraze mowing. Manganese treatments were applied every other fertilizer application (3 times @ 2 week intervals) as 2 pounds manganese sulfate/A. Nitrogen and manganese treatments continued in summer 2016 with 1 pound N/1000 ft2 and 2 pounds manganese sulfate/A applied monthly. Fungicide treated plots were sprayed with Velista (a.i. penthiopyrad) at 0.7 oz/1000 ft2 immediately after fraze mowing and again on 14 October 2015. Treatments were immediately watered in with 0.2 inches of post-application irrigation. SDS severity and green cover were evaluated every 7 days by visual estimation of percent disease area and digital image analysis, respectively, in Spring 2016. Area under the disease progress curve (AUDPC) was calculated with the trapezoidal rule. All data were subjected to analysis of variance, and where applicable, means were separated with Fisher’s Protected LSD (0.05).
Trial 1: Fraze mowing at either 0.16 or 0.32 inches did not statistically increase or decrease SDS severity in Spring 2015 compared to the initial rating. Plots treated with ammonium sulfate had lower SDS severity than urea on two of the four rating dates, reducing disease by 12-22% from the initial 2014 rating. Over the initial season of study, fraze mowing did not increase or reduce an established SDS epidemic. Even after year two, results for this smaller initial trial were inconclusive.
Trial 2: Fraze mowing at 0.32 inches alone and in combination with other treatments decreased SDS severity compared to non-fraze plots. Fraze mowing breaks up the thatch and mat layer, allowing new growth of rhizomes and stolons to occur. This new plant tissue should be pathogen free, and if protected from new infection should lead to recovery.
The evaluation of various nitrogen sources (calcium nitrate or ammonium sulfate) in this study was aimed at manipulating soil pH, which has been observed to play a role in the reduction of SDS. When ammonium (positively charged) is absorbed by plants, hydrogen ions are released in the rhizosphere; therefore, creating a more acidic environment near the roots. When nitrates are absorbed by plants, the exchange is with hydroxyl ions; therefore, raising the pH to a more basic environment. In most research, lowering the pH with an ammonium fertilizer source has subsequently reduced SDS severity.
In this trial, sprayable ammonium sulfate reduced SDS severity in the unfrazed plot compared to calcium nitrate, but no significant differences were noted among fraze-mowed plots. In a previous trial (Cottrill et al. 2016) at the site, ammonium sulfate did not reduce SDS on plots that did not receive aerification or fraze mowing. This seems to support the theory on acidic rhizosphere conditions immobilizing SDS, and the potential need to integrate ammonium fertilization with an aerification or thatch removal practice such as fraze mowing.
Manganese is a micronutrient needed for enzymes involved in the lignification of cell walls, a defense mechanism against diseases. Previous work has shown that the take-all patch pathogen (another soilborne disease) changes the form of manganese to make it unavailable to the plant root. This manganese deficit results in the plant being susceptible to infection. In these cases, and particularly in high pH or manganese deficient soils, supplemental manganese applications have resulted in lower disease severity not just for take-all patch, but also for summer patch on Kentucky bluegrass. After the first year of applications, manganese did not have an impact on SDS severity, so perhaps this mechanism is not the same. Treatments continued into the second season and will be evaluated.
Velista fungicide is a broad spectrum SDHI (Succinate Dehydrogenase Inhibitor – FRAC Group 7) that protects against several turfgrass diseases including SDS. Velista binds with SDH inhibiting a critical respiratory pathway preventing spore germination and mycelia growth necessary for infection. Getting fungicides to the point of infection is critical when working with soil-borne diseases such as SDS. As expected, Velista fungicide applications consistently reduced SDS severity compared to the untreated control. A numerical increase in fungicide control was also observed with an increase in fraze mowing depth from 0 to 0.32 inches.
In combination treatments of nitrogen source, manganese, and fungicide, SDS was numerically reduced with an increase in the fraze mowing depth. While there were no significant differences, the correlating trend of less SDS with increased fraze mowing depth provides further evidence that thatch/organic matter removal and subsequent rhizome and stolon regeneration could be an important part of disease control.
While fraze mowing still provides many other benefits of thatch removal, weed seed removal, and a clean spring transition of bermudagrass athletic fields by promoting new growth, the practice may also serve as a tool to reduce spring dead spot severity. As evidenced in our first trial, fraze mowing is not a stand-alone practice for control of SDS, but instead should be looked at as a component of an overall management program. As with most research, this work has led to more questions. Can aggressive fraze mowing with a fungicide alone be all that is needed? Does it become a multi-year approach? Is fraze mowing necessary every year? Continued work at the University of Missouri and other institutions will hopefully provide an answer to these questions and more as the new practice of “fraze mowing” becomes more standard and widespread.
*Bars with the same letter are not significantly different according to Fisher’s Protected LSD test.
Brad Fresenburg, PhD, is Extension Turfgrass Specialist, Division of Plant Sciences – University of Missouri. Lee Miller, PhD, is Associate Professor Turfgrass Pathology, Division of Plant Sciences – University of Missouri. Daniel Earlywine is Research Technician, Division of Plant Sciences – University of Missouri. References are available online at www.sportsturfonline.com.