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Virginia Turfgrass Journal – Wendell Hutchens and David McCall, Ph.D.

It is the middle of summer, the weather is warm, bermudagrass is thriving, and spring dead spot (SDS) (Ophiosphaerella spp.) is the last thing on any turfgrass manager’s mind. However, in short time thoughts of fungicide applications for the disease will be on the forefront of many turfgrass managers growing warm-season grasses in the transition zone as these applications are typically made in the fall for suppressing SDS. But when exactly in the fall is the optimal time to apply fungicides to suppress SDS and reduce the symptoms that appear the following spring? Moreover, what are some best management practices one can implement to optimize fungicide applications for SDS?

These two questions have intrigued the Virginia Tech Turfgrass Pathology lab over the past couple of years as many turfgrass managers throughout Virginia and the surrounding states have asked us these exact questions about managing SDS with fungicides. “When do I need to apply?” And “how do I get the most out of my fungicide application?” These are excellent questions that have not been fully answered even though years of research between our lab and other labs around the country have been devoted to solving these challenging problems. Fortunately, the past year of research in our lab has given us some insight on how to mitigate these issues in regard to fungicide management of SDS.

Fungicide application timing study

A study was conducted at two bermudagrass locations in Virginia to determine the optimal time to apply fungicides for SDS suppression based on soil temperature and time of year. Isofetamid [Kabuto, PBI Gordon] was applied once at the highest labeled rate (3.2 fl.oz./1000ft²) for SDS at specific soil temperatures throughout the year (Table 1).  The bermudagrass at the Chesterfield location was infested with O. korrae and the bermudagrass at the Blacksburg location was infested with O. herpotricha. Plots were assessed for percent SDS and treatments were compared in the spring of 2021 following a full year’s worth of applications (Figs. 1 and 2).

Spring treatments at the Chesterfield location did not suppress SDS, but fall applications made between 66 and 59°F 5-day average soil temperatures provided excellent suppression of the disease. Applications made during the peak of summer (80°F soil temperature) and in early winter (41°F) were also ineffective at the Chesterfield location. However, the results from this study are counter to what we saw under laboratory conditions, which showed maximum fungal growth near 80°F. In contrast, certain spring and summer treatments at the Blacksburg location had moderate efficacy against SDS. Similar to the Chesterfield location, the fall treatments had the greatest efficacy against SDS at the Blacksburg location, which holds true to the long-recommended practices. Spring dead spot was suppressed by nearly all timings throughout the year at the Blacksburg location.

Fungicide application methods study

Tebuconazole is notoriously inconsistent in its suppression of SDS, yet it is the most commonly applied fungicide because it is affordable and has some efficacy against the disease. So, our goal was to determine application methods that optimize tebuconazole efficacy against SDS on bermudagrass. The two factors we evaluated were post-application irrigation (PAI) and tank mixture of tebuconazole with a wetting agent. A study was conducted at two sites: Blacksburg, VA and Salisbury, MD. Treatments are listed in Table 2. Ophiosphaerella herpotricha was the causal agent of SDS at both locations. Plots were assessed for percent SDS and converted to area under the progress curve (AUPC) which encompasses the amount of disease over a certain period of time (Figs. 3 and 4).

At the Salisbury, MD location in 2020, the only treatments that did not suppress SDS were the ones that had only one application of tebuconazole without PAI. All other treatments provided similar suppression of SDS. At the Blacksburg location in 2021, all treatments suppressed SDS compared to the nontreated control. Two applications of tebuconazole with PAI suppressed SDS 86% more than one application of tebuconazole without PAI. All other treatments provided similar SDS suppression. The inclusion of a wetting agent did not impact SDS development at either location.

Conclusions

Our data fall in line with traditional recommendations in that fungicide applications for SDS should be made in the fall and that PAI can increase fungicide efficacy against the disease. However, the timing of application can vary by location. Our recommendation is to apply fungicides for SDS when 5-day average soil temperatures are between 55 and 70°F in the fall and immediately irrigate the fungicide in with 1/8” to 1/4” of water (Fig. 5). If symptoms still appear the following spring, late spring to early summer (mid-May) quick-release nitrogen applications are recommended to aid in bermudagrass recovery.

Table 1. List of isofetamid¹ applications for studies conducted in Blacksburg, VA and Chesterfield, VA.

¹Kabuto [PBI Gordon, Kansas City, MO]

Table 2. List of treatments for studies conducted in the fall of 2019 and 2020 in Blacksburg, VA and Salisbury, MD.

¹Tebuconazole rate = 1.1 fl.oz./1000ft²; single wetting agent application rate = 16 fl.oz./1000ft²; split wetting agent application rate = 8 fl.oz./1000ft²; irrigation amount = 0.25 in.

²Tebuconazole 3.6 [Quali-Pro, Houston, TX]

³Hydra-Last [Landscape Supply Inc., Roanoke, VA]