Dr. Pete's Turf Tips
Peter H. Dernoeden, Ph. D.
Spring dead spot (SDS) is the most damaging fungal disease of bermudagrass turf. This disease is caused by three root pathogens, but Ophiosphaerella korrae is the most common causal agent in the mid-Atlantic. These pathogens attack in soil on roots, stolons and rhizomes; they are not found on foliage. The intensity of SDS varies greatly from year-to-year, and it is not possible to predict those years when it will be severe. Indeed, turf pathologists are baffled by the unpredictable nature of SDS outbreaks. The disease generally is expected to be more common and severe during springs following cold and wet winters, but severe outbreaks of SDS have followed relatively mild winters. Thus, the intensity of epidemics are due to imperfectly understood environmental and soil conditions.
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As bermudagrass breaks dormancy in spring, circular patches of tan or brown, sunken turf a few inches to three feet or greater in diameter become conspicuous. Circular depressions may be seen in affected turfs prior to spring green-up. In older stands with chromic SDS, rings may develop with healthy turf in the center, indicating that the pathogen is moving outwards; much like older fairy rings. Sometimes patches coalesce and the damage is non-uniform and appears similar to winter-kill or winter desiccation. Rhizomes and stolons from nearby healthy plants eventually spread into and cover dead patches. This filling-in process is slow, a period which may last four to eight weeks. Unusually cool to cold spring and early summer weather further delay recovery.
Research conducted in Maryland showed that root infection by O. korrae occurs when soil temperatures range from about 50 to 75oF, but most root injury occurs between 55 to 65oF. These soil temperatures coincide with the time bermudagrass enters dormancy in late autumn. Temperatures below 50oF restrict growth of the pathogen and thus little injury would be expected to occur overwinter. Research also showed that O. korrae was unable to damage roots when inoculated plants were maintained at about 75oF, accounting for why SDS is not a major concern in regions where bermudagrass does not go completely dormant in winter.
Management practices that help reduce thatch (e.g., vertical cutting and coring) are very important in an SDS management program. The aforementioned practices are best performed in early summer after 100% green-up and when daily highs exceed 80oF to promote growth and recovery. Reduce or otherwise divert winter traffic on bermudagrass to minimize compaction. Potassium inputs also are important, despite soil tests showing sufficient levels. I have always recommended applying nitrogen (N) and potassium (K) in a 1:1 ratio whenever using N. Ammonium sulfate (applied at 1.0 lb. N/1000 ft2) and potassium chloride (applied at 1.0 lb. K/1000 ft2) applied on 14 to 21 day intervals (or lower rates more frequently) beginning at spring green-up speed recovery and help to alleviate SDS severity over time. The suppression effect provided by ammonium sulfate may take several years to develop. Acidification alone does not reduce the number of patches, but reduces their size. Late fall applications of high amounts of water soluble N have been shown to enhance SDS. It is likely, however, that modest (0.25 to 0.50 lb. N/1000 ft2) inputs of N in early autumn, especially from slow release forms, would not be expected to intensify the disease. More research is needed on the impact of late season applications of N on SDS severity.
Fungicides generally do not provide highly effective SDS control and often are erratic (≈ 30-75%). Even 30% control, however, would result in faster recovery. Research is conflicted on frequency of application, but Maryland studies indicate that a single application about 30 days prior to anticipated frosts” packs the best punch” against SDS. In the mid-Atlantic, our first frost would be expected around mid-to-late October; thus a fungicide should be applied about mid-to-late September. A second application is recommended before dormancy where SDS is chronically severe. Spring fungicide applications have been ineffective in the eastern U.S., primarily because most damage occurs in the fall and early winter period.
Demethylation/ sterol inhibitors (DMI/SI) like Banner MAXX, Rubigan (no longer produced), Torque and others have been the primary fungicides used to suppress SDS. Strobilurins like Heritage, Insignia and others have limited activity. However, mixtures like Briskway (Heritage + difenoconazole), Headway (Heritage + Banner MAXX) and Lexicon (Insignia + Honor) have been reported to boost SDS control in South Carolina. Velista also has been shown to be effective. A fungicide(s) should be applied in the highest water dilution (≥ 2 gal. water/1000 ft2) possible at ≥ 35 psi to help move fungicide down to stolons and between leaf sheaths to make contact with vital growing points. Currently, there are no data to guide us on the possible benefits of watering fungicides into the rootzone. Since bermudagrass generally loses most of its existing root system at spring green-up, it would seem prudent that protecting stem bases and stolons, which can live for one or more years, is the correct target for a fungicide.