Controlling Pythium blight on overseeded bermudagrass greens

From September through November in Florida and other warm-winter regions, the growth of bermudagrass tends to slow down and many superintendents start to think about overseeding their greens. Understandably, many of them fret over establishment of the overseeded turfgrass. One reason is Pythium blight. This fast-moving disease can destroy an established grass stand within 24 hours after the first symptoms become visible, especially under favorable environmental conditions.

Typical initial symptoms include irregularly shaped, purplish areas on blades and a dark, water-soaked appearance. These leaves will become soft and slimy, and soon mat together. Early in the morning or when humidity remains high, the leaves may be covered with the white, cobwebby mycelium of the pathogen, Pythium aphanidermatum.

If you are a superintendent overseeding a green and you're worried about Pythium blight, what should you do? The first logical choice is to choose a cultivar of overseeding turf-grass that has good resistance to this disease. Unfortunately, such a variety is not available. That leaves fungicides as the next best option. To test the efficacy of certain available fungicides for this purpose, we conducted a study on overseeded turf at the University of Florida's Ft. Lauderdale Research Center.

How we performed the study We arranged six treatments and an untreated control in a randomized complete-block design with four replications. Initially, we used chlorothalonil (Daconil Ultrex at 3.8 ounces per 1,000 square feet) in December 1999 over all plots to contain algae development. We then overseeded `Dark Horse' Poa trivialis on December 23, 1999, at 15 pounds per 1,000 square feet.

On all treated plots, we applied fungicides only one time - either immediately after seeding or after seedling emergence (see table, below). (In this study, we defined "emergence" as the full canopy of Poa trivialis that existed about 12 days after seed germination. The full canopy of grass created good conditions for inducing a Pythium-blight epidemic - ideal for evaluating fungicidal control.) We used three commercially available fungicides: Azoxystrobin (Zeneca's Heritage), fosetyl-al (Aventis' Aliette Signature) and mefanoxam (Novartis' Subdue MAXX).

We applied the fungicide sprays on January 4, 2000, with a backpack sprayer equipped with a flat-fan nozzle in 2 gallons of water per 1,000 square feet. We then inoculated the plots with Pythium aphanidermatum on January 5. Inverted plastic boxes placed directly within each plot increased the temperature and relative humidity to enhance infection.

We rated the plots for stand emergence and Pythium-blight development from January 9 through 12. We recorded stand emergence 12 days after overseeding, using a scale from 1 to 10, where 1 = no stand and 10 = a full stand of grass. Final percent disease severity was based on the total area of the turf plot infected by P. aphani-dermatum.

AUDPC, or Area Under the Disease Progress Curve, is a rating that tells you how much disease has occurred over time. It provides a good idea of how well treatments have suppressed overall disease development, and we employed it in this study.

What we found Pythium-blight development occurred throughout all the plots and was especially high in the control plots. All the fungicides we tested significantly reduced Pythium-blight development in comparison to the control (see table, page Golf 2). Azoxystrobin and fosetyl-al applied at seedling emergence (remember, we defined this term as 12 days after overseeding) had the lowest AUDPCs and final disease severities. However, they were not significantly different from mefanoxam, which provided control when applied both at seeding and seedling emergence, demonstrating good residual activity over 12 days.

The take-home message Environmental conditions are almost always favorable in Florida for a Pythium-blight epidemic to occur. Unfortunately, it is difficult to predict whether infection will take place at seeding, during emergence or after a full stand of the overseeded turfgrass has become established. Because Pythium blight has a short incubation period - the time from penetration of the host by the pathogen to the first appearance of visible symptoms - preventive fungicide applications are necessary to ensure control in an overseeded stand. Note that in our study, all fungicide treatments were, by definition, preventive because they all occurred prior to inoculation (even though azoxystrobin has labeling for curative treatment).

Further research is necessary to demonstrate if these results would hold true in other conditions and locations. In addition, more work needs to be done to determine what would be the best fungicide strategy for curatively managing this disease. We plan to conduct this research in the near future. Fortunately, several fungicides are effective in preventing this disease, including some not included in our study, so superintendents have several options at their disposal.

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