In this episode of Turfgrass Epistemology, I discuss and critically interpret the peer-reviewed article “Impact of Ferrous Sulfate Concentration on Clarireedia Isolate Growth and Dollar Spot Development” by Shelton et al. (2021), published in Crop Science. This paper is frequently cited in conversations about using iron—specifically ferrous sulfate—as a non-fungicidal tool for dollar spot suppression on golf course turf.

The study combines multi-site field trials with controlled in-vitro experiments to examine how ferrous sulfate rate influences dollar spot severity on creeping bentgrass fairways and putting greens, as well as how different Clarireedia isolates respond to increasing iron concentrations in culture. I walk through both components of the research, explaining what the data show about rate response, diminishing returns, and why previously recommended rates may be higher than necessary for meaningful disease suppression.

A major focus of the discussion is the nonlinear nature of the response. The results demonstrate that approximately 25–26 kg ha⁻¹ of ferrous sulfate was sufficient to achieve about 50% dollar spot suppression, with much smaller gains at higher rates and increased risk of turf injury under stress conditions. I explain why this matters for real-world management and how these findings challenge the tendency to assume that “more is better” when it comes to iron applications.

I also discuss the in-vitro portion of the study, which shows that ferrous sulfate has a direct fungitoxic effect on Clarireedia, but that the concentration required to suppress mycelial growth varies by isolate and by host type. This section is important for understanding why field performance can be inconsistent and why pathogen biology, management history, and turf system context all influence outcomes.

Throughout the video, I place this paper within the broader dollar spot literature and explain what it does—and does not—justify in practice. Ferrous sulfate is not a replacement for fungicides, nor is it a magic bullet, but it can function as a partial suppression tool within an integrated disease management program when its limitations are understood.

This episode is especially relevant for golf course superintendents, turfgrass researchers, and advanced turf managers who are evaluating iron-based programs for dollar spot control and want to separate evidence-based conclusions from oversimplified recommendations.

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