Those brown circular patches in your St Augustine turfgrass lawn are probably brown patch. Brown Patch forms distinct circles or joined lobed circles of dead leaves with a “halo” of yellow grass. It does not kill the turfgrass, it only affects the leaves. The results are a patchwork of crop circles.
What to expect from Brown Patch
Brown Patch is a cold season disease of fall and spring. Brown Patch forms circles and/or lobed circles of tan/green weak turf that is flattened – like a round weight had been placed on the turf. The rings start out fist size and can grow to be 20 feet in diameter or larger. The outer edge of the ring (active area) is a multi-colored, mainly yellow in color, and is very distinct. Brown Patch responds to fertilizer - especially readily available nitrogen – by getting worse and to potassium by declining.
Also, expect St Augustine to fully recover when cultural conditions change – i.e. the weather warms or cools.
Cause and Cure
The fungus that causes brown patch is natural part of the turf ecosystem recycling matter and cannot be eradicated. During the cooler weather of fall and spring, the fungi will attack living tissue - usually just the leaves. The Good News: Except for the worse cases, Brown Patch can be controlled with good cultural practices!
Cold vs. Deadly Disease
My experience with Brown Patch has me categorizing it as a “cold” as opposed to a “deadly disease”. I think the “cold analogy” is a good one because:
Brown Patch generally will not kill your grass – unless it is near death, to begin with.
It is a temporary condition – Brown Patch thrives at moderate soil temperatures and as the soil temperature rises (or lowers), Brown Patch will cease to be a problem.
Many lawns are not effected by Brown Patch, and others have chronic problems with Brown Patch (not everyone catches a cold).
The causal agent is part of the environment – it is a soil born disease and cannot be eliminated. Since it is not life-threatening, I manage the Brown Patch with biological methods to reduce occurrences and treating only severe occurrences with commercially manufactured fungicides.
Since the disease responds to fertilizers and is not lethal, manipulating fertilizers is the best management practice for controlling Brown Patch. To reduce the impact of Brown Patch during the cool season months, APL Pest Control uses slow release organic sources for nitrogen in very sparse amounts and high amounts of potassium to keep nitrogen levels low and potassium levels high, This is safer then applying chemical fungicides. However, in severe cases, I will use a commercially available fungicide as a curative measure. In the 700 or so lawns I service, I only have about 5 that are problematic for Brown Patch and require fungicides; therefore the manipulating of the fertilizer elements is a good management practice for controlling Brown Patch.