Many homeowners are experiencing the typical late fall-early winter surprise of large brown areas in what they thought was a perfectly green and healthy stand of turf.
Two main events are happening to cause these brown areas. One is crabgrass die-out and the second is a pernicious disease descriptively termed “brown patch” or more recently, ominously renamed “large patch.” Large brown blotchy areas have taken up residence in what appeared to be a perfectly healthy and green stand of turf.
Let’s review why these brown spots are there.
In south Florida we have about five species of crabgrass. Some are perennial and some are annual. The seeds seem to germinate year-round. One estimate says that each crabgrass plant can produce over 700 tillers and over 200,000 seeds. This makes crabgrass the cockroach of the weed domain.
When temperatures dip to around 40 degrees and lower, some of the tropical crabgrasses species turn brown. We were spoiled by the very effective results a selective grass herbicide called Asulox provided as a post-emergent (after the weed has established) application. It has been taken off the market. Since then, the public and landscape maintenance companies have been suffering from Asulox withdrawal. An immediate replacement for Asulox hasn’t been recognized. See turf care fact sheets at:
What to do
Thus, crabgrass management boils down to these options:
Keep your turf vigorous. A thick turf will shade out or reduce crabgrass invasions.
Raise your crabgrass tolerance threshold. Accept the fact that there will be some crabgrass in your lawn. Just attack it before it takes over large areas.
You may choose to R&R, Roundup and replace, that is, applying Roundup — a glyphosate herbicide that kills anything green — and replacing the area with sod during the rainy season. Sod will establish better if you wait and do the job once we are into our rainy (hopefully) season.
Attempt the use of pre-emergents. This means about three extra applications per year and the results will never be as good as up north because the pre-emergents break down within a short period (3 to 6 weeks or so) in our heat and rainfall. Pre-emergents “pre”-vent the crabgrass seedlings from growing into a plant.
We are seeing brown patch disease, caused by the pathogen, Rhizoctonia solani, in St. Augustine and zoyziagrass lawns. Symptoms begin as 6-12 inch diameter off-color patches that turn yellow with reddish-brown margins, they progress to brown, then to straw-colored patches. These patches may merge and may expand several feet in diameter.
Often, there may be rings of yellow-brown turf with green centers. Also, turf at the outer margin of an infected patch will be dark and wilted, this is the active zone of the pathogen as it spreads outward. To distinguish this disease from other ailments, check the basal area of the leaf blades for a black rot at the juncture of the stem and leaf blade.
This disease becomes active below 80 degrees F.; 73 degrees is optimum. Surprisingly, it is inactive during the hot humid summer conditions that one would typically think a fungus would thrive in. It shuts down when temperatures go above 90 degrees.
Infection occurs during periods of high humidity or excessive moisture for 48 hours or more. Soluble nitrogen fertilizer applications of greater than 1.1 pounds of actual nitrogen per 1000 square feet put the turf at risk. Thatch buildup, more than 0.75 inch, also creates a disease favorable habitat.
What to do
Avoid excess soluble nitrogen from November to April. Use slow-release sources of nitrogen and apply a balanced fertilizer containing equivalent amounts of potassium, preferably as a slow-release form also. Reduce late fall-winter watering to once per week (0.5 to 0.75 inch per week for St. Augustinegrass) to lower the available moisture level.
Keep records of areas where this disease appears. These high activity areas can be targeted with fungicides first; it is not always necessary to spray the entire lawn. However, fungicides work best when they are applied as a preventive, that is, before the fungus is active.
Applications should reduce spread if the infection is caught early. Commonly recommended fungicides are propiconazole, thiophanate methyl, mycobutanil, or azoxystrobin. UF/IFAS turf pathologist Phil Harmon recommends Headway™. Some products come in a “hose-end” package and can be applied with a garden hose.
Doug Caldwell, Ph.D., is the commercial landscape horticulture extension agent and landscape entomologist with the University of Florida Collier County Extension Service. E-mail: email@example.com