HERBICIDE DAMAGE? My client’s St. Augustine grass lawn shows irregular die out.

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The dead grass in your client’s lawn is an unusual and extreme example of possible pesticide misapplication. Nevertheless, the application of chemicals to residential turf is more challenging than in field crops. The small, irregular shape of lawns makes it difficult to achieve accurate and uniform application. Because the equipment is frequently hand-held, ground speed is difficult to control. Of the three variables in spray applications (width, pressure, and speed), speed is typically the least precise. In new situations, always use your rinsed pesticide equipment to go over the area with one application of water only, with a consistent pattern. Measure the total volume of water applied, and put this number into your calculations to make the spray concentration agree with the label rate

While this will assure accurate overall application, it does not adequately assure uniformity. (Weed System Equipment Co., Keystone Heights) advises against broadcast applications from a hand-held, single low-volume nozzle sprayer (i.e., back-pack type sprayers), and suggests that multiple nozzles should be mounted on a boom, with flat fan nozzles held at a height to achieve 30% overlap.

Applicators who have to use back-pack type sprayers, because of tight situations, should use a turf colorant so they can tell where they have sprayed. Low-volume nozzles are designed only for a straight motion, not side-to-side or circular.

Most common in lawn applications is the single high-volume multi-stream nozzle (shower-head type nozzle), usually swung back-and-forth on a hand-held gun attached to the end of a hose. The spray pattern is wide enough for the experienced applicator to achieve adequate coverage. If you are training a new applicator, you can help the person practice with water on a hard surface (e.g., blacktop or concrete) and watch the evaporation pattern, which will readily show uniformity problems.

A string-line or extra garden hose can be easily placed in the lawn, which demarcates sections, and helps the applicator maintain a true course. More realistically, commercial applicators would use the mowing pattern, the dew, or footprints, as a guide for direction and trueness of course.

Curiously, while most lawns are treated by side-to-side swinging of hand-held hose nozzles, books on turfgrass cover the subject of pesticide application by calculations based on straight-line movement of either motorized equipment, or rigidly hand-held equipment.

The shape of the dead grass is consistent with hand-application of a toxic liquid, e.g., an herbicidal spray. Notice the regular zigzag pattern typical of a person’s arm swinging side-to-side in harmony with a walking gait. I am aware of no other biological phenomena producing this regular pattern. The narrow width of dead grass is consistent with the spray pattern from a single, low-volume nozzle, with inadequate overlap to fill the area.

The sharp definition of the dead grass is consistent with the effects from a contact herbicide, not a systemic. Likewise, there are no symptoms on the perimeter plants, nor on the young citrus growing in the center of the dead grass in the background. If the contact herbicide interpretation is right, the area could probably now be replanted by plugs or sod. However, it is safer to wait a couple weeks to see if scorching of leaves or twisting of young shoots occur on other plants in the area, in which case there may still be a pesticide residue in the soil.

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