History.

Prior to its release in 2001, rimsulfuron (TranXit GTA) was distributed to athletic field managers and golf course superintendents for research demonstration purposes.   While the product was providing acceptable control of perennial ryegrass, two superintendents in Virginia noticed an adverse side effect on nearby creeping bentgrass putting greens.   It appeared as if the chemical had been tracked from the treated area onto greens via mower tires (image).   This greens injury raised concerns about lateral movement, or tracking, of rimsulfuron to undesirable locations.   A proposal was submitted to the Virginia Turfgrass Council and funded through the Virginia Turfgrass Foundation to investigate potential ways to prevent or remedy injury tracks on creeping bentgrass greens.

Implementation of tracking study.

Researchers at Virginia Tech designed and implemented three field studies to simulate typical mower tracking patterns on golf courses.   Plots consisted of Prosport perennial ryegrass (collar) and Penncross creeping bentgrass (green).   Upon trial initiation, two rates of rimsulfuron (1 or 2 oz/A) were applied to the collar in the afternoon.   The following morning, while dew was still present, a greens mower was driven across the collar and over the creeping bentgrass portion of each plot (image Shawn).   To prevent contamination, the tires were rinsed after driving through each plot.   Preventive treatments were made for each chemical rate and consisted of 1) no irrigation, 2) 0.25 cm of irrigation on perennial ryegrass two hours after rimsulfuron treatment, 3) 0.25 cm of irrigation on creeping bentgrass 15 minutes after tracking on the following morning, and 4) irrigation of both species as described above.   To determine ways to alleviate the problem remedial treatments were made after symptoms appeared, and consisted of 1) gibberilic acid, 2) chelated iron, and 3) both chelated iron and gibberilic acid.   Trials were monitored and rated on a five-day interval until all tracks had disappeared.

Implementation of persistence study.

Rimsulfuron labeled with carbon 14 radioisotope was applied to perennial ryegrass and annual bluegrass grown in aqueous solution in a controlled environment that mimicked spring conditions on the golf course.   A fine water mist was applied to plants each night to simulate dew patterns.   At various times after treatment, plants were harvested and radioactive rimsulfuron washed from the leaf surfaces.   Radioactivity inside plants and on leaf surfaces was quantified with a liquid scintillation spectrometer.   To determine if rimsulfuron was stable on the leaf surfaces and still in the herbicidally-active form, portions of leaf washes were separated using thin layer chromatography and a radiochromatogram analyzer.

Results of tracking study.

Data were compiled from the three studies and pooled together to show that tracks remained visible the longest when no irrigation was applied or when irrigation was applied only to the green.   Irrigating either the collar alone or both the collar and green provided the shortest discoloration period.   Recuperation times for irrigating both collar and green, collar alone, green alone, and no irrigation were 6, 10, 18, and at least 26 days, respectively.   After 25 days, only plots that received no irrigation still had visible tracks.   Remedial treatments with gibberilic acid and chelated iron showed no improvement in turf color.   In fact, when gibberilic acid alone or in combination with chelated iron was applied, tracks remained visible for longer than when no remedial treatment was made.   These growth stimulants tended to increase the growth rate and darken green color of surrounding turf and effectively made the injury track more evident.   We suggest increasing mowing frequency to twice daily to minimize the height differences between injured and noninjured turfgrass.   Lightly spray injured areas with turf paint to mask discoloration.

Results of persistence study.

Work in the laboratory tended to explain observances in the field.   It’s interesting that rimsulfuron can be characterized both by rapid absorption into the plant (over 25% in just 10 minutes) and considerable persistence on leaf surfaces.   For example, 50% of applied rimsulfuron was easily extracted from leaf surfaces with water after 24 hours incubation.   Thus, half of the applied rate is still available for tracking the next morning.   As much as 25% or more of applied herbicide could still be water extracted from leaves 3 days after treatment.   These data tell us two things:   1) If rimsulfuron is suspected of being tracked onto the green, a rapid irrigation will not prevent the appearance of tracks because rimsulfuron moves into the plants too quickly, and 2) Waiting for rimsulfuron to “go away” or break down on the leaves is not a viable option.   We must wash rimsulfuron from the treated leaf surfaces prior to tracking through the area if we are to prevent injury tracks.