Proceedings of the Sixty-first Annual Meeting of the Northeastern ...

105
METHODS TO ASSESS ENVIRONMENTAL INFLUENCE ON TURFGRASS
RESPONSE TO MESOTRIONE. S.D. Askew, M.J. Goddard and J.B. Willis, Virginia
Tech, Blacksburg.
Mesotrione is expected to be available for turfgrass markets Spring 2008. Of
several turfgrass species known to tolerate mesotrione treatment, perennial ryegrass
(PRG)(Lolium perenne) and fine fescue (Festuca spp.) are among the most sensitive.
The visual manifestation of turfgrass injury from mesotrione is striking and ranges from
a dull yellow to bright white. It is hard to predict when turfgrass will be discolored by
mesotrione. In several field trials since 2001, we have observed turfgrass responses to
mesotrione 4 SC at 0.14 to 0.28 kg ai/ha range from no effect to completely white
foliage. Studies were conducted in 2006 at three field locations and in growth chambers
in Blacksburg, VA to evaluate the influence of various environmental conditions on PRG
and hard fescue (Festuca longifolia) response to mesotrione.
In growth chamber studies, mesotrione was applied at 0, 0.14, 0.21, and 0.28 kg
ai/ha to PRG maintained at daytime and nighttime temperatures of 13 and 7, 18 and 13,
24 and 18, and 30 and 24 C, respectively. PRG was placed in 15 cm x 15 cm pots and
maintained at 2.5 cm in height. Chlorophyll b and a + b levels decreased by 750 µg g -1
fresh weight and 2970 µg g -1 fresh weight, respectively, for each 100g increase in
herbicide rate. Carotenoid levels were strongly temperature dependent and followed a
quadratic response with peak carotenoid production (820 µg g -1 fresh weight) occurring
near 25 C, the plants growth optimum.
In field trials, weather stations (Spectrum Technologies) were installed at each
site to monitor soil moisture, soil temperature, air temperature, dew period, solar
radiation, and photosynthetically active radiation every 30 minutes. Mesotrione was
applied at 0.14 kg ai/ha each week between March 8, 2006 and September 4, 2006. A
sample of leaf tissue was collected just prior to treatments, 5 days after treatment (DAT)
and 10 DAT. At each evaluation timing, leaf tissue was assayed for cuticle wax weight,
carotenoids, and chlorophyll a and b. Plots were also evaluated for color both visually
and with digital image analysis. Principle component analysis (PCA) techniques and
regressions were used to determine correlations between measured environmental
conditions and observed treatment responses. The PCA analysis indicates some
correlation between carotenoids and several environmental conditions. However,
correlation between visual injury responses due to mesotrione and environmental
conditions will likely be complicated.
As with other carotenoid synthesis inhibitors, the greatest amount of white tissue
occurs during maximum plant growth, as white tissue is typically evident on new leaves.
Several factors, however, can influence plant growth including a combination of abiotic
factors or even biotic factors such as disease. Thus, turfgrass injury response will be
difficult to predict based on environmental conditions before and after treatment.
Turfgrass growth rate will likely be the best predictor of injury responses, with increase
growth resulting in increased chances for white foliage. Preliminary studies also
indicate that conditions that promote foliar absorption of mesotrione, such as high
humidity, will also contribute to turfgrass injury. Our future work will evaluate absorption
as a contributing factor.
87