Search

Annual bluegrass (Poa annua L.) and roughstalk bluegrass (Poa trivialis L.) are among the most troublesome grass weeds on golf courses throughout the United States. Herbicides for selective control of these weeds in cool-season fairways are limited and ineffective. Methiozolin is a new isoxazoline herbicide that controls annual bluegrass on putting greens and shows promise for possible weed control in fairways. Kentucky bluegrass (Poa pratensis L.) is among the most common turfgrass species used for golf fairways in the Northern United States and its response to methiozolin has scarcely been tested. A 2.5-yr field study was conducted at four Virginia locations to evaluate methiozolin efficacy for selective annual bluegrass and roughstalk bluegrass control in creeping bentgrass (Agrostis stolonifera L.) or Kentucky bluegrass fairways. Another study evaluated the response of 110 Kentucky bluegrass varieties to three rates of methiozolin. Annual bluegrass has long been presumed to impact putting green trueness, or the ability of the greens canopy to provide a smooth and directionally-consistent ball roll. Although much research has evaluated the impact of greens management on ball roll distance, no peer-reviewed research has evaluated how canopy surface factors, such as weedy annual bluegrass, will influence ball roll direction. Laboratory and field research was conducted to elucidate and overcome experimental errors that may be limiting assessment of ball directional imprecision caused by greens canopy anomalies. Techniques to minimize experimental error were employed in field studies at two Virginia golf courses to determine the influence of annual bluegrass on ball directional imprecision, bounce, and acceleration. Study results suggest that annual bluegrass patches in a creeping bentgrass putting surface can cause subtle increases in ball directional imprecision and bounce but several sources of error must be controlled before these effects can be measured. By using a mechanical putter to avoid directional errors associated with simulated-putt devices, selecting golf balls with balanced centers of gravity, eliminating legacy or "tracking" effects of repeated ball rolls via canopy brushing, and scoring ball direction 30 cm prior to terminal acceleration, we were able to detect an increase in ball directional imprecision of 8 mm m⁻¹ when balls rolled over a single patch of annual bluegrass compared to adjacent rolls on visually-pure creeping bentgrass. In herbicide efficacy studies, methiozolin-only treatments did not significantly injure creeping bentgrass or Kentucky bluegrass, reduce quality, or reduce normalized difference vegetative index regardless of application timings and rates. In general, fall applications of methiozolin reduced roughstalk bluegrass and annual bluegrass cover more than the spring-only treatments. At 1 year after the last treatment, methiozolin at 1500 g ha⁻¹ applied four times in fall at 2-wk intervals for two consecutive years controlled roughstalk bluegrass and annual bluegrass ≥85% and more consistently than other herbicides or treatment regimes. Spanning 110 Kentucky bluegrass varieties, a commercially-acceptable threshold of 30% Kentucky bluegrass injury required between 3.4 to more than 10 times the methiozolin rate needed for annual bluegrass control. Results indicate that annual bluegrass increases directional imprecision and bounce of golf balls rolling across a greens canopy. Methiozolin could be a viable herbicide for managing annual and roughstalk bluegrass in Kentucky bluegrass and creeping bentgrass fairways but weed control efficacy may be dependent on application timing. By measuring small differences in ball directional imprecision as influenced by greens canopy factors, future research efforts will aim to help turf managers choose appropriate greens management techniques. Ph. D.

Light reflectance from plants can be used as a non-invasive predictor of health and yield for many cropping systems, and has been investigated to a lesser extent with managed turfgrass systems. The frequent agronomic inputs associated with maintaining golf course grasses allow for exceptional stand quality under harsh growing conditions, but often expend resources inefficiently, leading to either stand loss or unnecessary inputs in localized areas. Turfgrass researchers have adopted some basic principles of light reflectance formerly developed for cropping systems, but field radiometric-derived narrow-band algorithms for turfgrass-specific protocols are lacking. Research was conducted to expand the feasibility of using radiometry to detect various turfgrass stressors and improve speed and geographic specificity of turfgrass management. Methods were developed to detect applied turfgrass stress from herbicide five days before visible symptoms developed under normal field growing conditions. Soil volumetric water content was successfully estimated using a water band index of creeping bentgrass canopy reflectance. The spectral reflectance of turfgrass treated with conventional synthetic pigments was characterized and found to erroneously influence plant health interpretation of common vegetation indices because of near infrared interference by such pigments. Finally, reflectance data were used to estimate root zone temperatures and root depth of creeping bentgrass systems using a gradient of wind velocities created with turf fans. Collectively, these studies provide a fundamental understanding of several turfgrass-specific reflectance algorithms and support unique opportunities to detect stresses and more efficiently allocate resources to golf course turf. Ph. D.

Annual bluegrass is a winter annual grass that is problematic on golf putting greens due to its light green color, prolific seedhead production and intolerance to stress. On creeping bentgrass putting greens, herbicides for annual bluegrass control are limited. A new herbicide, methiozolin, developed by Moghu Research Center, LLC, in Daejeon, South Korea, safely and selectively controls annual bluegrass in creeping bentgrass and several other turfgrass species. Methiozolin typically controls annual bluegrass over several weeks, allowing desirable turfgrass time to grow into areas previously infested by annual bluegrass with little surface disruption. The mode of action of methiozolin is unknown, but has been proposed to act as either a cell wall biosynthesis inhibitor (CBI) or an inhibitor of tyrosine aminotransferase (TAT). Field studies were conducted at Virginia Tech to investigate strategies promoting surface recovery on putting greens following atypically rapid annual bluegrass loss resulting from methiozolin application, intensive core-cultivation as well as potential interactions with plant growth regulators (PGR's), like ethephon. In the rapid annual bluegrass removal study, all treatments receiving additional fertility via synthetic fertilizer with or without trinexapac-ethyl or biostimulant recovered 1 to 3 weeks more quickly than treatments that did not include additional fertility. Addition of the PGR trinexapac-ethyl inconsistently regulated speed of canopy recovery, both increasing and decreasing recovery speed. Under normal maintenance conditions, methiozolin does not negatively influence putting green recovery, however, if the putting green is exposed to droughty conditions, methiozolin can reduce recovery time by several weeks. Core-cultivation should be avoided in conjunction with methiozolin and ethephon applications because when this procedure was conducted on the same day as herbicide application it significantly damaged creeping bentgrass, reducing cover to 19% at 2000 g ai ha⁻¹, compared to the non-treated at 62%. Regarding the question of methiozoling mode of action, laboratory studies supported the claim that addition of exogenous 4-hydroxyphenylpyruvate (4-HPP) alleviates symptoms of methiozolin exposure in lesser duckweed, a model monocot species, but feeding various turfgrass species and annual bluegrass exogenous 4-HPP did not alleviate symptoms. Creeping bentgrass secondary root length and density was not affected by methiozolin, although annual bluegrass, Kentucky bluegrass and perennial ryegrass secondary root lengths were reduced. Based on these data, it does not appear that TAT inhibition is a primary mode of action of methiozolin in turfgrass. Studies were conducted to determine if methiozolin inhibited cell wall biosynthesis in desirable turfgrass species and annual bluegrass. All species exhibited decreased enrichment of ¹³C in cell-wall sugars form ¹³C-glucose in response to methiozolin and a known cell wall biosynthesis inhibitor, indaziflam. Indaziflam and methiozolin at 0.01 µM inhibited ¹³C enrichment of all sugars less than methiozolin at 1.0 µM, for xylose, arabinose and glucose, but not galactose. Addition of 4-HPP increased incorporation of ¹³C into xylose, but had no other influence on ¹³C incorporation into other cell wall sugars. Lack of species specific response indicates that cell wall biosynthesis inhibition is probably not the source of interspecific species responses observed in the field. Ph. D.

Annual bluegrass (Poa annua L.) is the most problematic weed to control in creeping bentgrass (Agrostis stolonifera L.) putting greens. The objective of this study was to transition a mixed putting green stand of annual bluegrass and creeping bentgrass to a monoculture by using fertilizers and plant growth regulators that selectively inhibit annual bluegrass. A 25 year old loamy sand rootzone research green, planted with \'Penn-Eagle\' creeping bentgrass, with roughly 45% initial annual bluegrass coverage was utilized. The biweekly application of ammonium sulfate (4.8 kg ha-1) with treatments of ferrous sulfate at rates of 0, 12.2, 24.4, and 48.8 kg ha-1 and in combination with seaweed extract (12.8 L ha-1) or paclobutrazol (0.37 L ai ha-1 spring and fall; 0.18 L ai ha-1 summer) were applied March to October, 2011 and 2012. Plots receiving the highest rate of ferrous sulfate resulted in annual bluegrass infestation declines from an early trial amount of 45% to a final average of 20% but also resulted in unacceptable late-summer events of annual bluegrass collapse. The ferrous sulfate medium rate resulted in a smooth transition from early-trial annual bluegrass infestation of 45% to an end of trial infestation of 20% and had the highest putting green quality. Previous research has reported that consistent use of paclobutrazol can effectively and safely reduce annual bluegrass infestations. In this trial annual bluegrass was reduced to 9% infestation after three months of application. Two unexpected observations from this trial were that ferrous sulfate, applied at medium to high rates, significantly reduced silvery thread moss (Bryum argentum Hedw.) populations and occurrences of dollar spot (Sclerotinia homoeocarpa F. T. Bennett) disease. Dollar spot control with ferrous sulfate has not previously been reported in the literature, so additional studies were designed to investigate this phenomenon further. A creeping bentgrass putting green study was conducted to determine if sulfur, iron, or the two combined as ferrous sulfate decreases dollar spot activity. Ferrous sulfate resulted in the highest turf quality and suppressed S. homoeocarpa infection, even during high disease pressure. Fe-EDTA suppressed dollar spot infection as well as ferrous sulfate but quality declined to unacceptable levels during the summer, due to Fe-EDTA only. Sulfur did not affect or increased S. homoeocarpa infection, indicating that a high and frequent foliar rate of iron is responsible for dollar spot control. An in-vitro study was conducted to determine if agar pH in combination with iron concentrations affects mycelial growth of S. homoeocarpa. Results from this trial indicated that 5.4 agar pH is an optimal pH for mycelial growth. The 10 to 100 mg iron kg-1 concentration had little effect on mycelial growth at 5.0 and 5.5 pH, but increased growth at 4.5 and 6.5 pH. As the iron concentration was increased from 10 to 100 to 1000 mg kg-1, mycelial growth decreased or stopped. Our final conclusions are that seasonal biweekly foliar applications of the medium rate of ferrous sulfate (24.4 kg ha-1) safely and effectively reduced annual bluegrass infestation out of a creeping bentgrass putting green, while also effectively suppressing silvery thread moss and dollar spot incidence. Master of Science

Creeping bentgrass (Agrostis stolonifera L.) is a major low-cut cool-season turfgrass used worldwide. The objectives of this research were to: 1) to gain insight into the diurnal fluctuation of N metabolism and effects of cytokinin (CK) and nitrate; 2) to characterize the impacts of N and CK on creeping bentgrass under heat stress; 3) to investigate the simultaneous effects of CK and N on the antioxidant responses of heat stressed creeping bentgrass; and 4) to examine the expression pattern of the major heat shock proteins (HSPs) in creeping bentgrass during different heat stress periods, and then to study the influence of N on the expression pattern of HSPs. The transcript abundance of nitrate reductase (NR), nitrite reductase (NIR), plastidic glutamine synthetase (GS2), ferredoxin-dependent glutamate synthase (Fd-GOGAT), and glutamate dehydrogenase (GDH) and N metabolites in shoots were monitored during the day/night cycle (14/8 h). All the measured parameters exhibited clear diurnal changes, except GS2 expression and total protein. Both NR expression and nitrate content in shoots showed a peak after 8.5 h in dark, indicating a coordinated oscillation. Nitrate nutrition increased diurnal variation of nitrate content compared to control and CKHowever, CK shifted the diurnal in vivo NR activity pattern during this period. Grass grown at high N had better turf quality (TQ), higher Fv/Fm, normalized difference vegetation index (NDVI), and chlorophyll concentration at both 15 d and 28 d of heat stress than at low N, except for TQ at 15 d. Shoot NO3-, NH4+, and amino acids increased due to the high N treatment, but not water soluble proteins. High N also induced maximum shoot nitrate reductase activity (NRmax) at 1 d. CK increased NDVI at 15 d and Fv/Fm at 28 d. In addition, grass under 100 µM CK had greatest NRmax at both 1 d and 28 d. Under high N with 100 µM CK, root tZR and iPA were 160% and 97% higher than under low N without CK, respectively. Higher O2- production, H2O2 concentration, and higher malonydialdehyde (MDA) content in roots were observed in grass grown at high N. The activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), and guaiacol peroxidase (POD) in roots were enhanced by high N at 19, 22, and 24% levels, respectively, relative to low N. Twenty-eight days of heat stress resulted in either the development of new isoforms or enhanced isoform intensities of SOD, APX, and POD in roots compared to the grass responses prior to heat stress. However, no apparent differences were observed among treatments. No CK effects on these antioxidant parameters were found in this experiment. At week seven, grass at medium N had better TQ, NDVI, and Fv/Fm accompanied by lower shoot electrolyte leakage (ShEL) and higher root viability (RV), suggesting better heat resistance. All the investigated HSPs (HSP101, HSP90, HSP70, and sHSPs) were up-regulated by heat stress. Their expression patterns indicated cooperation between different HSPs and that their roles in creeping bentgrass thermotolerance were affected by N level. Ph. D.