Methiozolin, A New Annual Bluegrass [Selective] Herbicide

by Suk-Jin Koo, Ki-Hwan Hwang, Man-Seok Jeon, Sung-Hun Kim, Jongsoo Lim, Dong-Guk Lee and Nam-Gyu Cho

BACKGROUND: Selective control of annual bluegrass (Poaannual L.) has been difficult in turfgrasses. The potential of methiozolin in this area was investigated.

RESULTS:
Methiozolin was safe on established zoysiagrass (Zoysia japonica Steud.), creeping bentgrass (Agrostis palustris Huds.), Kentucky bluegrass (Poa pratensis L.), and perennial ryegrass (Lolium perenne L.) at 1000 g ha⁻¹, and controlled annual bluegrass with GR50 values of 23, 52, 104, and 218 g ha⁻¹ at PRE, two-, four- and eight-leaf stage, respectively, in the greenhouse.

When applied at early flowering, methiozolin suppressed >80% of annual bluegrass seed heads at 2000 g ha⁻¹.  ¹⁴ C-Methiozolin was readily absorbed by both leaves and roots, but translocation was mainly acropetal. No herbicidal activity resulted from application to the leaf only; however, application to the soil surface only showed equivalent herbicidal activity to that of broadcast application to the leaf and soil.  Methiozolin at 500 to 1000 g ha⁻¹ provided 80 to 100% control of annual bluegrass when applied in the fall with acceptable and temporary injury to creeping bentgrass, and about 60% control when applied in the spring with no bentgrass injury in the field.

CONCLUSION: 
Methiozolin is an excellent candidate for annual bluegrass management in turfgrasses.

1 INTRODUCTION
Methiozolin is a new turf herbicide being developed by Moghu Research Center, Korea. The herbicide is in the isoxazoline chemical family and was first invented as a rice herbicide candidate.¹ The molecule controlled barnyardgrass (Echinochloa spp.) and several other annual broadleaf and sedge weeds from 125 g ha⁻¹ in rice paddies while having good safety to transplanted rice up to 1.0 kg ha⁻¹.²  Koo and Hwang reported that the herbicide controlled annual bluegrass and large crabgrass (Digitaria sanguinalis) effectively in various cool and warm season turfgrasses.³ The mechanism of action of methiozolin and related chemistry is not thoroughly understood and appears to be novel.

Lee et al. reported several morphological responses in germinating barnyardgrass including cessation of shoot and root growth without twisting, colour changes, burning and other symptoms associated with known modes of action.⁴ Further, they showed methiozolin inhibited root growth of corn, a susceptible grass, at a very low dose with a GR50 of 0.03 μM. Using ¹⁴C-glucose ncorporation into corn root cell wall constituents, Lee et al. showed methiozolin inhibited biosynthesis of both cellulose and hemicellulose fractions greatly from 0.1 μMafter 24 h of exposure.  However, the morphological symptoms did not resemble those of known cell wall synthesis inhibitors such as dichlobenil.⁴

Recently, Grossmann et al. suggested methiozolin might inhibit tyrosine aminotransferase (TAT), an enzyme in the biosynthesis of plastoquinone, in duckweed (Lemna paucicostata L.).⁵ This suggestion was based on feeding of hydroxyphenylpyruvate, the product of TAT, to duckweed, which nullified growth inhibition of methiozolin. They also showed that methiozolin inhibited a recombinant TAT of A. thaliana, but with a very high IC50 value (about 200 μM). Grossmann et al. further suggested that other TAT isoenzymes in Arabidopsis or from other plants could be more sensitive and the primary in vivo target of methiozolin.   Although the mechanism of methiozolin was shown to involve inhibition of cell wall biosynthesis and potentially plastoquinone biosynthesis, the primary site of herbicidal action is still unclear.

Annual bluegrass is a common and major weed species in both cool and warm season turfgrasses. Annual bluegrass usually behaves as a winter annual, germinating in the late summer to early fall when soil temperatures fall below 21◦C, followed by prolific seed head  production during the late winter and spring.^6,7 In addition to the typical annual biotype, P. annua spp. reptans (Hausskn.) Timm, a perennial subspecies of annual bluegrass, is commonly found in creeping bentgrass putting greens throughout most of the United States.

The perennial biotype shows a stoloniferous prostrate growth habit and has been shown to be difficult to control by herbicides.⁸ Annual bluegrass is also managed by default as a turfgrass species in many golf courses, but is generally regarded as an undesirable species due to prolific seed head production, shallow root system, and susceptibility to many diseases and less tolerance to heat and cold.⁹ In a situation where annual bluegrass persists in more desirable turfgrasses, such as creeping bentgrass, it is very invasive yet relatively intolerant of most biotic and abiotic stresses, often dying unexpectedly, leaving surface voids, requiring higher cultural and chemical inputs to maintain it.  Furthermore, it is considered to be the number one weed problem in turfgrass both in frequency of occurrence and difficulty of control.^10,11

Options for selective chemical control of annual bluegrass are limited, especially in cool season turfgrasses. A few herbicides, including bensulide¹² and ethofumesate,¹³ or plant growth regulators (PGRs) such as ethephon, paclobutrazol or flurprimidol have traditionally been used for annual bluegrass suppression.^14–17 Herbicides that have been introduced more recently include acetolactate synthase inhibitors, such as sulfosulfuron and bispyribac-sodium, which control annual bluegrass and/or roughstalk bluegrass (Poa trivialis L.).¹⁸ However, these options have had limitations in application timing, reliability or efficacy, possibly due to the wide genetic variability associated with annual bluegrass. 

Methiozolin has been extensively evaluated in turf including annual bluegrass control in creeping bentgrass putting greens.  McNulty et al. reported methiozolin at 0.5 and 0.75 kg ha⁻¹ applied in March 2009 reduced annual bluegrass cover by 77–80% at 2 months after treatment in creeping bentgrass putting greens.¹⁹ 

Methiozolin did not cause any injury on garden height (38–76 mm) perennial ryegrass, Kentucky bluegrass, or tall fescue, and suppressed seed head production of annual bluegrass 85% one week after application of >1500 g ha⁻¹.²⁰ In the test, methiozolin at>3000 g ha⁻¹ controlled annual bluegrass and roughstalk bluegrass>80 and >90%, respectively, and was eventually superior to two applications of bispyribac-sodium at 74 g ha−1.Methiozolin at 2000 g ha⁻¹ applied twice controlled 31 different annual bluegrass biotypes including 28 perennial biotypes in a pot test, and there was no biotype showing tolerance to methiozolin.²¹

Methiozolin did not injure several creeping bentgrass varieties such as A4, L-93 and Declaration when applied at 500–4000 g ha⁻¹ in two university trials; however,methiozolin at 2000–4000 g ha⁻¹ injured L-93 creeping bentgrass by 25–40% in hot and dry weather in a different location.²²

Despite various research, little has been published to date on the whole picture of methiozolin as a turf herbicide.  The objective of this research was to provide basic biological properties of methiozolin in terms of turfgrass safety, annual bluegrass rate responses in different growth stages, seed head suppression activity, site of uptake, and application window in the field.

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