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1997, No. 2
Plant Growth Regulators for Fine Turf Use
Plant Growth Regulators (PGRs) have traditionally been targeted for low maintenance turfgrass where a reduction in turf quality is acceptable.
However, the evolution of PGRs for turf use has continued and some newer PGRs are finding uses on highly managed turfs such as athletic fields, golf courses, and home lawns.
The PGRs that are gaining use in these situations are products that inhibit gibberellic acid (GA) biosynthesis.
GA is the plant hormone primarily responsible for cell elongation; slow or stop its production and you will see a marked reduction in plant growth.
Four products are commercially available and all inhibit GA production; however, only mefluidide (Trade name - EMBARK) also inhibits cell division and thus has a different mode of action than the other PGRs.
Flurprimidol (Trade name - CUTLESS) and paclobutrazol (Trade name - SCOTTS TGR) inhibit GA production at the same point in the GA biosynthesis pathway and have very similar responses.
Paclobutrazol is the more active compound requiring lower rates to get the same response as higher rates of flurprimidol.
Trinexapac-ethyl (Trade name - PRIMO) is the newest PGR and was released commercially in 1995.
Trinexapac-ethyl blocks GA biosynthesis later in the pathway and, in fact, blocks at the penultimate step in the GA biosynthesis pathway.
It has been speculated that since trinexapac-ethyl is more specific, it causes less "side effects" and thus may be a safer product to use.
This claim has not been proven or disproven; however, we have observed less visual turf injury with trinexapac-ethyl as compared to either flurprimidol or paclobutrazol.
PGR Use Guidelines
PGRs have been used for a variety of reasons including growth reduction, seedhead inhibition, Poa annua conversion, increased putting green speed, increased stress tolerance, and others.
The most basic use of PGRs is for turfgrass growth reduction.
Seedhead inhibition can be desired particularly where Poa annua populations are large; however, the PGRs that only inhibit GA production provide poor seedhead inhibition.
Mefluidide does inhibit seedhead formation and is used by some turf managers to control Poa annua seedheads in fairway turf.
While mefluidide is effective against seedheads, it tends to cause more turf injury than the other PGRs.
The trend among golf turf managers is to use less mefluidide and more specific GA-inhibiting PGRs.
Growth reduction is the goal of all PGRs, yet how is this reduction expressed? Intensity of growth suppression refers to the amount of growth compared to an untreated control.
Duration of growth suppression refers to the number of weeks that growth is reduced by the PGRs.
Typically, the currently available PGRs give 4 to 6 weeks of growth suppression.
Most of the PGR rates in current use do not give complete growth suppression but somewhere in the range of 40-60 % of an untreated control.
Usually growth is returning to normal levels by 4 weeks after treatment.
More severe growth reduction can be achieved with higher PGR rates, but these higher rates can lead to turf injury, usually expressed as a discoloration of the turf, that can dramatically reduce turf quality.
The trend among turf managers is to use lower rates and apply on shorter intervals.
Most PGR labels suggest 4 week intervals; however, by lowering the rate, the interval can be shortened to the point where some managers are applying PGRs on 10-14 day intervals.
This strategy yields very little turf injury and in fact often results in improved turf quality.
The repeated use of PGRs also will change the structure of the turfgrass leaf.
Research has shown that PGRs decrease the intercellular spaces within the leaf; the cells in the leaf become more tightly packed.
There are some data that indicate that repeated PGR use may increase turf wear tolerance.
One issue raised with repeated applications is the so-called "rebound effect".
As a turf comes out of growth regulation, an increase in growth rate above an untreated control is often observed.
The rebound effect is usually dismissed because the added growth occurs later in the spring or early summer when the
growth rate of the turf is slowing down so that the extra growth is not as apparent.
However, our research has indicated that when 3 to 5 applications are made during the course of a growing season, the total growth suppression achieved can be minimal, 10 to 20 % of an untreated turf.
Therefore, we prefer to think of repeated applications of a PGR as not so much achieving growth reduction but growth manipulation.
Growth is "pushed" from the spring into the summer.
Using a PGR gives turf managers growth management.
Other benefits may be obtained by using PGRs.
Some data indicates that PGR-treated turf can produce greater root mass, recover from injury faster, reduce water use rate, reduce disease incidence, and reduce Poa annua populations.
More research is needed to determine whether these observations are repeatable
under actual use conditions.
Plant growth regulators offer great potential for turf managers.
Time will tell whether that potential can be realized.
(BRUCE BRANHAM)
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