Golfdom, April 2016

Super Science LOOKING AT GDD The perils of PGR over regulation By Bill Kreuser Ph D T he benefits of plant growth regulators PGRs have been touted for decades and with good reason Research shows that PGRs can reduce clipping yield increase turf quality and performance and improve stress tolerance To sustain these benefits growth regulators need frequent re application throughout the year As a result many superintendents have tightened their application intervals down to a week or less during the summer Most will also use a lower application rate to increase safety Other superintendents like to combine different standalone PGR products to increase control and provide greater safety While the concepts are sound these PGR programs can result in too much of a good thing PGR over regulation Can turfgrass be over regulated by gibberellin inhibiting PGRs and what does that really mean It depends on the demand of the turfgrass stand For example turf grown under a fence line or around tree bases typically has lower demands for quality and performance It just needs to look decent and grow slowly to reduce the need to string trim On the other hand a golf putting green needs some amount of daily growth to recover from traffic disease or other pest damage In these cases too much growth suppression from a PGR can limit recovery and lead to undesirable turf quality and performance Too much growth suppression also can cause mild discoloration phytotoxicity and even segregation of grasses like creeping bentgrass and annual bluegrass which also is undesirable This would be a case where PGRs are over regulating turf growth A CASE STUDY OF OVERREGULATION Over regulation of turf can resemble other types of stress and often is confused with disease The turf has a blue brown color and putting greens have a patchy appearance On closer inspection the leaves generally are free of lesions but have a worn appearance with brown shredded leaf tips Occasionally leaves will have lesions from diseases like dollar spot or leaf spot Last summer I was asked to visit several golf courses with these symptoms and it was obvious that something wasnt quite right Fortunately for me one of the courses with these symptoms was helping the Nebraska Turf Program with a PGR interseeding study As part of the study half of the No 2 green was not treated with a PGR except for inside the study area The superintendent would first spray our research area with his normal mix fertilizer fungicide etc then would add the PGR mix and spray the rest of his greens In essence half of his No 2 green was a great no PGR check plot It was clear that his PGR program was the culprit There was a stark line between the treated and non treated part of this green Figure 1 Different biotypes of annual bluegrass and creeping bentgrass ranged in color from greenish brown to blue green The non treated side had a bunch of annual bluegrass seedheads while the seedheads on the treated side were still buried in the canopy and emerged several weeks later What was the superintendent spraying to get this response The superintendent was mixing paclobutrazol at the low labeled rate with the standard rate of trinexapac ethyl I was applying similar rates of both products alone and didnt see the response It could have been the mix because mixing the PGRs increases the amount of 30 Golfdom April 2016 Golfdom com PHOTO BY BILL KREUSER FIGURE 1 The foreground shows an annual bluegrass creeping bentgrass green treated with both Trimmit 2 oz acre and Primo Maxx 6 oz acre every five to 10 days The background section of the green had the same maintenance but did not receive the PGR mix The turf in the foreground has typical signs of PGR over regulation including patches of discolored turf leaf segregation and worn leaf tips