Role of Demethylation Inhibiting Fungicides In Poa Annua Seedhead Suppression Programs.

Increasing literature is available showing benefits of prophylactic fungicide applications for delaying onset and reducing rate of dollar spot progression. Demethylation inhibitors (DMI) are commonly used for this early season dollar spot prevention. These fungicides are typically applied in conjunction with annual bluegrass (Poa annua L.) seedhead suppression chemicals. DMI fungicides frequently have growth regulation properties and they could affect ABG seedhead suppression programs by altering suppression or turf tolerance. The use of growing degree days (GDD₅₀) is common for determining the timing for each application. GDD₅₀ for seedhead suppression and dollar spot are 50 and 140, respectively. These timings are often two to three weeks apart in the Mid-Atlantic region. Trials were established at the Virginia Tech Golf Course in Blacksburg, VA to evaluate these interactions. Experimental design was RCBD with four replications. DMI fungicides included fenarimol, metconazole, myclobutanil, propiconazole, triadimefon, triticonazole. PGR’s evaluated were mefluidide and a tank mixture of ethephon and trinexapac-ethyl (ETE). Initial PGR treatments were applied on 1 Apr 2010 and 29 Mar 2011 at 50 GDD₅₀. DMI’s were applied on 14 Apr 2010 and 20 Apr 2011 at 140 GDD₅₀. Data consisted of visually estimated turf quality, phytotoxicity, and percentage of seedhead coverage. Additionally, spectral reflectance data were used to determine normalized difference vegetative index (NDVI) and ratio vegetative index (RVI). In 2010, mefluidide suppressed 97% or better of ABG seedheads regardless of DMI. However, mefluidide also reduced plant quality and increased phytotoxicity compared to treatments excluding ETE + propiconazole. Seedhead coverage with ETE followed by DMI ranged from a 59% reduction (ETE + propiconazole) to a 24% increase (ETE + triticonazole). DMI alone saw no significant positive or negative influence on seedheads. Results from 2011 were less consistent, with little phytotoxicity from mefluidide and greater variability in seedhead suppression. Degree days accumulated much more rapidly in 2010 than 2011. The intervals between applications were 13 and 22 days, respectively. These results suggest that there is no correlation between DMI application and ABG seedhead suppression or phytotoxicity from PGR.