Methiozolin (MRC-01) has controlled annual bluegrass on golf putting greens in several research trials in the US and other countries but little is known about the method of selectivity between creeping bentgrass and annual bluegrass.  The proposed mode of action of methiozolin is a cell wall biosynthesis inhibitor and research is currently working on further elucidating this mode of action and the mechanism of selectivity.  In this study, we evaluated the effect of direct root exposure to methiozolin rates in nutrient solution on creeping bentgrass and annual bluegrass root regeneration in an aeroponics system. 

Studies were conducted in greenhouse and growth chamber environments with day/night temperatures of 29/21 C and supplemental light supplied by high-efficiency T-5 lamps generating 325 PAR on a 14 hr photoperiod.  Two aeroponics systems were used in replicating treatments.  The first consists of large chambers housing 6 annual bluegrass plants and 6 creeping bentgrass plants in a random arrangement.  The system uses a sump containing nutrient water that is pumped into another chamber where spray nozzles mist plant roots suspended from the chamber lid.  Excess nutrient drains out the bottom of the upper chamber back into the sump.  Plants started as single tiller shoots placed in foam plugs.  After 5 weeks, plants had 53 to 135 tillers and roots were 30 cm.  The plants were separated into size classes that were uniformly distributed between chambers.  Plant roots were cut to 2 cm and the study was initiated.  Nutrient sumps contained 38 L of nutrient water and were modified to contain methiozolin at 8, 16, 32, 65, 196, and 328 ppb.  These concentrations represent field application rates between 2.5 and 500 g ai/ha.  A comparison treatment included bensulide at a concentration to mimic the field application rate of 9 kg ai/ha.  After 21 days the first replicate was completed and data from the 6 subsample plants showed that a rapid drop in percentage root growth (% of NTC) occurred between 2.5 and 25 g ai/ha methiozolin and then percent reduction leveled at approximately 85% reduction of annual bluegrass root length and 60% reduction of creeping bentgrass root length.  The system was cleaned and prepared for the second replication and it was noted that plant roots would not grow in the chambers that previously held the higher methiozolin rates.  It became obvious that scrubbing and pumping with ammonia water was not sufficient to clean plasticware of methiozolin because methiozolin is not water soluble.  It was recommended we clean the system with an organic solvent such as methanol.  Before doing so, we allowed plants to grow in the system for three months to evaluate root regeneration.  After three months, creeping bentgrass roots showed a clear advantage over annual bluegrass.  In fact, many annual bluegrass plants died while creeping bentgrass plants continued to increase in size.  In the chamber that contained 338 ppb methiozolin before cleaning, no bentgrass or annual bluegrass roots were longer than the 2 cm cut length after 3 months.  A new system was created to allow for simultaneously replicated treatments.  This system uses smaller sumps of 500 ml capacity and pumps nutrient solution in tubing to dribble down plant roots rather than spraying plant roots with nutrient.  Results from this second replicated experiment are not available at the time of this writing and will be discussed at the annual meeting.  At this time, it appears methiozolin has a significant impact on root growth of both creeping bentgrass and annual bluegrass but there appears to be significantly more reduction of annual bluegrass than of creeping bentgrass.  We are not able to make conclusions at this time but a preliminary assumption is that differential root response to methiozolin may be one mechanism that allows for competitive displacement of annual bluegrass by creeping bentgrass on putting greens treated with methiozolin.