August 18, 2017Paul Giordano, Zac Reicher and Rob Golembiewski
Reports of dollar spot breakthrough with traditionally strong dollar spot fungicides have been reported throughout the parts the Northeastern U.S. as well as Ontario and Quebec. Turf pathologists have been recommending increasing fungicide rates and reducing spray intervals to achieve control. There are a number of reasons for this recommendation including:
Fungicide residual - Recent research by Dr. Rick Latin at Purdue University showed that more than 90% of the applied fungicide is depleted from within turfgrass leaves between 3 and 8 days. Furthermore, Dr. Paul Koch at the University of Wisconsin has demonstrated higher temperatures may result in quicker fungicide deterioration in turf leaves.
Disease inoculum is building - Even though disease symptoms are not visible, disease inoculum is likely building within the turfgrass system with favorable weather. This is especially true for dollar spot if early spring applications were not made, if low rates are used and/or intervals are stretched, and/or contact products have been relied on heavily throughout the season.
Turfgrass health is compromised - High heat naturally depletes carbohydrate reserves of cool-season turf and this is compounded by limited fertility, low mowing, and (potentially excess) growth regulation to control green speed or clippings in fairways.
Excessive plant growth - In areas not regulated, cool-season turf has been growing very fast throughout the summer due to higher microbial activity releasing nitrogen from soil and timely rainfalls. Fungicide residual activity in the turf leaves is likely reduced due to the growth and increased mowing.
Acute disease pressure is high - Combine the first four points with fluctuating soil moisture levels, high humidity, and warm temperatures, and dollar spot has been exploding on golf course greens, fairways and tees.
Other factors play a role - Spray volume, nozzle selection, droplet size, contact vs. systemic activity, and fungicide formulation can all impact length of control.
Bottom line - numerous factors play a role in reduced fungicide efficacy so be proactive and increase rates and tightened intervals when conditions are favorable for dollar spot. Though tempting, try to avoid skipping early season applications, and maintain higher rates and consistent intervals all year.