Reasons for Increasing Fungicide Rates and Reducing Spray Intervals in Summer Weather

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Reports of dollar spot breakthrough with traditionally strong dollar spot fungicides are often reported in areas experiencing tough early summer weather. Turf pathologists usually recommend increasing fungicide rates and reducing spray intervals to maintain control. There are many reasons for this recommendation including:

  • Fungicide residual – 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 grows quickly throughout the summer due to higher microbial activity releasing nitrogen from soil plus 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 explodes 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
  • More information – always refer to the label for specific recommendations. See the linked Bayer Solutions Guide and contact your Bayer Area Sales Manager if you need assistance

dollar spot activity

Periodic rainfall, high humidity, warm temperatures, and extended dew periods create extremely favorable conditions for dollar spot activity (Bayer).

  Dollar Spot Solutions Guide
Rob Golembiewski

by Rob Golembiewski , Ph.D., Green Solutions Team Specialist

Sunday, May 2, 2021

Zac Reicher

by Zac Reicher , Ph.D., Green Solutions Team Specialist

Sunday, May 2, 2021