Verdure: What to dew about dollar spot

A Penn State study looked at the influence of dew removal and mowing frequency on dollar spot control, testing the practices alone and in combination with fungicide use.


You have probably seen the classic plant disease triangle in nearly every turf disease presentation. The concept of the host-pathogen-environment interaction was known in plant pathology since the potato late blight epidemic of Ireland in the 1840s. Famous plant pathologist H.H. Whetzel, Ph.D., (1877-1944) is credited with articulating this concept into that triangle. Add time as a fourth factor, and you have a pyramid.

With foliar turf diseases, water films on leaf surfaces invite disease-causing pathogens to take up residence. Leaf wetness is a favorable environment to grow those foliar plant pathogens, which leads to various disease symptoms of spots and blights. At certain times of the year, we see small water droplets on the turf canopy during the morning sunrise. As turf leaf blades cool by radiating their heat, moisture in the nearby warm air condenses faster than it can evaporate, resulting in water droplets forming on those leaf blades — called dew.

What can we do about dew in relation to disease management? This question became Tanner Delvalle’s master’s thesis, under the guidance of Peter Landschoot, Ph.D. Dollar spot can be controlled successfully by many fungicide products. However, fungicide use on fairways may be limited due to economic, environmental and potential fungicide-resistance concerns. Delvalle wanted to know if daily dew removal, in conjunction with mowing practices and fungicide use, could improve dollar spot control in a creeping bentgrass fairway.

This work was located at Penn State’s Joseph Valentine Turfgrass Research Center in University Park, Pa. A field trial was conducted in summer 2009 and repeated in spring and summer 2010. The site was a 7-year-old mixed stand of 80% Penneagle creeping bentgrass and 20% annual bluegrass maintained at 0.510-inch (12.95-millimeter) fairway height of cut. The replicated trials consisted of two main plots of dew removed seven days per week, and no dew removed. Each main plot was subdivided into long strips for mowing two, four or six days per week. Those mowing plots were partitioned into sub-subplots as no fungicide or fungicide-treated (chlorothalonil [Daconil Ultrex 82.5 WDG, Syngenta], propiconazole [Propensity 1.3 ME, Sipcam Agro] and iprodione [Chipco 26 FT, Bayer Environmental Science]).

Dew was physically removed from the dew-removal plots by driving a Toro Reelmaster 5400-D across those plots with the mower units lowered but reels not engaged. Plots were mowed with the same mower according to the two-, four- or six-day schedule with clippings removed. Fungicides were applied only once at the start of each trial. Dollar spot occurred naturally, and pressure was considered moderate in all trials.

What did researchers find? It’s no surprise that removing dew every day resulted in much less dollar spot compared with not removing dew. This makes sense, since a disruption in leaf wetness is an unfavorable environment for the pathogen that causes dollar spot — recall that disease triangle. Plots mowed six days per week had much less dollar spot compared with plots mowed four or two times per week. Although it may not be economically feasible for most golf courses to mow fairways six days a week, this study showed the more days dew is removed from fairway canopies during peak periods of dollar spot activity, the better the suppression of new infections.

With all three fungicides tested, applications along with daily dew removal resulted in extending fungicide efficacy for as much as six to 10 days. This is a good example of how fungicides perform better when proper cultural practices are in place.

Dew removal can be achieved mechanically by mowing or dragging a hose or rope across the fairway to “knock down” the dew. There are dew-suppression products in the marketplace as well, but be sure to read the product label and ask to see field trial data to support the product claims.

Dew removal may not be something that has to be practiced throughout the entire growing season for benefits in dollar spot suppression to be realized. This practice yields the most benefits when dew formation is heaviest and remains on the canopy for long periods during the morning. Also, golfers should appreciate playing on a drier fairway.

Source: Delvalle, T.C., P.J. Landschoot and J.E. Kaminski. 2011. Effects of dew removal and mowing frequency on fungicide performance for dollar spot control. Plant Disease 95:1427-1432 (

Mike Fidanza is a professor of plant and soil science in the Division of Science at the Penn State University Berks Campus in Reading, Pa. He is a 19-year member of GCSAA.