InteractiveTurf Research - Fungicide Resistant Dollar Spot

Dollar spot is the most common disease on fine turf in the upper Midwest. In years past, this disease was fairly simple to control, but now has become much more problematic for many superintendents. There are many possible reasons for this, especially when reduced fungicide rates and low spray volumes are used on fairway turf (see "Is Dollar Spot Evolving into an Uncontrollable Beast?"). But another important reason for dollar spot control failure is reduced sensitivity of the fungal pathogen (Sclerotinia homoeocarpa, or Rutstroemia floccosum) to fungicide applications. Dollar spot is perhaps the most notorious disease to develop fungicide resistance, but the occurrence of fungal resistance is becoming more commonplace in other turf pathosystems (e.g. metalaxyl resistance in Pythium blight, strobilurin resistance in gray leaf spot), because of the increasing selectivity and single-site mode of action of our newer systemic fungicides.

When resistance is suspected in a dollar spot (or any other fungal) population, superintendents/turf managers should send samples to a research lab for confirmation. We have run a number of fungicide screens on dollar spot fungi collected throughout the Chicago District, and have found several sites where dollar spot has become quite tolerant of demethylase inhibitor (DMI) fungicides. Please note however, that a complaint of fungicide failure does not necessarily equal a case of fungicide resistance. If you have concerns, please don't hesitate to send samples to our laboratory for screening - - a few grass blades from the margins of 5-10 diseased areas should suffice (call or email for more info).

Resistance to the benzimidazole class of fungicides (e.g. thiophanate-methyl) is commonly observed among dollar spot fungal populations in Illinois. A pathogen only needs a single gene mutation to confer resistance to this class of fungicides, and this mutation doesn't make the strain less able to compete ("less fit") with non-mutated strains in the turf ecosystem. Therefore, resistance to this chemistry is long lasting and complete in a fungal population. In other words, higher or more frequent applications of fungicides in this class will not increase disease control and the resistance problem will persist for many, many years to come.

Resistance to the DMI class of fungicides has also been observed in Illinois dollar spot populations, but at a much lower rate (5-10%). With DMI fungicides, the appearance of resistance is more gradual and is dose dependent, unlike the benzimidazole class. Apparently, there are a couple of different biochemical mechanisms that allow dollar spot fungi to develop resistance. A good analogy is to look at two different types of light switches. The simple light switch is on and off, which would be benzimidazole resistance, a strain is either resistant or not. DMI resistance, on the other hand, is like a dimmer switch; a strain can have varying degrees of DMI tolerance (light), which can therefore be overcome by differing rates of the fungicide. The end result is fungicide failure in the form of a shorter control interval than what was previously observed. A DMI fungicide may last 21-28 days on a sensitive fungal strain, but only 14 days or less on a resistant strain. Because of this, DMI resistance is much harder to discern without a lab test.

The dicarboximide class of fungicides (which includes vinclozolin & iprodione) is another important chemistry for dollar spot control. The mode of action of these fungicides is not well known, and the rate of occurrence of resistant strains among dollar spot fungi (and other fungi) appears to be low. Also, recent research suggests that once dicarboximide resistance develops in a fungal population, these fungi are "less fit" and will not predominate once the fungicide selection pressure is removed. With dicarboximides, the theory is that the mutation that confers resistance to this class of fungicides makes the strain less fit to compete with the naturally occurring strain because the dicarboximide resistant strains are more susceptible to winter kill, and would therefore not recur year after year.

This study was conducted at the Cantigny research green, where we know from previous studies that both benzimidazole and DMI resistant dollar spot resides. Fungicides from five different chemical classes (see Table 1) were applied at different rates, tank mixes, and sequences to explore options for disease control for superintendents with this problem. Our related goals were to (i) see if "synergism" between these compounds could lead to better disease control, (ii) illustrate dose dependancy of DMI resistance, and (iii) to see if newer classes of DMI fungicides (i.e. tebuconazole, triticonazole) could control dollar spot known to be resistant to other, previously used DMIs (propiconazole, triadimefon).

Synergism is "the cooperative action of discrete agencies such that the total effect is greater than the sum of the effects taken independently" (so sayeth Webster's dictionary). In this case, a fungicide when paired with another fungicide would control disease better than each additively on its own (the old 1+1=3 formula). One fungicide we looked at in detail this year was Signature. Signature (fosetyl-aluminum) is primarily a Pythium fungicide and has no dollar spot control activity, but its mode of action is said to be as a "plant defense activator", and may aid against overall summer stress or decline. In our tests, we did see better control in 2001 with a Signature and chlorothalonil tank mix than with chlorothalonil alone (see Chart 1). Upon further review in 2002, the effect was not as evident (see Chart 2), although disease pressure was lower during the experimental period, as evidenced by the better chlorothalonil control.

In 2002 trials, we also tank mixed Signature with a sequence of fungicides which included 1) 26GT @ 3 oz, 2) Dac. Ultrex @ 3.8 oz, 3) Triton @ 1 oz, and 4) Compass @ 0.15 oz. Adding Signature to each of these applications as a tank mix did not cause better control of the fungicide resistant dollar spot than the same sequence of fungicides without Signature (see Chart 2).

A tank mix of Lynx, a different DMI fungicide chemistry, and Compass, a strobilurin, was also examined in 2002. Lynx by itself did not show adequate control of this dollar spot population, which was known to be resistant to three other fungicides in the DMI class. A Lynx+Compass tank mix had lower disease severity than Lynx alone, but was by no means adequate, and did not show true synergistic control in this trial (see Chart 3).

Triton, another DMI fungicide chemistry, was also tested on the resistant dollar spot in 2002 at three different application rates on a 21 day interval. None of the three application rates controlled dollar spot adequately during periods of high disease pressure, but higher application rates (e.g. 2 oz) did control the disease fairly during periods of low disease pressure (see Chart 4). This illustrates the dose effect of DMI fungicides on DMI resistant dollar spot populations, and shows why shifts toward DMI resistance can go largely unnoticed by a turf manager, especially when the high rates are being used.

One other tank mix that was examined in this study was Cleary 3336 + Vorlan. The thought here was that control could be greater with the tank mix than with just Vorlan. As evidenced by Chart 5 however, this thinking is incorrect if you are applying the tank mix to benzimidazole resistant dollar spot. The Vorlan in the tank mix is the only chemical doing the actual control, and the inclusion of the 3336 adds no real benefit for dollar spot control, but would help control brown patch & anthracnose. .

So what is a superintendent to do if he knows he has benzimidazole and DMI resistant dollar spot? A chart of the best performers in our studies is outlined in Chart 5. The only answer is to turn to other fungicide chemistries!. Dicarboximides, like Vorlan and Chipco 26 GT, provided consistent control of dollar spot throughout the study. Since the dollar spot can become resistant to this chemistry however, reliance on these fungicides alone for control is not a good idea. . Chlorothalonil, if applied at the recommended control interval, would have also provided adequate control, making it one suitable sequencing partner.

Unfortunately, these are the only choices you have on the market right now for dollar spot control. Some new chemistries are coming down the pike, but only having two fungicides in your rotation now is a risky proposition. If you know that you have a dollar spot population which has only a low level of resistance to the DMIs, (that's why it's important to have them tested!), a high rate of a DMI will still afford you some level of control and probably should be kept in your rotation (especially in a tank mix). An added benefit is the DMIs will control some other diseases (such as anthracnose) that the dicarboximides are ineffective against.

			Application Information
Trade name	Common name	Fungicide class	Rates (per 1000 sq ft)	Timing

Echo, Daconil	chlorothalonil	multi-site	3.8 oz	14 days*
Cleary 3336	thiophanate-methyl	benzimidazole	2 oz	21 days
Chipco 26 GT	iprodione	dicarboximide	3 oz	21 days
Vorlan	vinclozolin	dicarboximide	1 oz	21 days
Signature	fosetyl-Al	indirect plant activator	4 oz	21 days
Lynx Exp.	tebuconazole	DMI	0.3 oz	21 days
Triton Exp.	triticonazole	DMI	0.75, 1, 2 oz	21 days

*Under periods of high dollar spot pressure, the timing of applications should be 7-10 days for maximum control at this rate.

Chart 1. 2001 data from site with dollar spot known resistance to the DMIs and benzimidazoles.

* Observed severity during 2000 season.

Chart 2: Effect of fungicides tankmixed with Signature on disease severity caused by fungicide resistant dollar spot..

Note: Checks sprayed with chlorothalonil on June 26, August 7, & August 28.
* Treatment started as a check plot but was converted to a treatment on June 26.
** Sequence = #1 26GT @ 3 oz, #2 Dac. Ultrex @ 3.8 oz, #3 Triton @ 1 oz, #4 Compass @ 0.15 oz

Chart 3: Effect of Lynx, a new DMI fungicide, on disease severity caused by dollar spot with known fungicide resistance.

Note: Checks sprayed with chlorothalonil on June 26, August 7, & August 28.

Chart 4: Effect of Triton, a new DMI fungicide, on disease severity caused by dollar spot with known fungicide resistance.

Note: Checks sprayed with chlorothalonil on June 26, August 7, & August 28.

Chart 5: Best performing fungicides on DMI & benzimidazole resistant dollar spot.