P.V. Blenis, N.R. Knowles, D.K. Tompkins and J.B. Ross
Rings, or semi circles of dark green or dead grass, known as fairy rings, and caused by the fungus Marasmius oreades are a common problem on turfgrass in the Prairie provinces. The disease currently cannot be controlled except through disruptive and destructive means, such as removing the turf and cultivating the remaining soil. Chemical control is difficult because the fungus produces a water-repellent layer that prevents fungicide solutions from reaching the disease-causing fungus. The key to controlling this disease may therefore lie with developing a more efficient method for bringing water into contact with the pathogen.
Between 1990 and 1993, studies were conducted, with funding from Farming for the Future, to evaluate fungicides and surfactants for the control of fairy ring. In 1994 a new study was initiated to evaluate the organosilicone surfactant Sylgard, applied as a drench or through injection, to control fairy rings on a golf course.
Initially, we wished to evaluate a large number of surfactants to identify any that might be particularly effective in promoting soil water uptake by soils colonized by the fairy ring fungus. Thirteen surfactants were screened by immersing cores of infested turf in solutions of surfactant and water and then determining the amount of water that was taken up. The experiment was repeated using different times of immersion and different surfactant concentrations. These tests revealed that the organosilicone Silwet L-77 was significantly more effective than the non-organosilicones in increasing water uptake.
Greenhouse tests were conducted to evaluate three fungicides and the surfactants Silwet L-77 and Citowett Plus. Cores of infested turf were transplanted into the greenhouse and treated; the grass was cut over a 5 month period and weighed to determine treatment effects. In general there was no beneficial effect from the fungicides. The surfactants increased grass production, with Silwet L-77 being more effective than Citowett Plus.
Two organosilicone surfactants (Silwet L-77 and Sylgard) and the fungicide chlorothalonil were field-tested in 1992-93 at a single site in Edmonton. Twenty four rings were treated in 1992. In 1993, 12 new rings were treated, and 12 of the rings from 1992 were retreated. In both years, treatments were applied on June 24 and July 29 as a 4 L drench followed within one hour by a 6 L water drench; in the case of the surfactant + chlorothalonil treatment, the compounds were mixed prior to application. Rainfall was monitored and supplemented by watering, when necessary, to ensure that plots received at least 2.35 cm of water per week. Sampling was conducted to determine grass production, mushroom production, soil moisture content, and the presence or absence of the distinctive mycelium of the disease-causing fungus.
Although data were collected between 1992 and 1994, for the sake of brevity, only results from the last two years will be mentioned. The differences between the two different organosilicone surfactants were generally not significant. Also, there was generally little beneficial effect from the fungicide; applying the fungicide without the surfactants was much less effective than applying the surfactants without the fungicide. The surfactant treatment significantly increased grass production on plots treated in 1993 only and 1992 only by 54 % and 44 % respectively. On turf treated in both 1992 and 1993, there was numerically greater grass production for the water control than for the surfactants, although the difference was not statistically significant. Soil moisture content was significantly increased by the surfactants, regardless of whether they had been applied one or two years earlier. Mushrooms were produced in all infested plots that received no surfactant, regardless of whether fungicides had been applied. In contrast, only 9 of 72 plots treated with surfactant yielded mushrooms and the number of mushrooms produced per plot was greatly reduced. Mycelium was present in almost 95% of the cores taken from rings which received only water; in contrast, the fungus was evident in less than 10% of the cores from surfactant-treated rings. These results therefore, were strongly indicative of the potential for using organosilicone surfactants for the control of fairy ring.
In both greenhouse and field studies, surfactant treatments occasionally caused direct injury to the grass. To determine if reduced surfactant doses could control the disease without harming the grass, three surfactant doses (0, 21, and 85 mL/m2) of the two organosilicone surfactants (Silwet L-77 and Sylgard) were applied to 12 rings in 1993. The following were measured: weight of grass produced, the percentage of grass blades that appeared to have been injured by the surfactants, mushroom counts, the percentage of the surface of the ground that was covered by grass, and the water content and presence of mycelium in soil cores.
In general, there was no difference between the two surfactants. There was an extremely strong relationship between surfactant concentration and plant injury, with much more plant injury occurring at the higher dose. There was a strong tendency for grass production to increase with increasing surfactant concentration, although the lower concentration was nearly as effective as the higher. In contrast, the lower concentration resulted in greater grass cover than the higher concentration. The water content of cores taken from fairy rings treated with water was significantly less than that of cores taken from healthy turf outside of the ring. However, both doses of organosilicone treatment increased soil moisture such that it was not significantly different from uninfested turf, one year after treatment. Mushrooms were produced in 12, 11, and 1 of the 12 plots that received the water control, 21 and 85 mL/m2 surfactant treatments, respectively. Mycelium was present in nine of twelve cores that had been treated with the water control but in none of surfactant treated cores, regardless of dose. The results showed that the lower dose was better than the higher dose in causing less phytotoxicity and promoting greater grass cover but less effective in increasing grass productivity and decreasing mushroom production.
Trial Work Conducted in 1994
Last summer, Sylgard, applied as a drench or by injection, was tested at the Carstairs Community Golf Course in Carstairs, Alberta. Forty two rings were divided into seven groups of six rings each, such that all rings in a group were of approximately equal severity. The following six treatments were randomly assigned to the six rings per group: drench with water, drench with Sylgard, inject with water, inject with Sylgard, drench and inject with water, drench and inject with Sylgard. For the drenching treatment, the Sylgard dose was 51.7 mL/m2 which was intermediate between the two doses (21 mL/m2 and 85 mL/m2) that previously had been tested. However, the amount of water added to the surfactant was increased so that the concentration of the compound was reduced. The final concentration was a 2.2% solution This was done to ensure that there would not be phytotoxic effects of the application on the valuable golf course turf.
A Rogers 4903 Liquid pulse injector equipped with a 9 US gal/min triplex pump, operating at 5,000 psi, was used for injection. The injector was pulled by a John Deere 955 tractor with a hydrostatic transmission that permitted travel at 0.5 km/h Three applications of 2 L/m2 of water or surfactant were made. The rings were treated on June 3 and August 10, 1994. Application rates of surfactant were 13.2 mL/m2 for the injection treatment. Combined rates were 65 mL/m2 for the injection plus drench.
Throughout the summer measurements were made on turf area cover (National Turfgrass Evaluation Program standard rating system), phytotoxicity of the treatment using the ECW Western Canada Section rating system, soil moisture content, thatch levels and the presence of distinctive M. oreades mycelium.
There were no significant differences between the treatments for any of the variables measured. There is no obvious explanation why the surfactant treatments, which had worked so well at the Edmonton site, failed to have a beneficial effect on the golf course. However, it is possible that the level of supplemental watering was greater at Edmonton than at Carstairs. A second possibility is that thatch layers, which were rated severe, adversely affected the even distribution of the surfactant. It could also be that application rates of the surfactant were simply not high enough to thoroughly penetrate the area.
Further experiments will be conducted to further test the product. Increasing the concentration of the product and examining its phytotoxicity will be an important focus.