DEVELOPMENT AND EVALUATION OF STRATEGIC FUNGICIDE PROGRAMS FOR CONTROL OF
SUMMER DISEASES IN BENTGRASS
 
John Royals and Dr. Bruce Martin, Graduate Committee: Dr. Steve Jeffers, Dr. Jim Camberato, Dr. Bert McCarty, and Dr. Larry Grimes

 

ABSTRACT
 

     Bentgrass (Agrostis palustris Huds.) is grown widely on golf course putting greens in the Southeastern United States. Brown patch (Rhizoctonia solani Kühn) and dollar spot (Sclerotinia homoeocarpa Bennett) are major diseases of bentgrass in warm humid regions. Fungicide use is necessary to maintain a viable putting surface when bentgrass is utilized. Over a three-year period, seven strategic preventive and two curative fungicide programs were evaluated for disease control efficacy during summer. Preventive programs consisted of fungicides applied at low label rates on a 14-day schedule and were designed to deploy the most efficacious fungicide to control the most likely disease that was expected to occur at a particular time during the summer (Figure 1). Dollar spot was expected to be most severe early and late in summer while brown patch was expected to be most severe in mid-summer. Curative applications, evaluated separately, were made after disease symptoms were present. The experiment was conducted on a green built to USGA specifications with four cultivars of bentgrass (Penncross, A-1, L-93 and Crenshaw) arranged in a Latin square design with four replications. Strategic preventive fungicide programs were more effective in controlling disease and maintaining good turf quality than the curative fungicide programs. Preventive fungicide programs 2, 5, and 8 were generally the best programs over the three-year period in reducing the Area Under the Disease Progress Curve for brown patch and dollar spot. These strategic fungicide programs were best because they utilized our guidelines for fungicide selection by placing the most efficient fungicide for controlling the disease that was likely to occur during that period. For dollar spot control, fungicides Chipco 26GT, Daconil Ultrex alone or in combination with Banner Maxx or Eagle were very effective (Figure 2). For brown patch control, the fungicides Heritage or Daconil Ultrex in combination with Chipco Signature were very effective (Figure 3).
 

     To determine if and when different species of Rhizoctonia occurred during the summer, symptomatic areas of brown patch in some fungicide-treated plots were sampled twice each month of the study. Three hundred and eleven isolates of R. solani, R. zeae and R. oryzae were recovered in 1999, 2000, and 2001. Identification was confirmed by staining nuclei of the hyphae and by cultural characteristics of colonies on agar media (Photo 1). Rhizoctonia solani was isolated in greater numbers in June and August  (10 in 1999 and 78 in 2001). However, in 2000, greater numbers (19 isolates) of R. solani were collected in July. This most likely was due to cooler temperatures encountered in 2000 compared to 1999 and 2001. Rhizoctonia zeae was isolated in greater numbers in July and August (14 in 2000 and 7 in 2001). However, in 1999, R. zeae predominated in June isolations with a total of 84 isolates collected. In 1999, 2000 and 2001, R. zeae was recovered from brown patch symptoms in plots treated previously with thiophanate methyl (fungicide program 7). Disease caused by this pathogen increased in these plots after thiophanate methyl application. This is the first field report documenting a lack of control of R. zeae by thiophanate methyl (a benzimidazole class of fungicide).

CONCLUSIONS

 

     It makes a significant difference in how fungicides are deployed during hot, humid summers in the Southeast on creeping bentgrass putting greens. The best programs were effective in control of dollar spot early and late in summer, and highly effective in brown patch control in midsummer. During mid-summer stress, programs containing Heritage or Daconil + Aliette Signature were best. Rhizoctonia solani and R. zeae were documented as significant pathogens in the field, but R. zeae was not controlled by thiophanate methyl in mid-summer (Photo 2).