Status of Sugarbeet Leafspot Fungicides Studies done by Dr. Bill Bugbee, USDA sugarbeet pathologist, have shown that the Cercospora leafspot fungus has developed tolerance to Super Tin in some fields. Tolerance is more common in southern Minnesota and the southern Red River Valley than farther north, but it has been found in some of the more northern parts of the Red River Valley as well. Strains of the fungus resistant to Topsin M have also been found in some fields. Recently, he found some strains with tolerance to SuperTin and resistance to Topsin M. Although the terms resistance and tolerance are often used interchangeably, in this discussion each has a specific meaning. Resistance is used to indicate that the Cercospora fungus is unaffected by a level of fungicide that previously prevented growth in the laboratory. Tolerance is used to indicate that the Cercospora fungus makes reduced growth in the laboratory on a level of fungicide that previously prevented growth in the laboratory. Resistant strains of Cercospora are not controlled by field applications of a fungicide. If tolerant strains are present, a reduced level of control will occur. Two of Dr. Al Cattanach's fungicide trials located near Wahpeton were planted on or adjacent to fields where tin tolerance was identified last year. Super Tin, when used alone, did not provide the level of control we have expected with the tin fungicides. Tank mixes of tin and mancozeb did not seem to improve effectiveness much over the tin fungicide alone. A tank mix of 3/8 lb Topsin plus 3.75 oz Super Tin provided better control than the tin alone. Let me stress that Topsin should not be used alone, since resistant strains could prevent it from controlling Cercospora. In the tank mix, however, each product might provide some protection against resistance or tolerance to the other product. This is due to totally different modes of action of Topsin and Super Tin. The presence of strains of Cercospora that have double resistance (resistance to Topsin M and tolerance to Super Tin) raises some questions for next year. We do not know how competitive these strains with double resistance will be, and whether they are capable of building up in a commercial field. The data suggest that Super Tin + Topsin M tank mixes must be used very carefully in 1996, with the number of applications of this tank mix kept to a minimum (perhaps no more than two). Recommendations for 1996 will be found in the 1996 Sugarbeet Production Guide, better known as the "Pocket Guide". Application Equipment for White Mold Fungicides DuPont company conducted two commercial field trials using Benlate for control of white mold. Commercial equipment was used, and the comparisons in disease control that I took last week should be of interest for next season. These trials will be harvested in several weeks, and the yield data will be available later. The trials compared a Willmar air assist and a Brittonya sprayer. The Willmar delivered 17.5 gal/A. The Brittonya was used with drop nozzles to deliver 16 gal/A as a directed spray; both high pressure (175 psi) and low pressure (40 psi) were used. The Brittonya also was used to deliver 24 gal/A as a broadcast spray at high pressure (100 psi) and at low pressure (40 psi). Hollow cone nozzles were used at Northwood and flat fan nozzles were used at Hatton. Two fields were sprayed: a field of Norstar navy beans at Northwood and a field of Upland at Hatton. The experiments were not replicated, but plot size was 24- 32 rows by 3/8 mile long at Northwood and 3/4 mile long at Hatton. The field at Hatton had 36 inch rows; the field at Northwood had 30 inch rows. Both fields were at 70-100% bloom when sprayed on July 21. A second spray was applied broadcast on August 1 (11 days later) to half of the high pressure directed spray and to half of the high pressure broadcast spray treatments. I made white mold counts on August 17 (Northwood) and 18 (Hatton). Fifteen assessments of white mold incidence were made in each treatment. Assessments were usually 50 paces apart, and consisted of 22 observations (11 for each of two rows over 10 feet of row). There were highly significant differences between treatments at both locations. The results follow. % White Mold Incidence* Treatment Northwood Hatton 1. Willmar 0.38 ab 0.12 ab 2. Dir/High + Broadcast 0.08 a 0.00 a 3. Dir/High 0.00 a 1.0 b 4. Dir Low 0.24 a 10.1 c 5. Untreated 7.8 c 45.1 f 6. Broad/Hi + Broadcast 0.02 a 16.8 de 7. Broad/Hi 2.6 ab 12.0 cd 8. Broad/Low 11.8 c 21.7 e ------------------------------------------------------------ * means followed by the same letter are not significantly different (5% level) Treatments: 1. Willmar air assist 2. Directed high pressure (175 psi) spray, followed by a broadcast spray 11 days later 3. Directed high pressure spray 4. Directed low pressure (40 psi) spray 5. Untreated 6. Broadcast high pressure (100 psi) spray, followed by a broadcast spray 11 days later 7. Broadcast high pressure spray 8. Broadcast low pressure (40 psi) spray Summary and Conclusions All treatments at both locations except the broadcast low pressure spray at Northwood had significantly less white mold than the untreated check. The Willmar air assist and the directed high pressure spray provided the best white mold control. The directed low pressure spray was slightly less effective under the disease pressure at Hatton. Broadcast high pressure spray provided good white mold control, but at Hatton the level of control was slightly less than that of the directed high pressure spray. The broadcast low pressure spray was least effective at Hatton and not effective at Northwood. A second spray applied 11 days after the first appears to have provided little additional benefit over the first spray alone. (Submitted by: Art Lamey, Extension Plant Pathologist)