Resistance and Tolerance of the Sugarbeet Cercospora Fungus to Fungicides 

     The Cercospora fungus developed resistance to the benzimidazole 
fungicides Benlate, Mertect (no longer registered on sugarbeets) and Topsin M 
in 1981.  Resistant strains still persisted in the environment in 1995.  
Resistant strains are not controlled in the field by rates of the 
benzimidazole fungicides which control the sensitive strains;  resistant 
strains grow normally in the laboratory at rates which prevent any growth of 
sensitive strains.  

     In 1994 tin-tolerant strains of Cercospora were identified by Dr. Bill 
Bugbee, USDA sugarbeet pathologist.  These were strains that were tolerant to 
the triphenyltin hydroxide (TPTH) fungicide Super Tin.  Tolerant strains are 
not well controlled in the field by rates of TPTH which control the sensitive 
strains; tolerant strains have reduced growth in the laboratory at rates that 
prevent growth of sensitive strains.  

     A survey in 1995 by Dr. Bugbee showed that tolerance to TPTH was 
widespread in southern Minnesota and the southern Red River Valley, and that 
resistance to the benzimidazole fungicides also was common in the same area.  
He also found some strains that were resistant to the benzimidazole fungicdes 
and tolerant to TPTH.   

     Data from field plots at Crookston, MN; Breckenridge, MN and Renville, 
MN indicated that tank mixes of Super Tin and Topsin M provided better 
control than Super Tin alone in plots where TPTH resistance was known to 
occur.  However, data from the field plots at Crookston where resistance or 
tolerance had not been detected previously showed more TPTH tolerance and 
more Topsin M resistance in plots where the two were used in a tank mix than 
in other plots in the same field .  Therefore, extreme caution must be 
exercised in the use of the benzimidazole fungicides: they should never be 
used alone, and they should be used in a tank mix only once (or at most 
twice) in a year.   

     Another promising option was a sequential application of Super Tin and 
mancozeb (alternating application on a 7 day schedule), which  also provided 
good control.  The alternating option provided better disease control than a 
tank mix of Super Tin and mancozeb, when used on a 14 day application 
interval (too long an interval for mancozeb).  

     In 1996 timely and thorough scouting will be essential to Cercospora 
control.  It will also be important to use the Cercospora model and to 
continue use of fungicides until early or mid-September, if weather continues 
to favor disease.    We do not expect to have a suitable stand-alone 
fungicide for Cercospora control for at least several years.  In the 
meantime, tank mixes of Super Tin with a different class of fungicide, such 
as mancozeb or a benzimidazole fungicide, will be necessary at least from the 
Moorhead factory and south.  Since we can expect TPTH tolerance to become 
more common in the central and northern Red River Valley, tank mixes also may 
be needed farther north in 1996 or in subsequent years.  When the legal limit 
of 15 oz of Super Tin has been used on a field, it will be necessary to 
complete the season with mancozeb. 

(Submitted by: Art Lamey, NDSU Extension Plant Pathologist)