While many Manitoba growers are familiar with the extensive number of fungicides available for dealing with white mould (sclerotinia) in canola, they may not realize that there are a lot fewer options to treat this disease in soybeans.
Only four foliar fungicides are registered for control or suppression of white mould on soybeans: Acapela (DuPont), Priaxor (BASF), Allegro 500F (Syngenta) and Delaro (Bayer CropScience).
If you choose to spray, good coverage is important, said Michael Wunsch of North Dakota State University at the recent Manitoba Agronomists Conference in Winnipeg. For good coverage, timing is critical. Wunsch said available data suggests there is most white mould suppression and increased yield in soybeans when growers apply these fungicides at the R1 growth stage.
“That is when growers are getting the coverage they need for the product to provide activity against the target disease,” he said.
Research in the U.S. and Canada has shown that there is an average reduction in white mould of 44 to 45 per cent with a fungicide application at the R1 stage, and yield loss can vary anywhere from 1.3 to 5.5 bu./acre for every 10 per cent increase in white mould severity. Assuming disease control of 45 per cent and yield impacts of 1.6 to 4.6 bu./acre on average, soybeans valued at $8 bu. would have an economic threshold of 16 per cent to 46 per cent white mould levels before a fungicide application would pay for itself. If soybeans are $10/bu., the threshold is 13 per cent to 38 per cent and at $12/bu., the threshold range is 11 per cent to 31 per cent.
Wunsch said shorter-maturing soybeans are less susceptible to white mould than longer-maturing varieties.
“We have seen that white mould impact drops with maturity, so growers are likely to lose the most yield for every percentage increase in disease when they have a longer-maturity soybean,” said Wunsch.