As with any fungicide application the first question you need to ask yourself is, “What disease am I targeting and is there disease pressure present?” Fungicides should not be sprayed in the absence of disease pressure.
While there are common foliar diseases that we see in soybeans it is very rare that these diseases cause any yield loss. Here’s a list of some of the more common foliar diseases in soybeans, some of which are included on fungicide labels:
1) Septoria brown spot – often shows up on the lower leaves, identified as small necrotic flecks, may be quite numerous in lower canopy; rarely causes yield loss
2) Bacterial blight – may look similar to Septoria brown spot, but with chlorotic halo surrounding lesions; cool, wet weather favours development of this disease; it’s caused by a bacterium, so a fungicide wouldn’t help anyway; more often seen in the spring when the temperatures are cooler; rarely causes yield loss
3) Downy mildew – similar symptoms to what we see in other crops, chlorotic spots visible on the upper leaf surface and cottony growth on the lower surface of the leaves; rarely causes yield loss
4) White mould AKA Sclerotinia stem rot – same causal agent as sclerotinia in canola, sunflower, dry beans, etc.; conducive conditions include wet, cool weather when the crop is flowering; soybeans are not as susceptible to sclerotinia as canola or sunflowers, so we don’t generally see the same losses from this disease in soybeans
Of all these foliar diseases, the main one with potential to cause loss in Manitoba is white mould. However, even in years where we see loss in canola or sunflowers due to sclerotinia we have not seen widespread loss in soybeans.
Another thing to consider when spraying fungicides is the potential development of fungicide resistance. There are more cases of pathogens developing fungicide resistance each year around the world and one of the candidates for this in Manitoba is Sclerotinia sclerotiorum, the causal agent of sclerotinia stem rot/white mould. Since fungicide applications is one of the main ways we control this disease in other crops, such as canola, it is important that we don’t overuse this technology to a point where it is no longer effective.