Should I spray a fungicide in my soybeans?

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.

Have a follow-up question?

Drift – Use Your Judgement

Submitted by:  Jeanette Gaultier, MAFRI Pesticides – Minor Use & Regulatory Specialist

You’ve adjusted your sprayer to account for the technical factors of drift reduction (nozzle selection, boom height, etc.) but many aspects of pesticide drift rely on applicator judgement.  Consider the following before/during application:

Neighbouring crops:  Don’t assume your neighbour’s canola, soybean or corn is herbicide tolerant.  Drift is often allowed to occur to neighbouring crops based on the wrongful assumption of a particular herbicide tolerance package.  Also, while drift should be avoided on all crops, remember that certain crops, such as flax, potato, sunflower and beans (to name a few), are especially susceptible to injury by certain herbicides.

Sensitive areas:  Herbicide injury to shelterbelts accounted for ~15% of 2012 drift incidents.  Know your buffer zones; product doesn’t have to go far to injure shelterbelts that are right next to fields.  Returning to spray field portions next to sensitive areas after a shift in wind direction is an unpopular recommendation but one that could save you grief down the road. 

Pesticide chemistry:  Remember that ‘low volatile’ doesn’t mean ‘no’ volatile.  Many low volatile (LV) Group 4 herbicide formulations still have relatively high vapour pressures.  Use an amine formulation if it’s very hot or your field is next to a particularly sensitive crop (e.g. potatoes).  Also keep in mind that many herbicide chemistries are effective at low doses (e.g. Groups 2, 14, 27).  If it only takes a little to kill a weed, it only takes a little to result in drift injury.   

Insecticides and fungicides drift too:  MAFRI deals with a few insecticide/fungicide drift incidents a year.  Most often it’s a case of insecticide drift during a temperature inversion causing product to ‘hang’ in nearby farmyards.  While this doesn’t happen often, it should be avoided at all cost since pesticide exposure to humans and livestock can have serious consequences.

Check out SPRAYcast Manitoba ( to generate a 3 day forecast of optimal application times. 

Always refer to the product label.

Have a follow-up question?

How do I time an FHB application in a “stagey” crop?

This is a difficult question and there really is no easy answer. However, there are a few things you should consider:

  1. The ideal timing is early flower. Generally, wheat begins to flower about 3 days after head emergence and flowering usually lasts between 3 and 5 days. Target to hit as many heads as possible at the early flower stage.
  2. The majority of yield is determined by the main heads, so make sure you’re targeting early flowering on the main heads (ie. not the tillers)
  3. Consider the forecast. If your crop is really stagey and there is really no one time to hit the majority of the heads, think about the ideal conditions for infection – in the last week, has there been free moisture (precipitation or high humidity) for a 12 hour period and have temperatures ranged from 16 to 30°C? If not, consider holding off on an application until conditions are more favourable for disease. Use FHB risk models, such as the one produced by MAFRD ( as a guide.
  4. It’s better to be early than too late. Fungicides are designed to be used preventatively, not curatively.

Additional resources:

Submitted by:  Holly Derksen, Field Crops Pathologist, MAFRD

Have a follow-up question?

Can I re-crop my torn up winter wheat field to spring wheat? What is the risk of WSMV?

 It all depends on whether or not the green bridge was broken! Given the conditions across most of the province last fall the risk for Wheat Streak Mosaic Virus (WSMV) is likely lower than it has been in previous years, but it is not non-existent.

WSMV and its vector, the wheat curl mite, overwinter in Manitoba on host plants. While there are other minor hosts, the major concern for overwintering lies with winter wheat. Knowing this, there are preventative steps that can be taken to lower the risk of overwintering. The virus/mite combination needs green tissue in order to survive, so in the fall growers should ensure the green bridge is broken between the ripening of the spring wheat crop and emergence of the winter wheat. Ideally, this break would be approximately two weeks in length. If this green bridge is not broken, symptoms of WSMV may be observed in the winter wheat crop in the spring once it resumes active growth. If this winter wheat crop is torn up and replanted, it is suggested that the field be re-cropped to something other than spring wheat to prevent this disease cycle from continuing.

In many cases this year the winter wheat field that is being re-cropped did not emerge last fall and germination this spring was poor. If your winter wheat crop only germinated this spring the risk of WSMV overwintering in that field is lowered significantly. However, it is still important to scout for WSMV symptoms as there may have been some plants that germinated in the fall that overwintered the disease/vector or the overwintering may have occurred on one of the more minor hosts. If WSMV is found, the field should be either planted to something other than spring wheat or the green bridge must be broken (ie. two-week period free of green tissue required) to prevent further spread of the disease.

Have a follow-up question?

Should I be worried about aster yellows in my winter wheat and carry-over of the disease into next spring?


Aster yellows is spread by the aster leafhopper which feeds on a wide variety of hosts, including canola, flax, carrots, perennial landscape plants, and cereals (including winter wheat). However, the populations of aster leafhopper peaked in mid-summer this year and since then numbers have significantly declined. There is a chance that some of the insects may overwinter in Manitoba, but not enough to cause an issue with aster yellows in early spring next year. In 2013, like every year, the level of aster yellows will depend on when the populations of leafhoppers blow in from the southern US and to what extent they are infected with the disease.

Have a follow-up question?