Determining Whether to Spray a Fungicide

Determining whether it is worth while to spray a fungicide for disease can be a difficult thing to asses. The first thing I always say about fungicides is you need to look at them like they are insurance. No one knows they are going to get hail, but they still get hail insurance. Maybe this is an extreme example, but it atleast gives you an idea of how these products need to be viewed. Based on my experience, if conditions are good (or under irrigation) I strongly recommend a fungicide because time and time again they atleast pay for themselves and protect yield at the very least.  I will mostly focus on cereals, but some of the notes can be used for a number of crops.

The first thing you need to look at is the conditions. Is there moisture and the proper temperatures? Typically if you have good growing conditions, you have good conditions for disease to develop. Most diseases, generally speaking, enjoy lots of moisture and that 20 degree celcius temperatures range. Also remember, when you crop canopies over, it has its own microclimate in there which can be very different from the conditions you feel when you step outside, so get out to your field and see what it is like under the canopy. Heavy dews are more than enough for disease spores to germinate. Remember to check the forecast moving forward too.

Next thing to ask is how good does your crop look? If you have a great crop coming it typically means you had/have the conditions for disease to develop, and you want to protect that crop. So keep that in mind. Where I am originally from in west central Sask, in 2010 there were great growing conditions during the year and great looking crops. The area hadn’t heard of Sclerotinia before, let alone seen it, and it moved in and had 50bu/ac looking lentil crops go less than 5bu/ac. Disease can move in fast, the guys that sprayed preventatively actually had a crop.

If your crop is wheat on wheat for example then your field is a good candidate for a fungicide application. Disease such as tan spot and septoria over winter on stubble from the previous year so the pressure is going to be even higher on those fields so go out and scout those first.

When it comes to leaf disease in cereals especially, always remember to take off some flag leaves or penultimate leaves (second from top) off and hold them up to the sun, if you see some light “pinhole” looking speckles that is the beginning of disease. You have about 8 days from the time a disease spore lands on a leaf until it has reproduced again and you see the big, yellow/brown spots on your leaves. This is when the rain splash, dew, animals etc move the spores up the leaves. I have always been told that assuming there are good disease conditions, whatever your leaf below looks like that’s what the leaf above it is going to look like in 5-7 days. So for example if you have 20% infection on the penultimate leaf, a week later there will be around 20% infection on the flag leaf.

With a “curative” fungicide such as some of the active ingredients in the group 3 triazole family (Caramba, Prosaro, etc) these products have the potential to suppress the disease up to about day 5 or 6 in the lifecycle of the disease, so if you have a lot of small pinholes on your leaves you can guarantee more are coming and it probably is best to apply a fungicide to ensure you maintain a high yield potential. Always remember it is best to spray preventativley before there signs of heavy disease pressure, once it moves in the damage happens fast.

There is also the plant health benefits some products bring to the table such as the strobilurin family of fungicides (group 11) so these can help pay for the fungicide and further help a plant yield better and fight disease. Some of these benefits include, increased nitrogen use efficiency, water use efficiency, and decreased ethylene producttion. Examples of these products include Headline and Quilt.

A great leaf disease basic formula I got from Steve Larocque`s Beyond Agronomy Newsletter is as follows to determine yield loss:

Percent Loss= .66 X % of flag leaf area infected + .50 X percent of penultimate leaf area infected divided by 2.

Example: 10% Flag leaf infection and 20% penultimate leaf infection on what looks like will be a 50bu an ac crop.

                .66 X 5% + .5 X 20% divided by 2

                = 6.65% yield loss X 50bu = 3.3 bu an acre loss

Assuming $7.50 a bushel Red Spring prices 3.3 X 7.50 = 24.75 an ac LOSS.

Note: Some leaf diseases are more aggressive than others, such as rust being more yield robbing than tan spot. This is a ball park.

Sclerotinia yield losses in canola typically look like this:

% infection divided by 2 = yield loss.

Example: 15% infection on a 40 bu an acre canola crop = 7.5% yield loss = 3 bu an ac

Using a $13 a bushel prices = 13 X 3 = 36 bushel an acre yield loss.

There are also the quality issues the diseases cause, especially with Fusairum Head Blight. You lose yield AND quality.

Always know the target disease you are going after, that way you can talk to your neighbours or local agronomists or reps about what product is going to be the most effective on what disease and at what stage. For example, just because you do not have any leaf disease, does not mean that fusarium headblight isnt going to be an issue so an application may be warranted at head emergence. Also, remember fungicides don’t last for the rest of the plant life cycle, typically after 14-21 days (depending on product) the fungicide will begin to wear off and the plants will be susceptible to disease again. Note also that spraying a fungicide after heads have begun to fill (in cereals) typically isnt economical, the disease has done most of its damage. You are just revenge spraying at that point. The flag leaf and penultimate leaf of a wheat crop combine to photosynthesize and produce upwards of 60% of the total yield so that shows how important protecting those 2 leafs are.

Always remember the disease triangle. Host, Pathogen, Environment. There needs to be all three present for disease to develop and flourish. For example if you are concerned about Fusarium Headblight in Durum then Durum is the host, if you have had FHB in the area before then the pathogen is present and if conditions are in that 20 degree range and damp for a period of time then you have the environment. This means there is a good chance your crop will have some FHB in it.

This was all over the place, I apologize, hopefully there is still something everyone could take away from it.