Fighting the Fungal Foes

Tractor with pesticide fungicide insecticide sprayer on farm land top view Spraying with pesticides and herbicides crops

By Harry Brook

Crops are up but moisture levels are low. The crop is hurting from the heat and some retailers may suggest you use fungicide to protect yield potential. Is this a good idea? Crop diseases tend to be a problem under cool or warm, moist conditions. For most diseases, you spray to prevent the spread of the disease on the leaves or stem. Should you be concerned about this under conditions of moisture stress?

Probably not. That doesn’t mean you ignore your fields, but you should be checking them on a regular basis for signs or symptoms. Often, disease will announce its presence by spots on lower leaves or damaged areas. Don’t confuse dried, dead lower leaves as being killed by disease if the main problem is heat and drought. Under dry conditions it is very doubtful you’ll get any value from an application of fungicide.

Another point about fungicides is that they do not move very far from where they are applied. Even so-called translocating products only move millimeters or centimetres from where the droplets land. And fungicide is a preventative, not a curative. If you’ve already lost 50% of leaf area, that is not coming back. All a fungicide can do is protect the healthy plant tissue that is alive. This makes it all the more important that it be applied in a lot of water. Often, labels of fungicides mention using at least 20 gallons (200 L/ha) of water per acre or more. You only protect what you can cover.

Under drought conditions, some might be tempted to apply fungicides for later diseases such as sclerotinia or fusarium head blight. Just be aware of the weather conditions required for these diseases to be a problem. The single biggest nutrient required by all our crops and plants, in general, is water. Without that, every other issue is a distant secondary concern.

If we get more moisture and things turn humid and wet this July, you may consider using fungicide for specific diseases. However, you need to know the diseases you are trying to protect from and evaluate the risk associated with the weather, combined with any physical presence of the disease. All leaf diseases should only be sprayed for if they are moving up the canopy and onto the productive top leaves.

Diseases that need to be decided before there are any symptoms are pretty rare. The two most common of this sort of disease are sclerotinia of canola and Fusarium head blight in cereals. In both these cases, decision trees have been developed to help evaluate the risk of either disease causing a problem. They aren’t perfect but they are useful tools when you’re sitting on the fence, deciding to spray or not.

Fungicides are useful tools to combat diseases caused by fungi. They are totally useless when applied to address a problem from a bacterial infection or environmental damage due to weather. Sometimes the damage from these can be confused with disease symptoms.

And finally, remember that overuse of a particular pesticide family or mode of action can lead to resistance in susceptible organisms. We already see this resistance showing up to Group 11 fungicides, the strobilurins. That is a lot of our most common fungicides and used in many mixes.

Fungicides are useful tools to combat specific disease issues. Use them wisely to ensure they provide you a reasonable return for your investment and are effective into the future. Make sure you have good reason to use them.

Harry Brook is Flagstaff County’s Agricultural Fieldman. He can be reached via email at: or by phone at: 780-384-4138.