Why Use Fungicide?

fungicides harry brook

By Harry Brook

Due to very wet conditions this year, there is a lot of fungicide being applied to crops right across the board. You see tracks in the field of almost all crops, from cereal to canola to peas. Is this a good idea?

Diseases caused by fungi come from spores released by diseased plant material that was infected last year. It overwinters on the crop residue then, when conditions are right, with good moisture, it sends out spores into the air to infect the host. Fungal diseases tend to be very specific as to their host crop so that a pea disease will not infect a cereal or canola crop and visa versa.

One way to reduce disease is to bury crop residue, but that leads to soil erosion. Most producers now use conservation seeding which keeps much of the crop residue on the surface. This leads to a greater likelihood of diseases being an issue. Another, non-chemical way to reduce the chance of disease is to have a varied crop rotation. Limiting the number of times a crop is available to be infected will limit infections.

Fungicides do not translocate well. If the label says it translocates, this means it will move millimetres from where it was applied, not throughout the whole plant, as herbicides tend to do. Because of this, it is crucial that coverage be as complete as possible. Water volumes need to be in the 15 L/acre + range to get that coverage.

Almost all fungicides can be classed as protectants. That means, if applied thoroughly, they will prevent the fungal spores from germinating and infecting the plant tissue. The whole goal is to preserve the leaf material, that is your photosynthetic factory, to fill the seeds. If you see symptoms appearing in the lower leaves and conditions are wet, or humid, then spraying fungicide will preserve the plant material that is unaffected. Any infected tissue will not get better with fungicide treatment.

Think of it as a protective coating of the leaf surface. In thick crop stands it may be next to impossible to get good penetration into the lower canopy, where the disease usually starts. In wheat, the top three leaves, the flag leaf and previous two, are responsible for over 90% of the grain filling. Therefore, in cereals, the main leaves to keep disease free are the flag leaf and penultimate (next-to-last) leaf.

When choosing to spray for diseases, it is imperative to make sure you chose the right product, applied at the right time and for the right reasons. Know the diseases you spray for. Scout your fields and identify risks, stopping disease from stealing the profit from your crop.

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