Cereal Leaf Diseases - Frequently Asked Questions

 Agri-NewsThis Week

When should cereal crops be checked for leaf diseases?
Cereal leaf diseases should be evaluated in late June and early July when the crop is about to go into the flag leaf stage or is in the flag leaf stage. Although protecting the crop at this stage is critical and any fungicide applications must be made prior to heading, field observations any time after crop emergence can provide valuable information for making current or future management decisions. Early season symptoms may indicate seed-borne disease and the potential need for seed treatments, or stubble-borne infection indicating problems with current crop rotation practices.

If disease symptoms are present early, will these spread to the flag leaf?
Good crop growing conditions also favour the development and spread of leaf diseases. A thick crop canopy and frequent rains or humid conditions will allow leaf diseases to spread along leaves that are already infected, and disease progression from lower leaves to the top of the plant. On the other hand, even if leaf diseases are present early in the growing season, the disease will not advance if conditions are not favourable.

Why is protecting the flag leaf so critical?
The flag leaf and the leaf below the flag drive grain filling. As much as 50% or more of grain yield is due to the sugars and starches produced by these two leaves. If this leaf area is lost, kernels will not fill properly, causing yield and quality losses.

Is there a way to estimate yield loss due to leaf diseases?
Researchers in Europe developed yield loss estimates for scald in barley. They found that yield loss was proportional to the area of the flag leaf and second last leaf infected by disease, according to the following equation:
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Yield loss (%) = [2/3 x the area of the flag leaf infected (%) + ½ x the area of the second last leaf infected (%)] /2

For example, the yield loss with 30% of flag leaf infected and 50% of second last leaf infected would be (30 x 2/3 + 50 x 1/2)/2 = 22.5%

Although this equation was developed specifically for scald in barley, it can be used to roughly estimate the yield losses due to other leaf diseases in wheat or barley.

What fungicides are used to protect cereals from leaf diseases?
There are several good crop protection products available. These include Caramba, Folicur, Headline, Prosaro, and Stratego, but not all of these products are registered for all cereals or for all diseases.

W. Clive James, J.E.E. Jenkins, J.L. Jemmett. 1968. The relationship between leaf blotch caused by Rhynchosporium secalis and losses in grain yield of spring barley. Ann. Appl. Biol. 62:273-288.

Prepared by Neil Whatley, Ag-Info Centre, Alberta Agriculture and Forestry