Editor's note: Madeleine Smith is a small grains plant pathology specialist with University of Minnesota Extension.
The high incidence of tan spot in small grains this year has necessitated early fungicide applications. In recent years, it's become common practice to spray fungicides for leaf diseases in small grains, along with early herbicide applications the around five-leaf stage.This often happens when there are no obvious signs of disease.
This practice has been widely adopted with the relative increase in commodity prices and availability of inexpensive generic fungicides that are effective against a number of common leaf diseases in small grains.
Is it always necessary to spray fungicides early if there is no disease evident? Does it matter?
Yes, it could very well matter.
Every time a fungicide, or any pesticide, is applied, the fungi or pest must adapt or die. Most organisms will try to adapt rather than die; it's survival of the fittest. The time it takes to adapt can vary and depends on a number of factors such as host resistance, dose rates and environmental conditions.
There are many cases of fungi adapting and becoming resistant to fungicides in commodity crops, including small grains.
The idea that there will always be a new, more effective fungicide is not necessarily the case. Chemical companies must work harder to identify and optimize new chemistries; consequently, the interval between new fungicides coming to market is increasing. They are often more expensive than current market leaders because of the additional investment in bringing these products to market.
What can you do to balance the need for control with the long-term protection of fungicide efficacy?
- Scout the crop once a week if possible, especially around the critical spraying decision-making periods. Use disease-forecasting models to assess risk of diseases developing in your area.
- Be able to identify pests in your crop. If in doubt, consult your local Extension educator. Plant samples can also be sent to the University Plant Disease Clinic: http://pdc.umn.edu.
- If there is no disease, carefully consider whether this early stage merits fungicide. If not, don't apply. Remember, you usually have the opportunity for spraying later if it becomes necessary.
- Select the appropriate product and rate for any disease, and the dose. Always carefully read the labels. Consider using tank mixes with different modes of action of fungicide to reduce the selection pressure on the fungus using the recommended rates. There are now many premixes available to assist in using tank mixes.
Appropriate use of fungicides will give good control of many fungal diseases in small grains. By using these fungicides only when necessary and combining modes of action in mixtures, we can help extend fungicide efficacy.