March 28, 2008 

Early Applications May Not be Worth the Expense

Over the last several weeks there have been questions regarding the timing of wheat fungicide applications. Callers have asked:

1. Is there a benefit to applying a fungicide early (at jointing) followed by another application at flag leaf emergence to protect the flag leaf?
2. Should "split" applications be used, that is, a reduced rate early followed by another reduced rate at flag leaf emergence with the two rates adding up to the full label rate?

These questions have been discussed extensively by extension specialists in wheat growing states around the country. Data demonstrating a yield benefit from early and/or split fungicide applications on wheat are lacking. The few sets of data that have been shared among extension specialists around the country do not show consistency. In some instances the data show a slight yield benefit, in other instances there is no yield benefit.

Early Fungicides Not Always Warranted

The most consistent data are those that show a definite yield benefit when fungicides are applied at label rates to protect the flag leaf from diseases. Even then, if conditions do not favor disease development, there may be little or no yield benefit. Early applications may be beneficial in instances where conditions favor early season diseases such as powdery mildew and tan spot. However, early in the growing season, these diseases usually affect leaves in the lower canopy and these leaves do not contribute significantly to yield. Unless disease pressure is high or the variety planted is highly susceptible, there likely will be minimal or no yield benefit from early fungicide application. The residual effect of fungicides applied early usually will have expired by the critical grain filling period when the flag leaf should be protected.

Risk of Resistance Development

Concern has been expressed regarding the risk of developing fungicide resistance in fungal populations as a result of "split" applications (two or more applications at reduced rates that add up to the full label rate). Frequently exposing disease-causing fungi to lower than label rates of fungicides can lead to selection of fungicide-resistant mutants, especially if the same fungicide is used as a "split" application each year.

Wheat Prices and Your Decision

The current price of wheat apparently is a major factor in the decision to apply fungicides on wheat. The main purpose of applying fungicides is to control diseases. If fungicides are applied and there is no yield benefit because conditions did not favor disease development or a resistant variety was planted, there may be an economic loss regardless of the price of wheat.

When Early Applications are Recommended

It is recommended that wheat fields be scouted regularly for diseases. The highest yield and therefore economic benefit will be realized if fungicide applications are timed to protect the flag leaf.

Early fungicide applications may be justified when disease pressure is likely to be high based on

• favorable environmental conditions,
• high levels of fungal inoculum (for example, fruiting structures on wheat stubble in the case of tan spot),
• history of severe disease in a particular field,
• use of a highly susceptible variety, or
• a combination these factors.

An integrated approach that combines available disease management strategies into an integrated pest management (IPM) program can minimize losses and maximize yields and profits. These strategies include resistant varieties, crop rotation, judicial use of fungicides, and cultural practices such as irrigation and residue management.

Stephen Wegulo
Extension Plant Pathologist