Another Sign of Spring

While an ornithologist might say spring starts when the sandhill cranes have returned from their wintering grounds in the southern US and northern Mexico, a cereal pathologist may look for aecia of Puccinia cornonata on buckthorn to tell that spring is underway.  The picture below is from Steele County in SE Minnesota and confirms that the crown rust season of oats has started. That means that two of the three corners of the disease triangle are now in place.  Favorable conditions for the spores released by these aecia will be the third and final corner of the disease triangle for crown rust to start in oats. This past week, however, the risk models indicate that conditions have been too dry and that the resulting dew periods have been too short for crown rust to successfully infect oats at this point.  

To use the risk model, simply select the MAWN/NDAWN weather station nearest to you, select the flag leaf growth stage, and look for the number of consecutive days that were favorable for leaf rust to develop (leaf rust is a reasonable proxy for crown rust) in the previous seven to ten days. 

The optimum time to control crown rust is when the flag leaf has fully emerged. The first-seeded fields of oats are expected to reach the flag-leaf stage this weekend or early next week. Now is a good time to plan your oat scouting activity.  It would be a surprise if you find any crown rust at this time in the lower canopy, but if you do, consider using a fungicide when the flag leaf has fully extended, especially when the immediate forecast indicates higher dew points and/or precipitation. Both of those will yield longer leaf wetness periods, closing the third corner on the disease triangle.  If the canopy is completely disease-free, keep an eye on the risk models and postpone the decision until the crop reaches the heading stage in another week to ten days, and revisit the fungicide decision at that time.

Photo 1 -  Aecia of crown rust on buckthorn in Steele County, MN,
on May 29th, 2026. (Photo credit - Mark Bernard)