Current Wheat Rust Situation: April 12, 2012

Stripe rust of wheat.

First, let me point out that the period for fungicide applications in wheat is now over.  I continue to receive telephone calls regarding fungicide applications and once wheat reaches flowering (Feekes 10.5.1, or the appearance of a single flower anywhere on the head) then all fungicides (especially triazoles) should no longer be applied for the management of rust (leaf or stripe).  Please carefully read the fungicide label before making a fungicide application and understand this is a residue issue especially when triazole (propiconazole, tebuconazole) fungicides are applied at late growth stages.  I realize that one fungicide is labeled for application at 10.5.1 but rusts are not on the label at this growth stage and the application should only be made to prevent losses attributed to Fusarium head blight (or scab).  Rarely have we had an issue with scab in MS; however, we do from time to time observe scattered (albeit random) heads throughout a field where scab has occurred.

Over the past few weeks I have surveyed numerous fields throughout MS for the presence of leaf and stripe rust.  Frankly, I don’t remember a year prior to 2012 when we’ve had this much wheat rust.

Current counties containing either leaf or strip rust include:

Leaf rust (24 counties):

Adams, Bolivar, Carroll, Coahoma, Covington, DeSoto, Grenada, Issaquena, Jefferson, Lowndes, Noxubee, Oktibbeha, Panola, Quitman, Rankin, Sharkey, Smith, Stone, Sunflower, Tallahatchie, Tunica, Warren, Washington, Winston

Stripe rust (22 counties):

Bolivar, Claiborne, Coahoma, DeSoto, Grenada, Issaquena, Leflore, Lowndes, Marshall, Newton, Noxubee, Oktibbeha, Panola, Quitman, Sharkey, Sunflower, Tallahatchie, Tate, Tippah, Tunica, Warren, Washington

Heavy leaf rust, approximately 3 weeks following the application of a strobilurin + triazole pre-mix.

Over the past two weeks I’ve received numerous reports of wheat fields receiving a fungicide application in response to rust (leaf or stripe) and several days later continuing to find rust in the crop.  Frankly, I’m not sure I can come up with a valid explanation for this particular situation but I do think there are several factors contributing to these observations.  To make the situation even more interesting, in most cases the envirornmental conditions over the past few weeks have not been considered to be “conducive” for the continued sporulation of leaf and stripe rust.  First and foremost, the fungicide, regardless of specific product, did not fail to control the rust present in the field.  The likely factors contributing include:

1. Overwhelming rust in the field at the time of the application

2. Conducive environment continuing for several weeks following the fungicide application

3. Reinfection of the wheat since most wheat fields have some rust thus an excessive level of inoculum in the general area

4. Heavy dews contributing excessive free moisture for infection to occur