Wheat Disease Update: March 30, 2013

Stripe rust

Stripe rust continues to be observed throughout Mississippi as well as Louisiana and Arkansas.  At this time I would not say the disease is widespread (at least in MS) but symptoms can clearly be observed in some fields throughout the MS Delta.  As of this morning stripe rust has been confirmed in 10 MS counties: Bolivar, Coahoma, Holmes, Humphreys, Leflore, Sunflower, Tallahatchie, Tunica, Walthall (a single leaf), and Yazoo.

A few key points when making a decision to apply a fungicide:

1)      A fungicide application, made prior to heading, will not last through the end of the season.

2)      If you make an application using Folicur (or another fungicide containing tebuconazole such as Monsoon, Tebucon, Tebu-Crop, TebuStar), you are limited to a SINGLE application, 4 fl oz/A, in one season.  Please read and follow label directions carefully when choosing a product.

3)      Protecting the flag leaf is the most important thing to do once the flag leaf is exposed.  But, with that said, not all fields will require a fungicide application since in some cases adult plant resistance may take over and prevent yield loss.

4)      Know your wheat variety.  I have seen some fields with varieties that were considered to be resistant last year that do NOT have observable stripe rust.  Refer to https://www.mississippi-crops.com/wp-content/uploads/2012/08/wheat-disease-reactions.pdf  to determine the likely response of a particular wheat variety.  Observations are based on a scale of 0-100% so numbers represent leaf coverage by the particular diseases rated.  Remember, a fungicide provides the most benefit when disease occurs and provides the most benefit on more disease susceptible varieties.

5)      Scout for hot spots and determine the extent of the disease in a particular field prior to making a fungicide application.

6)      I have had several discussions this week regarding a delayed fungicide application (Feekes 10.5.1 (flowering), would be the latest application timing and Prosaro is the only product labeled for application at that particular growth stage) to protect against scab (Fusarium head scab, Fusarium head blight, head scab).  We don’t regularly have an issue with scab in MS.  In years when the disease occurs with any regularity, only one area of the state typically experiences much disease.  An extremely specific set of environmental conditions are required for scab to occur (rain at flowering).  Also, keep in mind, scab is the only disease on the label at the Feekes 10.5.1 application timing.  If you are concerned about the possibility of scab, or have experienced the disease in the past, you can use the scab prediction tool available at http://www.wheatscab.psu.edu/ .  Currently, the likelihood of scab developing in MS is extremely low, mostly because only a limited number of acres have even reached head emergence.

7)      Apply fungicides in a minimum of 5 gpa of water by air and 15 gpa of water by ground.  Coverage is key in protecting plants from any foliar disease.

Leaf rust

Over the past week leaf rust has been observed in a few Delta wheat fields.  Leaf rust differs from stripe rust in that pustules are more scattered, appear larger in size, and sporulation from the pustule will be more orange in color (see attached photo for comparison between leaf and stripe rust).  As a plant pathologist I am not near as concerned about leaf rust as I am stripe rust.  A similar situation could be stated regarding common rust in corn as compared to southern rust in corn.  As of this morning leaf rust has been confirmed in: Holmes, Sunflower, and Yazoo counties.

Powdery mildew

Powdery mildew can still be observed in some wheat fields in southern MS.  In addition, powdery mildew has been detected in the south Delta.  More than likely, those fields with observable powdery mildew were exposed to prolonged wet, cool conditions.  Scout fields for powdery mildew and keep in mind that in some instances “hot spots” of powdery mildew can be visible from the turn row but can easily be confused with stripe rust, nutrient deficiencies, freeze injury, and herbicide drift/injury.  Part the canopy back to observe for powdery mildew.  Even though the disease may be present lower in the plant canopy remember that yield loss is dictated by disease being present on the flag leaf.  If the flag leaf has emerged and it looks like the disease is going to remain in the lower canopy I don’t suggest you apply a fungicide.