Wheat Disease Update: March 25, 2016

For the most part the wheat season has remained quiet, at least in the pathology world. Due to the wet conditions that followed the 2015 season only a few fields of wheat were planted throughout Mississippi.  I was able to scout several variety trial locations (Fittler, Raymond, Stoneville) on Wednesday (March 25, 2016) and wheat growth stages varied wildly between the three locations (see: http://www.mississippi-crops.com/2015/02/12/identifying-wheat-growth-stages-using-the-feekes-scale/).  Wheat at Learned, south of I-20, ranged from Feekes 7/9 to Feekes 10.1.  Much of the difference at the Learned location was due to the differences in maturities between the varieties planted (71 varieties).  Wheat at Fittler ranged from Feekes 5 to 7/8.  The variety trial in Stoneville was planted in early December is well behind the other two locations (Feekes 4 to Feekes 6/7).  More mature wheat in Stoneville was observed in a graduate student cover-crop trial where the wheat was between Feekes 10.1 and Feekes 10.5.

Leaf rust is generally more “orange” than stripe rust and is randomly placed on the leaf surface with no defined pattern.

Leaf rust

To date, two fields have been observed to contain leaf rust. One field in the Canton, MS area and one field in east MS north of Columbus, MS.  Leaf rust observed on tillering wheat plants typically indicates the fungus overwintered in the field following a fall infection.  Normally, leaf rust is not much of a concern.  However, keep in mind that verifying the variety planted is important since the majority of the commercially available varieties planted in MS have good tolerance to the leaf rust fungus.

Observationally, leaf rust is more orange than stripe rust and generally does is not arranged in a pattern on the leaf. Pustules will be randomly scattered along the leaf surface and do not form the “striped” pattern that is associated with stripe rust as observed on flag leaves.

Wheat stripe rust as observed on the flag leaf. Stripe rust on tillering wheat may not develop the more characteristic “striping”.

Stripe rust

As of today, stripe rust has been observed around Canton, Fittler, and Stoneville, MS. Minimal infection was observed in a photo I received from the Canton, MS area early last week (March 21, 2016).  However, I was able to observe large hot spots of stripe rust in a commercial field that bordered the MSU wheat OVT planted at Fittler.  Wheat growth stages at Fittler were much less advanced than the wheat at Learned.  The infection in Stoneville was observed in wheat planted as a cover crop in October that had heads emerged and the flag leaf was infected.

Stripe rust as a disease can result in some confusion. In general, stripe rust can infect resistant varieties prior to the plant developing “adult plant resistance”.  Therefore, all wheat, whether of a resistant or susceptible variety can be susceptible to infection by the fungus from shortly after planting through the tillering stages until approximately Feekes 10.  The most important thing to do at this point is to scout fields for hot spots and make a decision on a fungicide application based on the presence of the disease.  In addition, if a fungicide application is deemed necessary make sure you do not expect more from the fungicide application.  For example, if the proper fertilization program has not been applied to the wheat do not expect the fungicide to compensate for fertility.  Fungicides should be used to reduce the yield losses associated with diseases such as stripe rust, not to replace a more important management practice.  Infection by the fungus at tillering growth stages can result in yield losses.  But, with that in mind, not all fields in an area are likely infected and a cheap fungicide application (e.g., propiconazole or tebuconazole) can be made at tillering growth stages to reduce the potential yield losses associated with the disease.  But, keep in mind, that only 4 fl oz/A of tebuconazole can be applied to a wheat crop in a single season.

Stripe rust in tillering wheat is generally observed lower in the plant canopy. In addition, the more characteristic “striping” pattern may not yet be visible.

When scouting fields for stripe rust in tillering wheat, focus on areas that appear to either be nutrient deficient or where water may have puddled for a period of time. In general, areas where either of these maladies may occur will appear to be yellow in color.  Stripe rust infected leaf material will typically be lower in the canopy prior to the emergence of the flag leaf.  Scouting for stripe rust in tillering wheat can be a hands-and-knees endeavor.

Bacterial leaf streak

As the name indicates, bacterial leaf streak is caused by a bacterium. Early-maturing wheat varieties planted south of I-20 were observed to contain substantial bacterial leaf streak last Wednesday (March 23, 2016).  In addition, wheat in Louisiana has been observed to have been infected following the last freezing weather.  Bacterial leaf streak is generally not observed until wheat plants reach growth stages where the head has emerged from the boot (Feekes 10.1).  However, in severe situations where the environment has dramatically shifted over a short period of time (e.g., windblown rain showers followed by freezing temperatures) bacterial leaf streak can be observed on younger wheat plants.

Note that bacterial leaf streak can easily be confused with leaf diseases caused by fungal organisms, such as Septoria leaf and glume blotch. The main differentiating characteristic between the two diseases would be the presence of reproductive propagules within the lesion.  Small, black pepper grains as observed through a hand lens would indicate a fungal disease rather than one caused by a bacterium.  For more information on telling the diseases apart please refer to: http://www.mississippi-crops.com/2012/04/15/septoria-leaf-blotch-leaf-blotch-stagonospora-leaf-and-glume-blotch-and-bacterial-leaf-streak-of-wheat/.

Bacterial leaf streak is generally observed following freezing temperatures as well as rainy conditions (Photo courtesy of T. Price, LSU AgCenter).

The symptoms associated with bacterial leaf streak are generally considered to be orange to brown lesions surrounded with a yellow halo. In addition, as the lesions mature and turn brown and necrotic they develop a “pitted” appearance as if something abrasive was rubbed on the leaf surface.