Wheat Disease Update: March 23, 2013

Characteristic pattern of stripe rust. Not aggregated lesions in a “stripe” pattern.

Stripe rust (yellow rust)

Additional stripe rust has been detected throughout the Delta over the past two weeks.  Some fields with observable stripe rust have sizeable hot spots around the Cleveland and Greenwood areas.  At this time it is difficult to determine if a planted variety will ultimately show adult plant resistance to the fungus.  Ideally, resistant varieties with adult plant resistance, can become infected at vegetative stages and the disease will not result in a yield loss.  However, if the race of the fungus that causes stripe rust has changed and the fungus now has the ability to overcome the genotype in certain wheat varieties that confer resistance to the fungus, then varieties that were observed to have been resistant in the past will no longer be resistant.  We’ll have to wait several more weeks to make that determination as adult plant resistance will not normally confer resistance until approximately boot (Feekes 10.0).  At present wheat growth stages range from Feekes 4 to Feekes 9/10 in some parts of southern MS.  The specific responses of wheat varieties in the MS Wheat Variety Trial from 2012 can be accessed at: https://www.mississippi-crops.com/wp-content/uploads/2012/08/wheat-disease-reactions.pdf.

Large yellow area in a wheat field associated with intense stripe rust infection.

With that in mind, scout fields for the presence of stripe rust.  Observing large, yellow areas in fields for the presence of stripe rust is a good scouting technique at this point.  I’ve had several reports of fields with yellow areas that were believed to have been the result of standing water, but upon closer inspection were the result of intense strip rust infection.  In addition, I have already observed several leaves that were brought to me as well as observed firsthand one field situation where the characteristic “stripe” pattern on the leaves was detectable.  Remember, stripe rust produces aggregated patterns on the leaves of young, vegetative wheat plants as opposed to leaf rust that typically produces lesions in non-aggregated patterns on the infected leaf.  Refer to: https://www.mississippi-crops.com/2013/02/28/wheat-stripe-rust-detected-in-mississippi/ for additional information regarding the differences between wheat rusts (leaf, stem, stripe).

If a fungicide application is warranted I suggest choosing a product that will not only be fairly efficacious against stripe rust but less costly.  I make this suggestion since we have a long way to go before wheat matures and it is possible, depending on several variables, that a second application could be warranted depending on what the environment does between now and physiological maturity.  But, keep in mind when making a fungicide application that we will not likely harvest the high yields we have been accustomed to over the past few seasons simply due to a wetter than normal winter.  Moreover, do not expect the fungicide to make up for cut fertilizer rates or even skipped fertilizer applications as the fungicide can only be expected to protect yield from disease causing fungi.  Refer to: https://www.mississippi-crops.com/wp-content/uploads/2012/03/stripe-rust-fungicide-table.pdf for additional help in choosing products that are labeled for application in wheat to prevent yield loss as a result of stripe rust.  The associated table is a year old and some commercially available products may not be represented on the table due to either the age of the information or the lack of location data from throughout the wheat growing areas.

Powdery mildew

Powdery mildew of wheat. Note the characteristic pale white, fluffy lesions.

Powdery mildew has been detected in several fields around the Hattiesburg, MS area.  Typically, depending on the particular field setting, wheat seems to grow out of a situation where powdery mildew could be much of a concern.  Rarely have I suggested a fungicide application in a situation where powdery mildew was observed.  However, wheat in old catfish ponds should be scouted for powdery mildew since the increase in nitrogen that typically follows catfish farming can greatly influence the presence and severity of powdery mildew.  In addition, in southern MS keep in mind those fields with trees surrounding them will maintain high levels of humidity that can also be conducive to the development of powdery mildew.