Wheat Disease Update: April 21, 2014

I was able to survey a substantial portion of the wheat crop production area during last Thursday and Friday (April 17 and 18). I’ve started rating wheat variety trials and in general the observable disease throughout the state remains extremely low. For specifics regarding the diseases observed, and those that continue to go without being observed in MS continue reading below.

Generally speaking, the growth stage of the wheat throughout the state ranges from wheat north of I-20 having reached the boot stage (approximately Feekes 10 to 10.1 in some limited fields) to wheat south of I-20 having reached flowering (Feekes 10.5.1) in most situations.

Comparison of bacterial leaf streak (left) and stripe rust (right). Symptoms can appear quite similar at first glance.

Bacterial leaf streak

I’ve observed quite a bit of bacterial leaf streak. Wheat in the Hattiesburg area has had quite a bit of bacterial leaf streak for a few weeks at this point. Generally speaking, when cooler temperatures occur some nights after the head has emerged the wheat plant appears to be more susceptible to infection from the bacterial organism regardless of the variety planted. Incidence and severity of this particular disease was the only disease that could be rated in all of the variety trials I observed last week (Beaumont, Fittler, Newton, Raymond). Earlier maturing varieties appeared to have more observable bacterial leaf streak.

The symptoms of bacterial leaf streak can easily be confused with stripe rust. Initial symptoms on leaf tissue appear as light yellow, water-soaked lesions. As the disease progresses the lesions will dry up and turn brown or necrotic. In contrast, the streaks (or stripes) observed as a result of stripe rust will be more subtle, more yellow, and the characteristic rows of pustules will rupture the leaf tissue and sporulation will easily be rubbed off the leaf. Bacterial leaf streak can result in lesions that tend to be small (1/8 of an inch or less) to long (great than an inch in length). Bacterial leaf streak can also be observed on the heads (or glumes) if they had emerged prior to the environmental event when infection occurred.

Don’t make a fungicide application as a result of an improper diagnosis.

BYDV can produce a range of symptom colors from yellow to purple.

Barley yellow dwarf virus

Over the past week I’ve received several calls and observed a lot of wheat fields with Barley yellow dwarf virus (BYDV) symptoms. In most situations I haven’t observed aphids at the same time. Once BYDV is observed no management practice is necessary. Even making an insecticide application this late in the season won’t remedy the presence of the virus. I suspect as more flag leaves emerge in late-planted wheat we’ll observe more BYDV. Generally speaking, the name of the virus causes some confusion. The dwarfing that can be associated with the disease is usually observed when early, fall infections occur shortly after wheat emergence. Late infection can be observed on the flag leaf. The coloring associated with the virus can range from yellow to purple depending on variety, strain of the virus, and likely additional variables.

Leaf rust

No leaf rust has been observed in MS. I’ve covered the area from Stoneville, to Beaumont to Newton to Fittler and haven’t observed any leaf rust in either the lower canopy or on the flag leaf itself.

Stripe rust

No stripe rust has been observed in MS.

Fusarium head blight (scab)

Follow the model information available at: http://www.wheatscab.psu.edu/. Presently, wheat in MS is not at risk to head blight or scab. Therefore I’m not suggesting a fungicide application be made to prevent scab. Generally speaking, rain around the time of flowering can result in scab. However, in the years that I’ve been in MS I’ve seen only scattered plants affected by scab. Rarely have I observed the disease throughout the state. In addition, keep in mind that the model on the webpage listed above is generated using environmental data collected from weather monitoring stations throughout MS. In my opinion, MS has a low number of weather stations that may skew the effectiveness of the model as presented.