by Boyd Padgett, Ph.D., Plant Pathologist, LSU AgCenter
I have received a few reports of leaf and stripe rust in commercial fields; however, not at high levels. I have not observed any rust in producer fields in Northeast and Central Louisiana. In my tests around the state (Dean Lee, Red River, Ben Hur, and Macon Ridge), I have observed leaf rust at low levels in my tests located at the Macon Ridge Research Station, at moderate levels at Ben Hur, and stripe rust at low levels in tests at Dean Lee. These tests are intentionally planted to SUSCEPTIBLE varieties, and are not representative of producer fields planted to resistant varieties. I have also observed powdery mildew in tests located at Ben Hur and the Red River Research Station. This disease is not considered to negatively impact wheat produced in Louisiana. However, if the disease is active (high incidence and severity) and present on the flag leaf prior to heading a fungicide may be justified. I HAVE NOT SEEN THIS SENARIO IN THE PAST 15 YEARS.
Producer fields: If rust incidence and severity is low (no pustules on the flag and confined to the lower canopy not active), most plants are fully headed (not flowering), and the variety is rust resistant, a fungicide is probably not needed.
Fungicides are justified if the wheat is at flag leaf to early heading and rust is active (spores are easily seen on the lower canopy). The following conditions are necessary for leaf and stripe rust development.
Stripe rust development is most aggressive when nighttime temperatures are 50 to 65oF in the presence of intermittent rain or dews. However, development can occur when temperatures are near freezing up to 70oF. Initial infections on seedling wheat may not have the characteristic striping pattern that occurs on more mature plants. Seedling infections often occur in ‘thumb-sized’ clusters on the leaves, as opposed to a random distribution that occurs with leaf rust. Infections may appear as linear rows of small yellow to light orange pustules (stripes) on the lower leaves during late winter or early spring. Striped patterns are typical of infections in older pants. If conditions remain favorable for development, pustules may cover the entire upper leaf surface, as well as portions of the head. A lifecycle (infection to reproduction) can be completed in 7 to 10 days when conditions are optimum for development.
Leaf rust is usually evident later in the season than stripe rust. This is because the leaf rust pathogen requires warmer temperatures for development than stripe rust. Initial symptoms of leaf rust begin as light yellow spots, usually on the lower foliage. As the disease develops, small pin-point pustules form on the upper leaf surface. Pustules are brick or dark red and occur randomly on the leaf. Similar to stripe rust, pustules can cover the entire leaf surface if conditions remain favorable for development. The disease develops optimally when nighttime temperatures are 50 to 70oF and leavers remain wet for 6 to 8 hours. Similar conditions will favor the development of leaf and glume blotch caused by Stagonospora and Septoria, respectively.
For more information concerning wheat disease management, contact your local LSU AgCenter county agent, LSU AgCenter specialist, or your agricultural consultant.