Time to apply a fungicide to control foliar diseases on Oklahoma wheat

Date:04-03-2012

The 2012 wheat crop in Oklahoma is approximately two weeks ahead of schedule compared to typical years, with heads emerging from the boot, heads in the boot or flag leaves fully emerged depending on location within the state.

To date, foliar diseases such as leaf rust and stripe rust are still at low levels, but are being reported across much of the state.

Bob Hunger, Oklahoma State University Cooperative Extension small grains pathologist, reports that powdery mildew is heavy in some fields but not in others, and tan spot, septoria leaf blotch and stagonospora glume blotch are present but not severe, although there are some exceptions to this in low- or no-till fields.

"Sometime between now and the next seven days to 10 days, producers will need to make a decision about whether or not to apply a foliar fungicide to protect the yield potential that exists at this time,” he said.

As of March 28, leaf rust has started to increase in prevalence and severity across Texas.  Spores of the fungus that cause this disease, and possibly also stripe rust, will be blown into Oklahoma and deposited on wheat plants. Temperature and moisture conditions have been ideal for both wheat growth and disease spread and infection.

"Planting a wheat variety last fall with resistance to the wheat rusts was advisable; however, if a susceptible variety was planted, then applying a foliar fungicide by ground rig or by aerial application is the only option available to protect the yield potential,” Hunger said. “Reports from the field indicate that yield potential is currently high for much of Oklahoma.”

Typically the optimum time to apply a foliar fungicide is when wheat is in the “boot” or the heads are just emerging. However, if foliar diseases are present before these stages, the fungicide should go on earlier to ensure that the flag leaf remains healthy and does not become infected during the time of head emergence and flowering.

"Over the years, I have seen many more instances where a fungicide was applied later than optimal, but only rarely have I seen where an application of a foliar fungicide was too early,” Hunger said. “This is related to the fact that fungicides are basically preventive rather than curative, so it is important to apply fungicides before disease is well established and severe on wheat plants.”

Fungicides typically provide full protection for at least two weeks with partial protection during the third week and sometimes even a little beyond that.

Foliar diseases, especially leaf rust, are most damaging when they are severe early on wheat.  For example, a leaf rust severity of 65 percent on flag leaves when wheat plants are flowering can reduce grain yield by as much as 30 percent.

"By comparison, the same level of infection when wheat is in the soft-to-hard dough stage will reduce yield by 3 percent to 7 percent,” Hunger said. “Thus, if you’re growing a variety susceptible to foliar diseases – and especially leaf rust – with excellent yield potential, consider using a fungicide to protect that potential.”

Producers looking for additional information on fungicide applications for control of foliar diseases in wheat should contact their local OSU Cooperative Extension county office and ask for Current Report No. 7668, “Foliar Fungicides and Wheat Production in Oklahoma.”