Guy Padgett, Price, III, Paul P, Harrison, Stephen A.
Boyd Padgett, Trey Price and Steve Harrison
Fusarium head blight (scab), caused primarily by Fusarium graminearum, devastated Louisiana wheat during the 2015 through 2017 growing seasons. The fungus causes shriveled seed, reduces yield and test weight, and produces the mycotoxin deoxynivalenol, which is toxic to animals and humans. Damage caused by this disease was, in part, responsible for the lowest wheat acreage recorded for Louisiana in recent years. Among the reasons this occurred was warm, wet weather that persisted during flowering that favored infection and sustained epidemics. Additionally, the lack of highly effective management practices, such as resistant varieties or effective fungicides, makes the disease difficult to manage.
Managing scab begins with knowing the conditions that favor infection and disease development. The fungus can infect corn; therefore, wheat grown in fields planted to corn the previous year have a higher risk of developing scab. Infected corn debris, as well as wheat straw and other host materials, can harbor the pathogen and serve as initial inoculum. Fungal spores produced on this debris are dispersed to nearby wheat plants by rain splash or wind. Other inoculum can be introduced into the field as windblown spores. Later in the season, plant to plant spread is possible. Infection can occur at any time from head emergence to harvest, but infection during flowering through the soft dough stage is most damaging. Conditions that favor infection are temperatures from 75 to 85 degrees and 48 to 72 hours of free moisture. Symptoms of the disease are shown in the accompanying photos.
While no single management practice is effective, combining a moderately resistant variety with a timely fungicide application for suppression can significantly reduce damage. Other practices that may aid in management include crop rotation with nonhost crops, tillage, mowing or shredding, and staggered planting or varietal maturity. At harvest, combine fan speed may be increased to blow out infected seed, which is lighter than healthy seed. Additionally, seed cleaning equipment may help remove affected seed but may not be cost-effective. These cultural practices alone will not completely manage Fusarium head blight. An integrated approach is required to lessen the impact of Fusarium head blight. It important to have a management plan in place before planting.
During the past several years, LSU AgCenter scientists have evaluated scab severity in variety trials and in inoculated, misted nurseries at several locations. These ratings can be found at LSU AgCenter Small Grains Breeding and Performance Trial Data. This information can be used to avoid planting a susceptible variety. However, varieties that have low scab ratings may still be genetically susceptible to infection because conditions during flowering did not favor infection and disease establishment. Determining genetic resistance should be based on results from multiple locations in years when scab was present. AgCenter scientists also collaborate in the U.S. Wheat and Barley Scab Initiative to identify varieties and management practices to control this disease. In addition, new and existing fungicides are evaluated to manage this disease.
It is common to see two to three years of scab epidemics followed by years with little to no disease. A risk assessment tool that is based on temperature and relative humidity is available online at the Fusarium Head Blight Prediction Center. It includes regional commentary to help determine risk at any location. This is the only practical way to determine the need to spray because scab symptoms may not show up for a week or more after infection occurs.
Boyd Padgett is a professor at the Dean Lee Research, Extension and Education Center. Trey Price is an assistant professor at the Macon Ridge Research Station. Steve Harrison is the Walker T. Nolin Professor in the School of Soil, Plant and Environmental Sciences.
(This article appears in the summer 2018 issue of Louisiana Agriculture.)
Symptoms of scab can appear 10 to 14 days after flowering as bleached heads, which can be seen from the turn row or head row. Photo by Boyd Padgett
Bleached heads are often mistaken as maturing wheat. Upon close inspection, affected wheat heads will usually have infected kernels showing a bleached appearance with pinkish, salmon or light orange coloration along the bracts at the base of the spikelets. Photo by Boyd Padgett
Discoloration is caused by millions of microscopic spores or reproductive structures of the fungal pathogen. Healthy kernels usually appear along with the diseased kernels on the same head. Photo by Trey Price
In extreme cases, the entire wheat head may be infected. At harvest, affected seed will be shriveled, off-color and much lighter than healthy kernels and are referred to as “tombstones.” Photo by Trey Price