Boyd Padgett, Stephen A. Harrison, Patrick Colyer, Henry J. “Rick” Mascagni and Clayton Hollier
During 2005, Louisiana producers harvested nearly 110,000 acres of wheat with an estimated value of $19.6 million. Effective disease management using high-yielding, disease- resistant varieties developed by LSU AgCenter scientists helps maximize profits in the wheat industry.
Before 1999, leaf rust and to a lesser extent leaf and glume blotch were the major diseases affecting wheat grown in Louisiana. Since that time stripe rust has emerged as the predominant disease. Losses from stripe rust escalated from zero in 1999 to 5 percent in 2005, compared with leaf rust losses of 1 percent in both 1999 and 2005. This increased incidence of stripe rust may be due to changes in the genetics of the pathogen population. Stripe rust development was thought to subside when nighttime temperatures reached the mid-60s, but new strains of this pathogen appear to be better adapted to Louisiana’s warmer temperatures.
The emergence of stripe rust as a major disease of wheat in Louisiana has demonstrated the need for effective management strategies. Some of the existing strategies for leaf rust are not effective for stripe rust. For example, wheat varieties with resistance to leaf rust are not always resistant to stripe rust. Stripe rust epidemics develop earlier in the season than leaf rust epidemics, and the timing of fungicide applications for managing leaf rust may not be effective for managing stripe rust. To address this emerging disease, LSU AgCenter scientists have conducted research since 2000 targeting stripe rust management. Genetic Resistance
Genetic resistance is the most effective means for managing some diseases but may be short-lived for rust pathogens because new pathogenic strains can evolve quickly and unexpectedly. Therefore, disease-resistant varieties must be continually developed to replace those that become susceptible. Each year LSU AgCenter scientists evaluate wheat varieties at seven research stations – Dean Lee at Alexandria, Red River at Bossier City, Rice at Crowley, Iberia at Jeanerette, Northeast at St. Joseph, Macon Ridge at Winnsboro and the Ben Hur Farm on the Central Research Station at Baton Rouge. Each location represents a unique growing region of the state. Varieties are evaluated for agronomic characteristics and response to naturally occurring diseases.
Disease incidence and severity is documented several times during the growing season for each variety entered in these trials. The information from these trials provides producers with a source of nonbiased information for variety performance and serves as the basis for variety recommendations provided by the Louisiana Cooperative Extension Service, which is placed immediately on the LSU AgCenter’s Web site at www.lsuagcenter.com.
In 2006, the results of these efforts provided producers with five agronomically adapted stripe rust-resistant varieties in South Louisiana and six agronomically adapted stripe rustresistant varieties in North Louisiana. Host response to leaf rust ranged from moderately susceptible to resistant. Four varieties ranked at the Recommended-1 status are moderately susceptible to susceptible to stripe rust and are in jeopardy of being eliminated from the recommendations. Fungicide Use
Fungicides are another tool available to producers for managing stripe rust and are necessary for varieties susceptible to stripe rust. Commercial and experimental fungicides are evaluated each year by LSU AgCenter scientists. Tests are conducted on experiment stations and in producer fields. The effectiveness of fungicides on disease progress as well as the influence of application timing on disease development is evaluated.
From 2001 to 2005, fungicide performance on stripe rust has been evaluated on four research stations (Macon Ridge, Northeast, Dean Lee and Red River) and in producer fields in Concordia Parish. In summaries from tests conducted from 2002 to 2005, stripe rust severity was less and yields were greater in wheat treated with the fungicides Headline, Quadris or Tilt than in wheat not receiving a fungicide treatment (Table 1).
In 2005 and 2006, the fungicides Quilt (a combination of Quadris and Tilt) and Stratego (a combination of Gem and Tilt) were similar in efficacy to Headline, Quadris and Tilt. Yields in wheat sprayed with Quilt, however, were greater than the wheat treated with the other fungicides (Table 2).
The impact of application timing on disease progress and yield varied among tests and years. This is probably related to when stripe rust epidemics began in the area. When stripe rust was present during late winter or early spring, an earlier application was slightly better than a later application. If epidemics developed later in the growing season, application timing was not as important. More research is needed to determine the source of this variation among disease severity and yield to application timing.
The incidence and significance of stripe rust in Louisiana wheat have increased since 1999. To address this, LSU AgCenter scientists have intensified research in this area. Their efforts have provided producers with disease-resistant varieties and cost-effective fungicide programs to manage this disease.
Acknowledgment: Louisiana Soybean and Grain Research and Promotion Board(This article was published in the fall 2006 issue of Louisiana Agriculture.)