Boyd Padgett, Myra Purvis, Steve Harrison, Rick Mascagni and Clayton Hollier
If not properly managed, diseases can reduce grain yield and quality, adversely affecting the profitability of wheat production in Louisiana. The major diseases of Louisiana wheat are leaf rust and stripe rust. Until recently, stripe rust was the major wheat disease in Louisiana, but leaf rust has re-emerged as the predominant threat. Symptoms of these diseases and conditions that favor their development are listed in Figure 1. Other diseases that occur less frequently or are not as widespread are bacterial streak, stem rust, leaf and glume blotch, barley yellow dwarf and head scab.
Effective disease management is crucial to a productive wheat cropping system, the foundation of which is high-yielding, disease-resistant varieties. Each year, LSU AgCenter scientists evaluate genetic resistance to plant pathogens in variety tests planted on research stations across the state. Varieties are monitored for agronomic characteristics and genetic resistance to pathogens. The results from these tests provide producers with a source of unbiased information for variety performance and serve as the basis for variety recommendations. Go to Variety Trials & Recommendations
When resistant varieties are not available or new races of plant pathogens emerge and threaten the crop, fungicides may be needed to combat disease epidemics. Therefore, LSU AgCenter scientists evaluate experimental and commercial fungicides for efficacy against these pathogens.
Field experiments targeting genetic resistance and fungicides were conducted on the Ben Hur Research Station, Baton Rouge; the Macon Ridge Research Station, Winnsboro; the Northeast Research Station
, St. Joseph; and the Dean Lee Research Station
, Alexandria, from 2003 to 2009. Selected fungicide treatments were applied to wheat varieties expressing varying degrees of genetic resistance to leaf and stripe rust (Table 1). Fungicides were applied at flag-leaf emergence, which is when the last leaf emerges just below the head or at 50 percent head emergence. Disease severity on the flag leaf and the flag-minus-one leaf (the leaf just below the flag leaf) was recorded in field plots several times during each growing season. Stripe rust was the predominant disease in 2004 and 2005, while leaf rust predominated from 2006 to 2009. In addition, grain yields and test weights were calculated for individual plots for treatment comparisons. Stripe Rust Evaluations
In studies conducted during 2004 and 2005, disease severity was less and yields were higher in stripe rust-susceptible wheat varieties treated with fungicides compared with nontreated wheat (Table 2). Stripe rust severity ranged from 50 percent to 16 percent in the initial ratings and 88 percent to 50 percent in the last ratings. Initial severity was lowest in wheat treated early at the flagleaf emergence stage; however, by late season, severity was similar among most fungicide treatments. Yields in treated wheat were 20 percent to 30 percent more than the nontreated wheat. There were minimal differences in yield among fungicide treatments based on application timing.
In four tests conducted during 2005, the fungicide Quilt was added as a treatment. Results were similar to 2004-2005. All stripe rust-susceptible varieties benefited from a fungicide application (Table 2). Based on early-season ratings, disease severity was lowest in wheat treated with Quilt; however, by late season, disease severity was high in all treatments. Disease severity was affected by application timing. Initial severity was less in wheat treated at flagleaf emergence. Yields from fungicide-treated wheat were 25 percent to 54 percent more than the nonsprayed wheat. Application timing also affected yield. Yields were higher in wheat treated with Quadris or Headline at flag-leaf emergence compared with applications at the 50 percent head emergence stage. There were minimal effects on yields when fungicides were applied in the absence of stripe rust (Table 3). Leaf Rust Evaluations
Leaf rust developed to damaging levels in tests conducted during 2005, 2008 and 2009. Disease severity in leaf rust-susceptible varieties was less in all wheat treated with a fungicide than in the non-treated wheat (Table 4). Disease severity in the initial rating was 46 percent in the non-treated and was lowest in wheat treated with Quilt at flag-leaf emergence. Disease severity in final ratings ranged from 84 percent (nontreated) to 43 percent (Quilt at flag-leaf emergence). Fungicides had minimal influence on test weights. Yields for all fungicide treatments were 17 percent to 27 percent higher than for the nontreated wheat. Disease severity was less in wheat treated with Headline at flag-leaf emergence compared with Headline applied at 50 percent head emergence; however, this difference in severity was not reflected in yields. Disease severity did not differ between the application timing of Quadris. Yields and test weights of the rust-resistant variety Terral LA841 did not differ among non-treated and fungicide-treated wheat (Table 5).
The residual activity of fungicides varied among products; however, most products provided at least four weeks of protection against both rust pathogens. It is important to note that residual activity can be eroded by harsh environmental conditions such as excessive rainfall or other conditions favorable for disease development. Generally, the most effective products are premixes of a strobilurin (azoxystrobin and trifloxystrobin) plus propiconazole. Propiconazole – the active ingredient in the fungicide Tilt – applied alone also provided satisfactory residual activity.
Leaf and stripe rusts can be managed effectively using high-yielding, diseaseresistant varieties. When these varieties are used, a fungicide usually does not provide an added benefit. When susceptible or moderately susceptible varieties are used, however, a fungicide may provide economic returns. While both rusts reached damaging levels in susceptible varieties, stripe rust caused more damage than leaf rust in these studies. This may be attributed to earlier initiation of stripe rust epidemics resulting in a longer period for disease development compared with leaf rust. Application timing was not a significant factor in most tests; however, in some tests the early timing (flag-leaf emergence) provided more protection than the later application (50 percent head emergence.). This was more pronounced when epidemics began prior to the later application.
Boyd Padgett, Professor, and Myra Purvis, Research Associate, Macon Ridge Research Station, Winnsboro, La.; Steve Harrison, Professor, School of Plant, Environmental & Soil Sciences, LSU AgCenter, Baton Rouge, La.; Rick Mascagni, Professor, Northeast Research Station, St. Joseph, La.; Clayton Hollier, Professor, Department of Plant Pathology & Crop Physiology, LSU AgCenter, Baton Rouge, La.
(This article was published in the summer 2010 issue of Louisiana Agriculture.)