LSU AgCenter plant pathologist Trey Price looks for signs of root disease as part of his disease management study at the Macon Ridge Research Station in Winnsboro.
Plant scientists agree that one of the greatest limits to yield in row crops in Louisiana is disease pressure.
LSU AgCenter plant pathologists Boyd Padgett and Trey Price have begun a three-year study to develop disease management programs for grain crops in central and south Louisiana.
Padgett said the climate in central and south Louisiana is conducive for the development of many diseases impacting soybean.
“From 2015 to 2019, diseases reduced yield an estimated 10%-20%,” he said. “Aerial blight, Cercospora leaf blight, frogeye leaf spot and target spot are just some diseases that can negatively impact yield and quality if not managed.”
In some instances, successful disease management must employ multiple practices. Therefore, management programs using multiple strategies need to be developed.
An effective disease management program begins with genetic resistance. Disease resistant varieties usually are no more expensive than susceptible varieties and could eliminate or reduce the need for a fungicide.
Padgett said his goal is to evaluate varieties entered in the LSU AgCenter Soybean Official Variety Trials not treated for disease resistance. He will then compare the results (disease and yields) of the official variety trials treated with fungicides to those not treated with fungicides.
“We also will evaluate the impact of disease development in varieties planted at various planting dates,” Padgett said.
In soybeans, genetic resistance is particularly important for managing Cercospora leaf blight and frogeye leaf spot, since strobilurin and thiophanate methyl fungicides are no longer effective due to pathogen resistance, he said.
To identify high-yielding disease resistant varieties LSU AgCenter scientists evaluate commercial and experimental varieties every year in official variety trials.
Trials are conducted on seven experiment stations. Trials in central and south Louisiana are normally treated with a fungicide to minimize the impact of disease.
Therefore, varieties in these trials at these locations cannot be evaluated for resistance to naturally occurring diseases. Additional official variety trials not treated with a fungicide are needed to evaluate disease resistance at some of these locations.
To evaluate varieties for resistance to naturally occurring diseases, an additional official variety trial (side by side if possible) will be planted on the Dean Lee Research and Extension Center and Doyle Chambers Central Research Station.
At each location one official variety trial will be treated with a fungicide, and one will not receive an application. To assess genetic resistance and the benefit of a fungicide, varieties will be monitored for disease development in each trial. To determine the impact of fungicide and genetic resistance on yield, trials will be harvested. This provides stakeholders the opportunity to evaluate the performance of each variety with and without a fungicide and identify disease resistant varieties.
Corn hybrids, grain sorghum hybrids, and small grain varieties entered in LSU AgCenter official variety trials conducted at the H. Rouse Caffey Rice Research Station, Doyle Chambers Central Research Station, Dean Lee Research and Extension Center and Iberia Research Station will be evaluated for disease resistance.
Price, who is also working on the project, said he is also looking at diseases in soybeans, corn, sorghum and wheat. At present, one of the main diseases in soybean that he’s working on is taproot decline. “One problem that we encounter with soybeans is the pathogens that remain on the residue in the field after harvest,” Price said.
“We are currently looking for fungicide options to slow the disease in soybeans,” he said. “In addition, we have a specialized variety trial that is inoculated with the pathogen to screen varieties in the field as well as yield impact trials.”
Corn producers may encounter many different foliar diseases throughout the state including common rust, southern rust and northern corn leaf blight.
“Other less common foliar diseases such as Curvularia leaf spot, gray leaf spot, anthracnose and southern corn leaf blight may occasionally significantly affect corn as well,” he said.
Curvularia leaf spot is an emerging issue in Louisiana and can be mistaken for eyespot. This disease is often seen after periods of extensive rainfall and appears to vary in severity among hybrids.
“We need to know if Curvularia leaf spot causes yield losses,” Price said. “Because if it does not, there is no need to manage the disease. So far, we have seen mixed results in our Curvularia yield loss studies.”
Wheat in Louisiana may be affected by a number of foliar diseases, such as leaf rust, stripe rust, stem rust and Septoria leaf blotch, he said.
Leaf rust, stripe rust, and Fusarium head blight are the major foliar diseases affecting wheat produced in Louisiana.
Stem rust and bacterial leaf streak have been sporadic problems in the state. Leaf and glume blotch and barley yellow dwarf are diseases that occur less frequently, but also may impact yields.
The occurrence of disease each year depends on which pathogens are present, environmental conditions, varieties, cultural practices and total acreage.
“It is imperative to continually develop management strategies for all potential diseases,” Price said. “The most efficient way to manage wheat diseases is to avoid them by planting resistant varieties.”
Fungicide application types, rates and timings for commercial and experimental fungicides are evaluated for efficacy against naturally occurring diseases in wheat.
Treatments are evaluated for their impact on disease development, grain yield and test weights at multiple locations.
One of the most common and destructive grain sorghum diseases in Louisiana is anthracnose, which may affect all parts of the sorghum plant.
The foliar phase of anthracnose is most observed and may cause losses up to 50% under optimal environmental conditions of cloudy, warm and humid weather.
“Hybrid selection is the best management option for anthracnose,” Price said. However, genetic resistance may not last long because the pathogen can develop cultivar-specific virulence.
Over the past several years in fungicide trials, researchers have observed different reactions to fungicide applications depending on the hybrid.
“For example, we will conduct the same test with the same application timing and fungicide treatments on two or more different hybrids,” he said. “When the data is analyzed, a given fungicide may perform very well on one hybrid and not the other.”
According to Price, continuing annual integrated disease management research must be done to keep up with changing environments, farming practices, fungicides, cultivars and economics.
LSU AgCenter plant pathologist Boyd Padgett checks wheat for disease pressure at the Dean Lee Research Station in Alexandria. Photos by Johnny Morgan