Managing Sunflower Rust
Sunflower rust was a sporadic disease throughout the 1990s and early 2000s. However, in 2008 an increase in incidence and severity caused producers major headaches. Management of the disease became a top priority in 2008, but fungicide recommendations for the United States were not well defined, and most commercially available hybrids were susceptible.
Making matters worse, the sexual stage of the rust pathogen was widespread, which can lead to the development of new races. Realizing the importance of the rust situation, the National Sunflower Association supported a graduate student in the Plant Pathology Department at North Dakota State University to conduct research on management of sunflower rust. Four years of research has provided answers to manage this disease.
Research goals were developed to answer questions about both the host and pathogen, in an effort to provide short-term and long-term management tools to sunflower growers. Objectives were to: (1) determine the best fungicide efficacy and timing for the management of sunflower rust; (2) determine the phenotypic (races) and genotypic diversity of the rust pathogen, Puccinia helianthi; and (3) identify rust resistance germplasm that can be incorporated into hybrids to manage rust in the future.
While the 2008 epidemic made it necessary for some sunflower producers to apply fungicides, very little information on fungicide timing and efficacy was available. From 2009 to 2012, a total of 21 fungicide efficacy and timing trials were conducted across five locations in North Dakota. Trials included both labeled fungicides Headline (BASF), Quadris (Syngenta) and tebuconazole (Folicur and generics; in our trials we used Tebuzol by UPI) and experimental fungicides. Fungicide timings included everything from vegetative herbicide timings to post bloom. Results of the trials provided the necessary information to create fungicide recommendations that have been adopted by both sunflower growers and companies.
Take-home messages from these trials were as follows:
Headline, Quadris and Tebuzol can effectively manage rust.
Timing of a fungicide application is more important than the chemistry.
A single fungicide application at R5 when rust severity on the upper four leaves is approximately 1% or greater is recommended to reduce disease and protect yield.
Single fungicide applications made at R6 did not effectively manage rust or protect yield.
This study demonstrated that labeled fungicides can effectively manage sunflower rust, and that timing of a fungicide application is more important than the fungicide choice (Headline, Quadris, Folicur generics). If rust appears in your field, scout frequently, observe the progression of disease, and pay special attention to the upper four leaves of the plant as the plant gets close to bloom.
The detection of the sexual stage of the pathogen created the potential for rapid and frequent race changes. Surveying the pathogen population was essential to find which races were prevalent and how virulent they were. Importantly, this information is necessary to determine which genes are most likely to manage rust in sunflower hybrids. A two-year survey was conducted in 2011 and 2012. It focused on North Dakota, but also included the states of California, Iowa, Minnesota, Nebraska, South Dakota and Texas, as well as the Canadian province of Manitoba. Results of the survey are currently being used by sunflower breeders within both USDA and the private sector.
Take-home messages from the survey were:
A total of 29 races were found over two years, and multiple races commonly occur in the same field. There were no obvious race differences among confection, oilseed or volunteer sunflower.
Race 304 was the predominant race detected in both years. Most resistant genes are effective against this race.
Race 777 was detected several times in 2012. This is significant because race 777 can overcome most resistance genes.
Results from this study were encouraging. Many races were found, but there are known resistance genes that can protect against most of them. However, a highly virulent race was detected several times, and race changes may occur in the future, which may challenge resistance genes. Periodic monitoring for race changes is very important for breeding implications, and searching for new resistance genes is critical for success in the long-term management of rust.
Identification of Resistance
A very important long-term tool for sunflower rust management is the use of resistant hybrids that are resistant to both common and highly virulent pathogen races. To identify potentially novel sources of resistance, screening sunflower germplasm to multiple races of rust is essential.
Identification of rust-resistant sunflower germplasm provides sunflower breeders with another tool in the arsenal that they can use to battle rust. A core-set of Helianthus annuus germplasm was obtained from USDA North Central Regional Plant Introduction Station in Ames, Iowa. The 112-line core-set is a cross-section of plant accessions that are thought to contain the greatest genetic diversity among the 1,500 cultivated accessions.
Both greenhouse and field studies were conducted to screen for rust resistance in this collection. We screened the core-set accessions to the rust races 300, 304, 336, 337 and 777 one at a time in the greenhouse. Then, the core-set accessions were screened at three field locations to a bulk sample of the races 300, 304, 336 and 337.
Results of the study identified resistance lines in the core-set of germplasm.
Take-home messages from the screening were:
The majority of the accessions tested were susceptible to all tested races of sunflower rust.
Two lines were resistant to all five races in the greenhouse and had low year-end rust severity in the field.
In some lines, certain plants were resistant while others were susceptible, indicating that resistance is present but not fixed in the lines. These lines will be investigated further and could prove to be very useful in the future.
Detecting sources of rust resistance is a crucial step in the long-term management of rust. Producing a sunflower hybrid with resistance to common and virulent rust races, while having sufficient yield, is the most cost-productive way of managing rust. It is essential to detect any novel resistance in sunflower due to the ever-changing ability of the pathogen.
Research efforts in the last four years have provided some answers for rust management in the present and lay a foundation for additional tools in the future. With more-concrete fungicide recommendations, a clearer picture of pathogen diversity and leads on genetic resistance to explore, the authors are optimistic about the future of rust management. Being able to incorporate both short-term and long-term management strategies is essential to offset the yield-robbing potential of sunflower rust.
The authors would like to thank the many people who collaborated on this project. The help from agriculture professionals, seed companies and chemical companies played a critical role in the success of the work in the last four years.Lastly, we would also like to thank the National Sunflower Association. Without the support of the sunflower growers and industry who are the NSA, none of this work would have been possible.
* Andrew Friskop conducted this research while pursuing his Ph.D. in plant pathology at North Dakota State University. He now is NDSU cereal crops extension plant pathologist. Sam Markell is NDSU extension plant pathologist, row crops. Tom Gulya is research plant pathologist with USDA-ARS at Fargo.
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