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Sunflower Rust

Wednesday, March 25, 2020
filed under: Disease

By Robert Harveson* 
Sunflower growe Rust is generally considered to be, universally, the most damaging disease of sunflower, and it is now found wherever the crop is grown.  Both the sunflower plant (Helianthus annuus) and the rust pathogen (Puccinia helianthi) are native to North America, and this fungus has followed the crop wherever it has migrated.  

The pathogen has a complex life cycle consisting of five distinct spore stages.  Unlike wheat rust, though, there is no alternate host to complete the pathogen’s life cycle; all spore stages of P. helianthi occur on Helianthus spp.  However, did you know that this pathogen has also played a significant role in the modification and production of the modern commercial sunflower?
Sunflower Arrives in Europe
It is thought that the Spanish first introduced sunflower from the New World into Europe through the Iberian Peninsula sometime around 1500.  The plant rapidly became popular — but originally it was as a curiosity and as an exotic ornamental plant, not as a food plant.  

Sunflower spread northward into England, Germany, Belgium and Holland before drifting into Eastern Europe in the 17th century.  Although it was first utilized as an ornamental plant, seed consumption became the preferred snack in the 1740s, replacing hazelnuts.  This dietary practice also assisted in the gradual, unanticipated selection for plants with large, single heads and larger seeds.  The British first noted the plant’s potential use as an oil crop in the early 1700s; however, it was not until the 1920s that the Russians really developed this trait.
Migration to Russia
Peter Romanov, also known as Peter the Great, was the Tsar of Russia from 1682-1725, and he has generally been credited with introducing sunflower into Russia.  After reputedly seeing the plants for the first time in Holland, he was so enamored with them that he brought the first seeds back to his homeland in the early 18th century.  After sunflower’s utility for producing vegetable oil for human consumption was recognized in Russia, the crop’s cultivation expanded rapidly.  
Sunflower Rust & Woronin
Russian botanist Mikhail Woronin conducted the first serious research on the rust disease prior to his pioneering studies of the cabbage club root disease for which he is better known.  During the late 1860s and early 1870s, rust became so severe that it threatened the entire industry in Russia, and growers approached Woronin for assistance.  He began studies into the pathogen life cycle and established the presence of both spore types.  He furthermore conducted the first epidemiological studies, determining that the incidence, severity and spread of the disease were increased with any system employing close row spacing and no crop rotation.  But those investigations, although useful, by no means solved the problem.
Early Breeding in Russia/Soviet Union
It was not until the early 1900s that an intensive breeding program began.  Improvement in the sunflower via plant breeding was first attempted by crossing different forms of the cultivated crop with one another.  The earliest record of this occurred about 1915 with the specific purpose of attaining resistance to rust; but other desired agronomic traits — like higher yields and oil content — also were pursued. 

In the 1930s, the Russian/Soviet breeder and plant hunter Nicholai Vavilov found and collected multiple accessions of wild sunflower in west Texas that resisted rust, increased yields and possessed tolerance to variable environments.  One of these accessions subsequently served as the source of rust resistance in the production of new cultivars.  
Return to North America
Sunflower’s homecoming as a crop in North America has often been credited to Russian Mennonite immigrants settling into Canada’s prairie provinces of Manitoba and Saskatchewan as early as the 1870s.  After its reintroduction into North America, sunflower was used primarily as a forage and silage crop in those areas, which were barely adequate for corn production.  However, the rust disease returned with the crop and continued to plague growers. 
Rust Pathogen’s Legacy
Breeding efforts for rust resistance in North America began in the mid-1930s, but were largely unsuccessful until some resistance was found in H. argophyllis and several wild forms of H. annuus from Texas.  

Early work at the Fargo, N.D., USDA Sunflower Research Unit consisted of disease surveys and selecting plants exhibiting resistance to rust. Those plants were then utilized in winter breeding nurseries in Hawaii and testing in North Dakota each summer.  The technique resulted in a resistant line in 1973, called “Sundak,” that was employed in creating the first commercial disease-resistant hybrids, increasing yields by up to 30%. 

More recently, sunflower rust was a sporadic disease throughout the 1990s and early 2000s.  In 2008, however, a spike in both incidence and severity generated big problems for many producers — particularly in the Upper Midwest.  Since then, substantial progress in both (1) hybrid with resistance to rust and (2) fungicide availability and efficacy has gone a long way toward reducing the yield impact from rust for commercial producers.  

Still, this disease continues to be a real concern and a yield threat, as new strains of the pathogen evolve.  In the 2017 National Sunflower Association crop survey, for instance, rust was found in all regions surveyed, with some locales approaching 100% in terms of the percentage of fields containing rust.  That didn’t translate into severe infestation in all cases; just that the disease was present.  The good news was that incidence, survey-wide, was just 38% in the 2017 survey, down from 62% and 2015 and 68% in 2013.

Research into the prevention and management of sunflower rust in ongoing, both among public scientists and private breeders.  Progress is ongoing in terms of both hybrid resistance and fungicide availability efficacy — and that’s a good story for a future article!                                  
* Robert Harveson is plant pathologist with the University of Nebraska’s Panhandle Research & Extension Center, Scottsbluff.
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