Understanding Sunflower Rust
By Robert Harveson and Samuel Markell
Rust, caused by the fungus Puccinia helianthi, can cause significant losses in both yield and seed quality on susceptible sunflower hybrids under conditions favorable for disease development. Rust is present to some extent each year in Nebraska and North Dakota in both cultivated and wild sunflower. However, it does not always occur early enough or in high enough levels to affect yields or require treatment with fungicides.
The pathogen has a complex life cycle consisting of five distinct spore stages — all of which occur on sunflower. The most damaging and commonly observed spore stage is the uredial — indicated by reddish-brown, cinnamon-colored pustules (uredia) containing thousands of spores (urediospores).
These “rust-colored” spores are the origin for the disease name and generally develop in mid- to late summer. This stage of the cycle is also called the “repeating stage” because it can repeatedly infect new leaves and plants throughout the season, as long as conducive conditions for disease prevail. As temperatures cool in the fall (<50° F), these spores are converted to dark, two-celled teliospores that serve as the overwintering stage of the fungus.
In early spring, teliospores germinate to produce basidiospores, which can infect sunflower seedlings. The basidiospore infections give rise sequentially to the pycnial and aecial spore stages. Pycnial lesions are circular and orange measuring 2-5 mm in diameter and surrounded by a yellow halo. They are found primarily on the upper leaf surface.
Aecia, which develop directly from the pycnia, are usually found on the lower leaf surface directly below the pycnia. The aeciospores, formed in developing aecia, then re-infect sunflower to create new uredia, completing the life cycle. The aecia are recognized as clusters of small yellowish-orange cups arranged in rings filled with spores.
Favoring Environmental Conditions
Environmental conditions favoring infection and disease development by the urediospore (repeating) stage include a minimum of two to three hours of leaf wetness and temperatures ranging from 55-85° F. New infections can occur every seven to 14 days depending on temperatures, with higher temperatures resulting in more rapid and frequent infections. For example, at 77° F, more than 90% of urediospores germinate if the leaf stays wet for three hours. At 68° F, the leaf must stay wet for more than eight hours to induce the same level of spore germination.
Rust Findings in 2009
The spring and summer of 2009 in western Nebraska and North Dakota were unseasonably cool and wet. In mid-May to mid-June of 2009, both the aecial and pycnial spore stages were readily identified from cultivated volunteer and wild sunflower species widespread throughout the sunflower production areas of western Nebraska. The aecial spore stage has similarly been noted for the last two years (2008and 2009) from North Dakota.
These observations are assumed to be very rare, and it is not known whether the high incidence of the early spore stages of rust is a common but generally unobserved phenomenon, or simply a consequence of the cool, wet environment experienced during the last several years.
The appearances of these early rust spore stages in 2009 provided us with new information on the likelihood for uredial infections to occur due to the early production of inoculum. In Nebraska, we did identify the uredial (repeating) stage occurring on volunteer sunflower plants in early June (before commercial plantings had started), with infections first appearing in commercial plantings in late June. So the major impact from these early infections was the need to treat for rust by mid-July, which is highly unusual for Nebraska.
Earlier infections have also been observed in North Dakota for several years, perhaps due in part to the widespread presence of these early rust spore stages.
• To reduce the chances of early infections, destroy volunteers and wild species to break the disease cycle before the uredial stage is formed.
• Since the pathogen overwinters as teliospores in infested residue, sunflower should not be planted two years in a row in the same field.
• Resistant hybrids are available in both confection and oilseed types.
• Fungicide usage can also be effective in rust management, and several fungicides are registered for use on sunflower for rust — including Headline (pyraclostrobin), Quadris (azoxystrobin), and Folicur (tebuconazole). Regular scouting for the presence of rust will help.
The economic threshold for applying fungicides is difficult to estimate. As a rule of thumb, uredial infections appearing on the upper four leaves closest to the head prior to petal drop will usually economically justify fungicide applications.
Current studies are currently being conducted jointly in North Dakota and Nebraska to evaluate both the most effective fungicides and the optimal timing of fungicide applications for reducing disease incidence and severity.
Robert Harveson is extension plant pathologist with the University of Nebraska’s Panhandle Research & Extension Center at Scottsbluff. Samuel Markell is extension plant pathologist at North Dakota State University, Fargo.
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