Rust (Puccinia helianthi)
Rust is the most economically important foliar disease of sunflower in the United States. The disease can be found most years in the Northern Great Plains, the High Plains and Southern Great Plains, but severity varies widely. Occasionally, rust can be found in non-traditional sunflower growing areas and in the seed production region of California. Oilseed hybrids tend to be less susceptible than confectionary hybrids. In extreme cases yield losses over 80% have been reported. Active scouting and management of rust is strongly recommended.

The pathogen that causes sunflower rust is specific to sunflower, and cannot spread to other crops. The sunflower rust fungus survives even the harshest winter as spores can survive on wild, volunteer or commercial sunflower residue.  Favorable conditions for rust infection include frequent periods of leaf wetness (fog, dew) and temperatures between 55 and 85 F.
A rust epidemic may begin in sunflower fields anytime in the growing season. An early epidemic of rust begins when the pathogen overwinters and completes its life cycle in (or in close proximity to) a sunflower field. When this occurs, yellow to orange raised bumps (pycnia) about 1/8 to 1/4 inch in diameter appear on the top side of leaves or cotyledons (Figure 1), and a similarly sized cluster of orange cups (aecia) appear opposite them on the underside of the leaf or cotyledons (Figure 2).
rust pycnia
Figure 1. Rust pycnia
rust aecia
Figure 2. Rust aecia
Small raised cinnamon-brown pustules (uredinia) filled with dusty-brown urediniospores typically occur within a week or two of aecia observation on leaf tissue. Pustules may be surrounded by distinct chlorotic halos (Figure 3).
rust uredinia
Figure 3. Rust uredinia
More commonly, rust appears in late vegetative or early-mid reproductive growth stages, when sunflowers are infected by uredinispores that are aerially dispersed from a more distance source (such as infected commercial or wild sunflowers several miles away).  Under favorable conditions, urediniospores can cause new infections to produce new uredinia and urediniospores every seven to 14 days and quickly result in severe infection (Figure 4).
rust severe
Figure 4. Rust – severe
When rust is severe, pustules can also be found on stems, petioles and bracts (Figure 5). The severity of a rust disease epidemic is linked to the number of urediniospore infection cycles in the season; thus, yield loss can be very severe when an epidemic begins in early vegetative growth stages, and frequent heavy dews or fog occur during the growing season.  At the end of the growing season, the pustules transition into their overwintering stage (telia) and turn hard and black.
rust pustules on stems
Figure 5. Rust – stem and petiole
Rust is best managed using multiple tools and techniques, including destruction of overwinter hosts (volunteers, wild sunflowers), avoiding planting next to a field with infested residue, selection of a sunflower hybrid with genetic resistance and application of an efficacious foliar fungicide. Scouting sunflower fields for rust is critical when considering a foliar fungicide application, as not every field gets rust every year and the timing of onset and severity vary. The action threshold for application is 1% rust severity on the upper-four fully-expanded sunflower leaves at or before bloom (growth stage R5) (Figure 6).
rust with 1% severity
Figure 6. Rust – 1% severity

However, when a confectionary hybrid is infected from overwintering infection, an earlier application may be warranted.  Notably, the action threshold was developed from research conducted on confectionary sunflower, and may be slightly lower for oilseed sunflower.
Figure 1. Rust pycnia (Sam Markell, NDSU)
Figure 2. Rust aecia (Sam Markell, NDSU)
Figure 3. Rust uredinia (Andrew Friskop, NDSU)
Figure 4. Rust – severe (Sam Markell, NDSU)
Figure 5. Rust – stem and petiole (Sam Markell, NDSU)
Figure 6. Rust – 1% severity (Andrew Friskop, NDSU)
Additional Resources
Fungicide trials for rust management on oilseed and confection hybrids: 2016-2018 (Research paper) 

Fungicide trials for rust management on oilseed and confection hybrids: 2016-2018 (Powerpoint)

Rust severity diagrams
Other NSA Resources
Disclaimer statements
Information based in part on and reproduced from Kandel, H., Endres, G. and Buetow, R. 2020. Sunflower Production Guide. North Dakota Extension Publication A1995. Informational updates made possible by the Sunflower Pathology Working Group, and is/was supported by the USDA National Institute of Food and Agriculture, Crop Protection and Pest Management Program through the North Central IPM Center (2018-70006-28883).
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