Sunflower Rust and Phomopsis Stem Canker Appearing on Sunflower

Sunflower rust (Puccinia helianthi) and Phomopsis stem canker (Diaporthe spp.) are showing up in SDSU sunflower research trials near Onida and Highmore. The sunflower plants are in the R4 – R5 development stage (prior to or during bloom). Current weather conditions are favorable for disease development and presence of inoculum. These diseases can work their way up the crop canopy and compromise yield.

Sunflower Rust
Unlike the pathogens causing wheat leaf rust or common corn rust, the sunflower rust pathogen can overwinter in South Dakota and spores are locally dispersed. However, the disease onset is dependent on the weather conditions and source of inoculum. The most common stage of rust is the uredinia which can occur on the upper or undersides of leaves (Figure 1). The uredinia pustules are filled with cinnamon-brown spores that can be rubbed off easily.

This year the weather conditions (moderate to warm temperatures and frequent rains) have been favorable for rust development. Therefore it is important to keep an eye on sunflowers, particularly on confection sunflowers since they tend to be more susceptible than oilseed hybrids. A fungicide application may be needed if rust is found on the top of the canopy (upper four, fully expanded leaves) at 1% severity or higher in R5 development stage (flowering). Both strobilurin (Aproach, etc.) and triazole fungicides (Proline, etc.) are effective on sunflower rust. For more information on fungicide applications to control rust, refer to the 2015 Alfalfa & Oilseeds Pest Management Guide. Please note that if rust shows up at R6 or later, fungicide applications are not beneficial.

Phomopsis Stem Canker
The disease is caused by a fungus that survives on crop residues and infects the next sunflower crop at flowering. The earliest symptoms are browning (necrosis) of the leaf margins (Figure 2). Irregularly shaped brownish lesion form at the nodes of the stem (Figures 3 and 4), which can streak up the plant. Often the disease is confused with Phoma black stem and Sclerotinia stalk rot; however the lesion produced by the Phomopsis stem canker fungus is much larger and typically has a sunken border. Additionally, Phomopsis stem canker can cause lodging. Current research indicated that several fungi are associated with Phomopsis stem canker, and there are no in-season options available for management. We are conducting research this year (in collaboration with NDSU and University of Nebraska) to develop management strategies. Fungicides and host genetics are currently being evaluated for management and the research is supported by the National Sunflower Association.

Phomopsis Stem Canker Sampling & Analysis
If you identify or suspect that Phomopsis stem canker is in your field, please consider sending samples to the SDSU Plant Science Department. Samples will be used in the Phomopsis stem canker research conducted by Ms. Taylor Olson (a graduate student supported by the National Sunflower Association). Ms. Olson is developing a diagnostic assay for detection and quantification of pathogens causing Phomopsis stem canker on sunflower.

Source: Febina Mathew, South Dakota State University