By Febina Mathew, Samuel Markell & Robert Harveson*
Phomopsis stem canker gained sudden importance in the U.S. sunflower industry in the last decade because of the 2010 disease epidemic that caused yield losses in commercial fields and industry nurseries in Minnesota, North Dakota and South Dakota. This disease became a hot topic among the researchers, industry personnel and farmers, since it made us aware that endemic diseases can suddenly re-emerge and challenge our the industry.
Phomopsis stem canker was first reported in the U.S. in 1984. Only one fungus, Diaporthe helianthi, was believed to the sole causal agent. However, in 2010 a newly described fungus called D. gulyae was identified, along with D. helianthi, as causing the disease in Minnesota, North Dakota and South Dakota.
Since the beginning of the disease epidemic, the prevalence of Phomopsis stem canker has varied from field to field, depending on the presence of the causal fungus, weather conditions and production practices. According to the 2019 sunflower production survey organized by the National Sunflower Association, Phomopsis stem canker was prevalent in about 70% of the fields surveyed in Northern Great Plains and Central High Plains.
Coincidentally, three fungi (D. helianthi, D. gulyae and D. longicolla (see Figure 1) were recovered from the diseased stalks sampled from commercial fields by NSA surveyors, extension agents and farmers. (The samples came from six counties in Minnesota, 12 counties in North Dakota and seven counties in South Dakota.)
Among the three fungi, D. longicolla was identified as a pathogen of sunflower for the first time in the U.S. — and is possibly the 20th fungus described causing Phomopsis stem canker in the world.
Symptoms of Phomopsis stem canker include brown-colored, irregular spots on leaves that extend from the leaf margin (Figure 2) and a light-to-dark brown-colored “canker” on the stem (Figure 3). These symptoms are observed when the sunflower plants enter the reproductive growth stages. As the disease continues to develop, the fungus causes the stems of the sunflower plants to become hollow and lodge (Figure 4).
These symptoms can be easily confused with those of other sunflower stem diseases such as Sclerotinia mid-stem rot and Phoma black stem. However, unlike Phomopsis stem canker, lesions of Sclerotinia mid-stem rot are tan-colored, and black sclerotia can be seen inside the stem of the sunflower plants. As for Phoma black stem, the lesions are black-colored, relatively smaller, and they appear glossy and shield-shaped when compared to Phomopsis stem canker (Figure 5).
In response to the 2010 disease epidemic, NSA has been focusing on developing strategies to manage Phomopsis stem canker. The current disease management options available to farmers include the use of commercial hybrids with partial disease resistance and rotating sunflower with corn and small grains.
Foliar fungicides are labeled to manage sunflower foliar diseases (which includes Phomopsis stem canker), and these have active ingredients from the triazoles (FRAC 3), strobilurins (FRAC 11) and SDHI (FRAC 7) families. But despite fungicides being labeled for sunflower foliar diseases, their efficacy data against Phomopsis stem canker was lacking until recently.
Trials conducted by South Dakota State University, North Dakota State University and the University of Nebraska between 2014 and 2019 demonstrated that fungicides can help reduce Phomopsis stem canker severity and mitigate yield loss. The best timing was generally when the miniature floral head was formed on the sunflower plants (R1 growth stage), but was influenced by rainfall and disease pressure. Further, our research showed that FRAC 11 fungicides were most effective, and a single application of pyraclostrobin at R1 growth stage reduced the severity of Phomopsis stem canker (by 37.2%) and increased yield (by 5.6%) compared to when the fungicide was not sprayed. This indicates that fungicides are not a silver bullet, but might be a useful tool until Phomopsis stem canker can be better managed with resistant hybrids.
Currently, the NSA is supporting additional research work evaluating multiple rates of fungicides containing triazoles, SDHI, and QoI in Nebraska, North Dakota and South Dakota. The NSA maintains a database of research presentations on www.sunflowernsa.com. For more information on fungicide trials, search for Phomopsis in the database.
- Along with developing fungicide recommendations for sunflower farmers, we are currently researching host genetics and the etiology of the causal fungal pathogen to manage Phomopsis stem canker. For example:
- We are examining the effect of sunflower growth stage on development of Phomopsis stem canker in Nebraska, North Dakota and South Dakota (NSA-funded). Preliminary results suggest that species of Diaporthe (D. longicolla and D. gulyae) can live on sunflower without producing the disease symptoms we typically observed during early reproductive growth stages of crop development. This study has given us a new perspective on the etiology of the pathogens causing Phomopsis stem canker, and further research is warranted on how the use of fungicides and host resistance can affect the survival of these fungi in the early growth stages of sunflower development.
- We are working on identifying genomic regions and the candidate genes associated with Phomopsis stem canker in the cultivated sunflower germplasm (South Dakota Oilseeds Council-funded). Preliminary results are promising, and suggest that several genetic markers may be linked with Phomopsis stem canker resistance that can be incorporated into commercial hybrids.
- We are assessing the sensitivity of isolates of D. helianthi and D. gulyae to QoI (pyraclostrobin), triazole (tebuconazole) and SDHI (fluxapyroxad) fungicides (NSA-funded). Preliminary results from the fungicide sensitivity studies using isolates of D. gulyae collected from Minnesota, Nebraska and the two Dakotas indicate that the fungus has not developed any resistance to tebuconazole.
The fungicide trials were funded, in parts, by the National Sunflower Association, the AFRI Foundational: Critical Agricultural Research and Extension (CARE) Program [grant no. 2016-08651] from the USDA National Institute of Food and Agriculture, and chemical companies. We also thank the students and personnel from Mathew’s lab (Nathan Braun, Brian Kontz, Renan Guidini, Ruchika Ruchika, Nabin Dangal, Mackayln Fulton), Markell’s lab (Bryan Hansen, Jessica Halvorson, Michelle Gilley) and Harveson’s lab (Clay Carlson, Alison Rickey, Tyler Patrick).
* Febina Mathew, Samuel Markell and Robert Harveson are plant pathologists with, respectively, South Dakota State University, North Dakota State University and the University of Nebraska Panhandle Research & Extension Center, Scottsbluff.