Stemphylium leaf blight (caused by a fungus, Stemphylium vesicarium) of onion was first identified in Ontario in 2008 and has since become the most economically important disease in onions. The main management method for this disease is regular applications of foliar fungicides. However, many products are no longer effective against this fungus due to fungicide resistance. This is putting more pressure on the remaining effective fungicides. Growers should be aware that some fungicides that they have relied on for managing Stemphylium leaf blight may no longer be effective.
Stemphylium leaf blight starts as yellow-tan, water-soaked lesions that develop into elongated brown to black spots (Figure 1b). As the lesions grow, whole leaves will die back, and plants lose their photosynthetic ability. Onion leaves may die prematurely resulting in a reduced yield. Additionally, onions going into storage are typically sprayed with a sprout inhibitor before lodging. Plants need five to seven green leaves to take up the sprout inhibitor. If Stemphylium leaf blight kills most of the leaves, then sprout inhibitor cannot be applied, resulting in reduced storage and shelf life.
Early in the season, the symptoms of purple blotch, caused by Alternaria porri, look similar to those of Stemphylium leaf blight. Purple blotch lesions are tan to white with purple centers, while Stemphylium leaf blight causes tan lesions with black centers (Figure 1A). Botrytis leaf blight, caused by Botrytis squamosa, also causes lesions on onions but are easier to distinguish as they are smaller and irregular in shape with a greyish/white appearance (Figure 1C).
Stemphylium vesicarium spores are dispersed by wind and spores are present throughout the entire onion growing season, with spore counts increasing during the season. Stemphylium leaf blight lesions generally appear at the end of June to mid-July and continue to develop during warm temperatures with adequate rainfall or irrigation until harvest. Stemphylium vesicarium also causes purple spot in asparagus.
Annual fungicide efficacy trials conducted on Stemphylium leaf blight by the Ontario Research Station -Bradford (formerly the Muck Crops Research Station) in the Holland Marsh have shown reduced efficacy of many fungicides since 2013. Stemphylium vesicarium can develop resistance to fungicides over short periods of time. Populations of resistant fungi are selected for by repeated use of products with the same mode of action. In a field, the fungal population is quite variable, and some individuals can be more tolerant to the fungicide. These resistant individuals can survive the application of fungicides, reproduce and pass on their resistant trait. These traits are called mutations. As the frequency of resistant mutations increase in the field, fungicides become less effective. The main fungicides for management of Stemphylium leaf blight in onions have been in the FRAC (Fungicide Resistance Action Committee) groups 3, 7, 9 and 11. Fungicides fall within the same group when they have a similar target site in the pathogen.
Recent research out of Cornell University and the University of Guelph have detected populations of S. vesicarium in onion fields with reduced sensitivity to FRAC groups 9, 11 and now, group 7 fungicides. All products labeled for use in Canada for Stemphylium leaf blight contain at least one of these fungicide groups. Approximately 50 samples were collected from 2018 and 2019 across the main onion growing regions in Ontario, and 90% were insensitive to azoxystrobin (group 11) and 57% were insensitive to pyrimethanil (group 9) in laboratory sensitivity studies. In 2020 samples, this increased to where there was at least 90% resistance to both fungicides. In New York, multiple resistance genes have been detected for group 7 fungicides, and as a result, some group 7 fungicides might be more effective than others. Reduced sensitivity in laboratory and field studies to the group 7 fungicides boscalid, fluopyram and fluxapyroxad have all been seen in New York. Until we better understand our local population, growers should use caution when relying solely on these FRAC groups for management of Stemphylium leaf blight.
Table 1. Registered products listed in OMAFRA’s Vegetable Crop Production Guide, 2021 (omafra.gov.on.ca/english/crops/pub838/pub838.pdf)
Certainly, there are other foliar diseases present in an onion field such as Botrytis leaf blight and purple blotch and these fungicides can still be effective on these diseases. When using fungicides, follow these best management practices to reduce resistance development;
- Use fungicide groups in rotation.
- Where possible, incorporate effective broad-spectrum, multi-site fungicides (group Ms such as chlorothalonil or mancozeb) as well as biofungicides as these have a low risk of resistance development.
- Limit the number of total applications per season per FRAC group.
- For group 7 fungicides, if using a solo formulation such as Sercadis or Aprovia, apply in strict alternation with other fungicide groups and only use in 1/3 of total applications.
- Luna Tranquility and Quadris Top are mixtures (see table 1), but only one group in each of these products is effective against Stemphylium leaf blight, so these should be treated as solo products.
- When using a group 7 in a mixture (such as Miravis Duo or tank mixing a solo group 7 with mancozeb), do not make more than 50% of your applications with group 7 containing fungicides.
- Avoid using a group 7 as the first foliar application if the seed was treated with a group 7 (such as Pen 240FS).
- Start applications early, before the disease becomes severe the crop. Research from Michigan State University has shown that starting fungicide applications at the 3-4 and 5-7 leaf stage were more effective than starting at the 8-12 leaf stage.
Research conducted in the Holland Marsh has shown onion cultivars have varying degrees of susceptibility to Stemphylium leaf blight, however all onion cultivars are susceptible. Other cultural management strategies include limiting the leaf wetness period by irrigating in the early morning and allowing the crop to dry out as soon as possible to avoid prolonged periods of leaf wetness. Always remove or bury cull piles and bury leaf debris from the harvested crop as soon as possible. As with any other foliar disease of onion, it is beneficial to rotate with non-host crops for a minimum of three years. While this may not be practical in muck growing regions with a high density of onion fields, such as the Holland Marsh, longer crop rotations can help in reducing spore load.
Stemphylium leaf blight continues to be a challenging disease to control for onion growers. Disease forecasting shows promise for better timed fungicide applications, but more research is needed and is on-going. Continue to use all cultural methods to reduce disease severity and follow resistance management guidelines when using fungicides. On-going research into fungicide resistance will allow for better recommendations in future years. Follow ONVegetables.com for up-to-date information about Stemphylium leaf blight of onions and other vegetable-related news.
Katie Goldenhar – Pathologist, Horticulture Crops
Travis Cranmer – Vegetable Crop Specialist
References
- Hay, F., Sharma, S., Hoepting, C., Strickland, D., Luong, K., & Pethybridge, S. (2019). Emergence of Stemphylium Leaf Blight of Onion in New York Associated With Fungicide Resistance. Plant Disease, 103(12), 3083–3092. https://doi.org/10.1094/PDIS-03-19-0676-RE
- Muck Vegetable Cultivar Trial & Research Report 2020. https://bradford-crops.uoguelph.ca/system/files/Muck%20Vegetable%20Cultivar%20Trial%20Research%20Report%202020.pdf
- Stricker, S., Tayviah, C., Gossen, B., & Mcdonald, M. (2020). Fungicide efficacy and timing for the management of Stemphylium vesicarium on onion. Canadian Journal of Plant Pathology, 1–13. https://doi.org/10.1080/07060661.2020.1804461
- Hoepting, C. & Hay, F. (2021). Fragile FRAC 7 fungicides; on the brink of losing their utility for control of Stemphylium leaf blight in onion production in NY. VegEdge, 16 (15), 8-10 https://rvpadmin.cce.cornell.edu/pdf/veg_edge/pdf195_pdf.pdf
- Fungicide Resistance Action Committee, SDHI resistance management recommendations. https://www.frac.info/frac-teams/working-groups/sdhi-fungicides/recommendations-for-sdhi
