Update on Stemphylium Leaf Blight of Onions in Ontario

Marion Paibomesai and Michael Celetti, OMAFRA and Michael Tesfaendrias, University of Guelph-Muck Crop Research Station

Adapted from HortMatters article Vol. 12, Iss. 18

Stemphylium leaf blight is a foliar disease of onions and garlic caused by the fungus Stemphylium vesicarium. Symptoms start as small yellow to tan, water-soaked lesions that develop into elongated spots that turn dark olive brown to black when spores develop. Leaves may be completely blighted as the lesions coalesce (Fig. 1 and 2). The symptoms of stemphylium leaf blight are easily confused with purple blotch, which is caused by Alternaria porri (Fig. 3). These foliar diseases of onions can prematurely defoliate the crop which can compromise bulb quality and make the crop more susceptible to secondary diseases that affect bulb quality (i.e. storage rots caused by bacterial pathogens).

The stemphylium fungus typically invades damaged, diseased or dying leaf tissue and under ideal environmental conditions causes serious leaf damage. Development of this disease is favoured by warm (18 – 25°C) humid conditions and long periods of leaf wetness (16 hours or more). In 2010, this disease started to appear in mid-July. Since the pathogen is likely to enter leaves that have been physically damaged or infected by other diseases it is important to maintain healthy plant stands and control other common foliar diseases of onions such as downy mildew and Botrytis leaf blight.

Use similar cultural control methods that are used to manage other major foliar diseases of onions. Bury leaf debris left from last year’s crop through deep cultivation, rotate crops with non-host crops for 3 years and remove culls and volunteer plants from the field.  If possible increase plant spacing to facilitate air movement and quicker drying. Irrigate crops during the late morning or early afternoon to allow leaf surface to dry quicker and reduce the potential of infection.

In 2010 and 2011, stemphylium leaf blight was observed in many fields in the vicinity of Holland Marsh/Bradford, Ontario. This disease was first seen by scouts in 2008 in a few fields, more infected fields were identified in 2009, and over the last two years yield losses were associated with this disease. In NY State, this disease is a problem in mid-July and August. In Michigan, the disease is an occasional problem, but can be more difficult to control than other foliar diseases of onions when present. Over the last couple weeks, stemphylium leaf blight has been starting to show up in the Holland Marsh area.

Research from the US has shown that fungicides registered for the control of purple blotch will also be effective on stemphylium leaf blight; however, further investigation is required to improve management of this disease. Currently there is only one product registered in Ontario, as such there is a need to register more products for resistance management purposes! Quadris Top (azoxystrobin + difenoconazole) is currently registered for suppression of stemphylium leaf blight with a maximum of one application when specifically targeting stemphylium leaf blight. Please refer to label for directions for use. With little to no options for controlling this disease, stemphylium has been identified as a top onion minor use priority for Ontario in 2011 and 2012.

In 2011, the Muck Crops Research Station ran an efficacy trial evaluating fungicides for control of stemphylium leaf blight on dry bulb onions. (note: not all fungicides tested are registered for use on onions!). The disease pressure was high during the 2011 season with stemphylium leaf blight symptoms starting to develop mid-to-late June. Treatments were applied 6 times approximately every week during the season, starting at the beginning of July. All treatments reduced the severity of stemphylium leaf blight and Fontelis, Luna Tranquility and Inspire were the most effective at reducing stemphylium leaf blight when compared to the untreated control (Fig 4A). Each plot was rated for percent foliar disease (rating 0 = 0% foliar disease to rating 9 >85% foliar disease) and according to this all treatments except Bravo and Mancozeb had lower percent foliar area disease than the untreated check (Fig 4B).

Looking at yield and size distribution of the onions, there were no significant differences between the treatments; however, a higher percentage of small onions and lower marketable yield were correlated with percent total leaf length with stemphylium symptoms.

For the 2012 season, the Muck Crops Research Station will be running another efficacy trial evaluating fungicides for control of stemphylium leaf blight with hopes of registering new products to control this disease in the near future.