Marion Paibomesai, Vegetable Crops Specialist, OMAFRA
Michael Celetti, Plant Pathology Program Lead, OMAFRA
White rust (Albugo occidentalis) is a major fungal disease of spinach in the United States that occurs sporadically on spinach grown in Ontario, but when it does appear it has the potential to cause economic damage by making the spinach unmarketable. Symptoms of the disease first appear as yellow spots on the upper side of the leaf similar to downy mildew. However when the leaf is flipped over to expose the underside of the leaf, a cluster of white pustules are observed instead of a mat of grey or purplish downy growth as seen in downy mildew. Ideal conditions for white rust infection are wet and cool (16-21°C) especially when warm days are followed by cool nights with the chance of dew. This disease frequently shows up in Ontario spinach crops during August but can occur any time.
Management practices for white rust are similar to the practices used for downy mildew. There are some varieties (i.e. Regal, Samish, etc.) that are tolerant to white rust. Avoid planting fall crops in or adjacent to fields where an infected spring crop was grown and practice a 3-year crop rotation. Serenade ASO (Bacillus subtilis) is registered for suppression of white rust on spinach in Ontario; however, other products that are used to control downy mildew may also have activity on white rust. Please follow the label for precautions and directions of use.