Onion Purple Blotch

Purple blotch, caused by Alternaria porri, is probably one of the most common diseases of onion and is distributed worldwide. The fungus overwinters as mycelium in onion leaf debris. During periods favourable for sporulation (leaf wetness or relative humidity of 90 percent or higher for 12 or more hours), inoculum becomes wind-borne and spreads to new foliage. Infection is highest at 25°C. Older plant tissue is more susceptible to infection by purple blotch. Thrips feeding is thought to increase susceptibility of onion tissue to this disease.


Purple blotch symptoms are first observed as small, elliptical, tan lesions that often turn purplish-brown. Concentric rings can be seen in lesions as they enlarge. A yellow halo surrounds lesions and extends above and below the actual lesion itself for some distance. Lesions usually girdle leaves, causing them to fall over. Lesions may also start at the tips of older leaves.

Model TomCast

Purple blotch and Tom Cast: TOMCAST is derived from the original F.A.S.T. (Forecasting Alternaria solani on Tomatoes) model developed by Dr. Madden, Pennypacker, and MacNab at Pennsylvania State University (PSU). The PSU F.A.S.T. model was further modified by Dr. Pitblado at the Ridgetown College in Ontario into what we now recognize as the TOMCAST model.
A Disease Severity Value (DSV) is the unit of measure given to a specific increment of disease (early blight) development. The use of the DSV: A DSV is a numerical representation of how fast or slow disease (early blight) is accumulating in a  field. The DSV is determined by two factors; leaf wetness and temperature during the "leaf wet" hours. As the number of leaf wet hours and temperature increases, DSV accumulate at a faster rate. Conversely, when there are fewer leaf wet hours and the temperature is lower, DSV accumulate slowly if at all. When the total number of accumulated DSV exceeds a present limit, called the spray interval or threshold, a fungicide spray is recommended to protect the foliage and fruit from disease development.

The spray interval (which determines when you should spray) can range between 15-20 DSV. The exact DSV a grower should use is usually supplied by the processor and depends on the fruit quality and end use of the tomatoes. Following a 15 DSV spray interval is a conservative use of the TOMCAST system, meaning you will spray more often than a grower who uses a 19 DSV spray interval with the TOMCAST system. The tradeoff is in the number of sprays applied during the season and the potential for difference in fruit quality.