What’s the Risk? Late Blight Update

By Amanda Tracey (OMAFRA) and Cheryl Trueman (Ridgetown Campus, University of Guelph)

Late blight (Phytophthora infestans) has yet to be reported in Ontario, Ohio, or Michigan but has been reported in Pennsylvania and New York. The hot, dry conditions across the province have reduced late blight risk. The application of broad spectrum contact fungicides, like mancozeb (Dithane Rainshield, Manzate Pro-Stick and Penncozeb) or chlorothalonil (Bravo and Echo), generally provide good protectant activity against late blight and also control other diseases such as early blight, Septoria leaf spot and anthracnose. Most late blight specific fungicides, like cyazofamid (Torrent), dimethomorph (Forum), amectotradin + dimethomorph (Zampro), mandipropamid (Revus), and oxathiapiprolin + mandipropamid (Orondis Ultra), do not control these diseases. Late blight specific fungicides should be used when risk is high. Click this link to see late blight symptoms and some look-alikes. Growers should monitor for changing weather conditions that favour the disease (15-21°C, cool nights with warm days, and moist weather) and follow sites such as ONvegtables.com and USAblight.org for new reports within the Great Lakes Region, as these conditions increase disease risk.

Better prediction of high-risk periods for late blight, and when to apply late blight specific fungicides, may be achievable with more rapid detection of P. infestans in tomato growing regions. We are currently testing the Spornado passive spore trap in four commercial processing tomato fields in Kent Co (Figure 1). The traps contain a cassette that passively collects air particles. The cassettes are changed twice weekly and tested for the presence of P. infestans, the organism that causes late blight. So far, we have had no positive detections for P. infestans. Tomecek Agronomic Services are collaborating on this project with funding from the Ontario Tomato Research Institute.

Late blight is a ‘community disease’ and because of its aggressiveness, it is important to know if it is present in a region. If you suspect the disease, please contact us at amanda.tracey@ontario.ca / 519-674-1699 or ctrueman@uoguelph.ca / 519-674-1500 x63646 to confirm identification. Your name and specific location will remain confidential.