Late blight in Ontario tomatoes, potatoes

Janice LeBoeuf, OMAFRA Vegetable Crop Specialist, Ridgetown

Yesterday (August 4), late blight was confirmed in tomatoes near Simcoe, Ontario and in potatoes in the Alliston area.  Reports of late blight were also received from another location near Simcoe and one near Milton.

This may indicate that late blight spores have reached much of southern Ontario.  Tomato growers should implement a protectant fungicide program immediately and scout their fields carefully. Late blight can devastate a tomato crop very quickly if conditions are favourable.  Remember that conventional tomato growers following a regular fungicide spray program (by calendar or TOMcast) for early blight, septoria leaf spot, and anthracnose, are also protecting the crop from late blight infection.

Recommended protectant fungicides for tomato late blight in Ontario:

chlorothalonil (Bravo 500)

mancozeb (eg. Dithane DG, Manzate Pro-Stick, Penncozeb 80 WP)

metiram (Polyram DF)

famoxadone + cymoxanil (Tanos 50 DF) – has some kickback activity

mandipropamid (Revus) – has some translaminar movement

pyraclostrobin (Cabrio) – use high rate for late blight

During wet cool periods, a fungicide should be applied every 5 – 7 days to protect against late blight. If the weather conditions become dry, the spray intervals may be extended.

It is important to scout your fields regularly and know the symptoms of late blight.  You can review previous Veg Crop Update articles on late blight in the ONvegetables archives.

Note:  Organic producers may be interested in an article by Meg McGrath (Cornell University) on managing late blight in organically-produced tomato.

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