I have had reports of late blight on tomatoes from Chatham-Kent, Middlesex, and Norfolk counties. In many cases, these are from plants that have not had fungicide applications, however there appears to be at least some disease development in a few fungicide-treated fields as well.
The cool, wet weather is favourable for disease development, so commercial growers in the region should be using a tight fungicide schedule and applying products that are strong on late blight.
Cloudy and high humidity or wet conditions are favourable for late blight. The pathogen prefers cool temperatures. The disease is suppressed by hot, dry weather, but it can continue developing and spreading when suitable conditions return.
When late blight is found in the area:
- Tighten up spray intervals – 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.
- Scout fields often. Know the symptoms.
Fungicide recommendations (updated June 2012):
- chlorothalonil (Bravo, Echo), mancozeb (Manzate, Dithane), metiram (Polyram) – contact fungicides – have been very effective
- Tanos, Revus – translaminar, some “kickback” activity – have some ability to move into the plant – can add to the fungicide program, but rotate chemistries and use in combination with protectants for resistance management
- Acrobat – translaminar – has some ability to move into the plant – must be tank-mixed with another late blight fungicide from a different chemical family – can add to the fungicide program, but rotate chemistries
- Presidio – translaminar, some “kickback” activity – registered (in tank mix with Bravo), but note that after using it, rotation to any crop except brassica (head and stem), bulb vegetables, cucurbit vegetables, fruiting vegetables, leafy vegetables (except brassica), root and tuber vegetables (except carrot and sugar beet) is prohibited