Tomato late blight management in Ontario

As reported last week, late blight has been confirmed in a tomato field in Essex County.  It was found in a conventional processing field, so it had been receiving a typical fungicide program, but in an area that has received rains in recent weeks.

Late blight is becoming a bigger concern in recent years.  New strains are moving into the Great Lakes region, some of which are much more aggressive on tomato than previous strains.

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 options

  • chlorothalonil (Bravo, Echo), mancozeb (Manzate, Dithane), metiram (Polyram) – contact fungicides – have been very effective
  • Tanos, Revus, Cabrio – translaminar – 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
  • Presidio – 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

I posted some links to previous articles that give additional information about late blight.  Please report any occurrences so that we can keep the tomato community informed about its development and spread and so that we can collect samples to learn about the strains, fungicide sensitivity, and the biology of this evolving disease.  Thank you to those who have already submitted samples.

Note:  Organic producers may be interested in an article by Meg McGrath (Cornell University) on managing late blight in organically-produced tomato.  Note that crop protection products mentioned in her article relate to US registrations.  Consult the Canadian labels and your organic certifier for registered products that can be used in organic production.

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