Late blight update

Previous 2012 late blight updates: May 31, June 4, July 20.

Article updated, July 30, 2012:  OMAFRA Potato Specialist, Eugenia Banks, is now reporting that late blight has not been confirmed in the area of Uxbridge, Ontario, as reported last week.  The report was the result of a misdiagnosis. To date, there are no confirmed reports of late blight in Ontario.

There has been one late blight find in Ontario (July 19 near Uxbridge).  In neighbouring states, the closest finds have been in Clinton and Onondaga Counties, New York.  There have been no reports to date from Michigan or Ohio.

Remember that conventional tomato growers using a recommended fungicide program for early blight, septoria leaf spot, and anthracnose, are also protecting the crop from late blight infection. 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.

If 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. Refer to Late Blight Look-Alikes for photos of late blight and possible look-alikes on tomato.

Fungicide recommendations:

• 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

Past OMAFRA articles on tomato late blight are found under the late blight tag to the right.  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.

Contact OMAFRA at 519-674-1690 or janice.leboeuf@ontario.ca if you suspect you have found late blight in Ontario. Those in the US can contact their state cooperative extension service or report online at http://www.usablight.org/.

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.