Late blight update – June 24

To date, I know of no confirmed cases of late blight in Ontario or nearby states, but there’s a report today of it showing up in Wisconsin. Nevertheless, our experience of recent years would indicate we are likely to see it in Ontario tomatoes at some point in the season.

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, tomato growers should:

• 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 the Tomato Late Blight Photo Gallery and Late Blight Look-Alikes for photos of late blight and possible look-alikes on tomato.

Fungicide registrations for tomato:

[Ratings are shown for late blight activity based on information from Dr. Tom Zitter, Dept. of Plant Pathology and Plant-Microbe Biology, Cornell University, Ithaca, NY. 0 = no effect or not labelled; + = poor; ++ = OK to good; +++ = very good. P, p = protectant; C, c = curative; A, a = antisporulant (capitalized indicating stronger activity)]

• chlorothalonil (Bravo, Echo, group M5) – contact fungicides – have been very effective protectants in tomato [++, P]
• mancozeb (Penncozeb, Manzate, Dithane, group M3), metiram (Polyram, group M3) – contact fungicides – have been very effective protectants in tomato [++, P]
• Acrobat 50 WP (group 40)– 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 [++, P, c, A – use as preventative]
• Cabrio EG (group 11) – translaminar – has some ability to move into the plant – strobilurins generally rated lower than the other targetted late blight materials by researchers – preventative only – tank mix with stronger late blight product [+, P, a]
• Presidio – translaminar, some “kickback” activity – registered (in tank mix with Bravo) [+++, P, C, A]
• Torrent 400SC (group 21) – protectant, contact (limited locally systemic) – should be tank-mixed with a non-ionic or organosilicone surfactant [++, P]
• Reason 500 SC (group 11) – locally systemic and translaminar – has some ability to move into the plant – must be tank-mixed with mancozeb or Bravo 500 [not rated]
• Revus (group 40)– 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 [++, P, c, a]
• Tanos 50 DF (group 11/27)– translaminar, some “kickback” activity – has 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 [++, P, C, a – use as preventative]
• Tattoo C (group 28/M5)– systemic + protectant, some “kickback” activity – has some ability to move into the plant – includes a systemic ingredient plus a protectant ingredient from a different chemical family [rating for propamocarb 720 g/L: ++, P, C, A (Tattoo C is propamocarb 375 g/L + chlorothalonil 375 g/L)]
• Zampro (group 45/40)– two active ingredients – protectant and systemic, translaminar – can move into the plant [not rated]

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/. This helps us alert others of late blight in the region and if possible, we will try to collect samples for late blight researchers studying the disease. Identifications of late blight genotypes for Ontario are provided by L. Kawchuk, AAFC Lethbridge, Alberta, Canada.

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.

Tomato Late Blight Photo Gallery