There have been a number of new reports of late blight in the US in the last week and a half. With rainfall, fog, and cooler temperatures, conditions have improved for the disease.
- August 15 – New Haven County, CT (tomato)
- August 15 – Suffolk County, Long Island, NY (tomato)
- August 15 – Tolland County, CT (tomato)
- August 13 – Centre County, central PA (tomato)
- August 13 – Fulton County, southern PA (tomato)
- August 12 – Barron County, northwest WI (potato)
- August 10 – Pike County, southern OH (tomato)
- August 9 – Waldo County, ME (tomato)
- August 9 – Hancock County, ME (potato, tomato)
- August 9 – New London County, CT (tomato)
- August 9 – Penobscot County, ME (potato)
- August 9 – Cumberland County, southern PA (tomato)
- August 9 – York County, southern PA (tomato)
- August 8 – Centre County, central PA (tomato)
- August 6 – Tompkins County, western NY (tomato, US-23)
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.
Figure 1. Late blight symptoms on tomato leaves appear as water soaked grey-brown lesions often surrounded by a light green or yellow halo.
Figure 2: Early late blight symptoms on tomato
- 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 firstname.lastname@example.org 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.