In the past week there have been eight new reports of late blight in NY and the New England States, several in counties where late blight had not yet been reported this season (map). A common theme in these reports is that plants looked fine one day and were heavily infected 2-3 days later. This could mean that the early stages of infection were missed, or that there were a large number of spores in the air that were deposited on leaves while they were wet from rain or dew.
The isolates of the late blight pathogen that have been identified in the Northeast to date this season continue to be exclusively US-23, which is susceptible to mefenoxam, the active ingredient in Ridomil, giving conventional farmers an excellent control option. See my pervious post for more information on fungicide options.
Stay vigilant! Scout tomatoes and potatoes regularly and thoroughly, focusing on parts of the field that are shaded or poorly drained, maintaining high relative humidity longer. Trials have demonstrated that copper fungicides are the most effective of those allowed in certified organic production. Be sure you are getting excellent coverage, and reapply often when weather is favorable for late blight. Keep in mind that leaf wetness from heavier dews at this time of year are enough to favor infection even if the weather is not rainy.
The late blight isolate from tomato in Onondaga county has been identified as US-23, which is fully susceptible to mefenoxam, the active ingredient in Ridomil. This isolate has been the predominant one identified in NY and the Northeast this season. Mefenoxam can practically eradicate infections caused by susceptible isolates. For other fungicide options, click on the 2012 potato and tomato fungicide rosters on the right side of this page.
From Bill Fry, Department of Plant Pathology and Plant Microbe Biology at Cornell: “In between Ridomil sprays apply one of the highly effective LB fungicides such as Presidio, Ranman or Gavel, all rated Good-Excellent against both foliar LB and tuber LB. Previcur Flex is Good against foliar LB but only Suppresses tuber LB. Tankmix Presidio and Previcur Flex with chlorothalonil, mancozeb, etc.”
From Carol MacNeil, Cornell Vegetable Program: ”Note regarding other fungicides: Curzate is only rated Fair-Good against foliar LB and Poor against tuber LB, needing daytime temperatures of 75 degrees or less, and active growth, for effectiveness. If used, tankmix Curzate with chlorothalonil/mancozeb, etc. Revus Top is only rated Fair for both foliar and tuber LB. Strobilurin fungicides are not LB materials, but combination products (Quadris Opti, etc.) provide some protection in low pressure situations. The phosphorus acid fungicides (Fosphite , ProPhyt, Phostrol, etc.) only have suppressive activity against LB in low pressure situations, don’t control early blight, and should never be used alone.”
Several fixed copper fungicides are OMRI listed for use on certified organic farms. Because coppers offer only protectant activity, they work best if applied before late blight is present. Be prepared and have some on hand so there is no delay if late blight is reported in yours or an adjacent county.
Track late blight infections across the US and sign up for email or text alerts of late blight in your area at the USAblight web site.
Late blight was confirmed today on tomato in Onondaga county. This is the first confirmed report from upstate NY. Scout your potatoes and tomatoes carefully for late blight symptoms, and, if you have not already, strongly consider applying a protective fungicide if you are in or adjacent to Onondaga Co.
New reports have also been popping up in Massachusetts in the past week. If you have not already, sign up for email or text alerts from your area on http://usablight.org.
While it will be at least several days before the isolate responsible for the Onondaga report is identified, nearly all the late blight isolates identified so far this year have been at least partially susceptible to mefenoxam, the active ingredient in Ridomil. Mefenoxam will stop a susceptible isolate dead in its tracks. See the links to the tomato and potato fungicide rosters to the right for more fungicide options.
For organic farmers, fixed copper fungicides can be very effective if applied before symptoms appear, and reapplied regularly. Several copper fungicides are allowed for certified organic production, search for “fixed copper” on the OMRI web site (http://www.omri.org)
In the past week, several new reports of late blight have come in from Pennsylvania,two from Long Island, and one from Connecticut. Scout your potatoes and tomatoes regularly to detect early symptoms and take appropriate management steps. Submit samples for isolate identification. All the samples identified so far from the Northeast have been at least partially susceptible to mefenoxam, a fungicide that is very effective against susceptible water molds.
Track reports and report finds on the USA Blight web site: (usablight.org).
This post is to let everyone know that I will not be using this web site to report and map late blight outbreaks in the future. The USAblight web site now serves that function. If you have not already, please sign up for email or text alerts on USAblight. I will be using this website to provide general management information as the season progresses.
That said, I’ll take this opportunity to report that late blight has been found in one of the earliest planted potato fields on Long Island and in a potato field in New Jersey. The Long Island report is suspected to be from potato seed, and late blight was detected in another shipment of seed earlier this season.
Potato growers should scout fields especially carefully, looking for characteristic lesions on the stem or leaves. Many good photos are posted on Dr. Meg McGrath’s web site to help with identification. If you do find late blight please submit a sample through the usablight web site to Dr. Bill Fry’s lab for isolate identification. Some of the more recently identified isolates are susceptible to mefenoxam, the active ingredient in Ridomil, which could provide excellent control of a susceptible isolate.
Many locations in NY have reached or are approaching the threshold for the first fungicide application for late blight based on an accumulation of 18 Blitecast severity values since the first potentially-infected green tissue in an area. Go to the NEWA late blight forecast to track severity values in your area. Start accumulating severity values on the date that green tissue emerged in your area, usually cull or volunteer potatoes.keep looking »