July 2009


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Vern Grubinger <[log in to unmask]>
Reply To:
Vermont Vegetable and Berry Growers <[log in to unmask]>
Wed, 15 Jul 2009 11:50:43 -0400
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Late blight is now confirmed at multiple locations in Vermont as well as 
in every other northeast state. It appears to be spreading rapidly. Hot 
dry weather may help slow it but cool mornings mean dew on the plants 
which is not good. Many homeowners are now reporting symptoms of the 
disease as well. Here are condensed recommendations, adapted from UMass 

Fields with significant disease should be burned down with herbicide or 
plowed under ASAP. Each infected field is producing spores that will 
move to other farms in the area. Fields showing little or no symptoms 
should be protected as follows with fungicides, which are the only 
practical tool for managing late blight this year. Destruction of all 
infected plant material will help manage the disease for next year since 
it needs plant tissue to overwinter. Be especially vigilant about 
digging all infected tubers and destroying, or burying deeply.

Fungicides - Conventional.
Protectant sprays work well, with chlorothalonil (ie. Bravo or 
comparable material) continuing to provide excellent control. However if 
you are late in getting either a chlorothalonil, metiram or mancozeb 
spray on, then the combination of Curzate (cymoxanil, fungicide group 
27) mixed with a protectant is helpful to provide some kickback 
activity. Curzate works well when plants are actively growing and 
temperatures are cool, conditions that exist now for both potatoes and 
tomatoes. Previcur Flex has similar activity, and should be mixed with 
chlorothalonil. The pathogen has developed resistance to some fungicides 
such as metalaxyl and mefenoxam (Ridomil, Ridomil Gold), so these are no 
longer effective. If environmental conditions remain conducive for late 
blight, apply a fungicide from a different mode of action class every 
5-7 days such as Ranman, Forum, Tanos, Gavel, Reason (each mixed with a 
protectant), Revus Top,or a phosphorous acid fungicide (ProPhyt, 
Fosphite, Phostrol).

Fungicides – Organic.
There are some OMRI (Organic Material Review Institute) approved 
products that list late blight as a target disease. The information 
available usually indicates that they are not as efficacious as 
“conventional” materials, but if good coverage of crop foliage is 
maintained they offer some hope for protecting an uninfected crop. The 
OMRI approved materials include basic copper sulfate (NuCop 50w and 
Champ WG are OMRI and the biological fungicide Sonata. Use a 5-10 day 

here are links to many images of late blight on tomato

and potato

if you have any doubts send plant sample to: Ann Hazelirgg, UVM Plant 
Diagnostic Clinic, Carrigan Ave., Burlington VT 05405-0082