*Articles on Organic & Sustainable Farming
(June 24,2010)

*Late Blight Website
Here's a good website on late blight by Cornell University, College of
Agriculture and Life Sciences. On this site you will find photos of late
blight in various stages, as well as new articles related to late blight. **.

Tomato/Potato Late Blight

Late blight has been confirmed in tomatoes on an organic farm in Harrison
County, and a conventional farm in Knox County, OH.  Harrison County is in
east/central Ohio, one county west of the Pennsylvania border, and Knox
County is in Central Ohio.    This follows confirmations of late blight in
potatoes or tomatoes in Michigan, Pennsylvania, Kentucky and New York.
Weather conditions (cool-warm with high moisture – rain or heavy dew) in
Ohio have been very favorable for late blight.

If you are not sure that the disease symptoms you see on tomatoes or
potatoes are caused by late blight, you may send a sample to Sally Miller or
Fulya Baysal-Gurel, Department of Plant Pathology, The Ohio State
University, OARDC, 1680 Madison Ave., Wooster, OH  44691, ph. 330-263-3838,
or to the C. Wayne Ellett Plant and Pest Diagnostic Clinic, OSU, Kottman
Hall, 2021 Coffey Road, Columbus, OH 43210, ph. 614-292-5006 (c/o Nancy
Taylor) for diagnosis.  Please go to our website ( to download the sample
submission form.  Those in the Fremont area may take their samples to the
OSU-OARDC North Central Agricultural Experiment Station, which is now
equipped with microscopes connected to the OSU Wooster and Columbus labs via
the internet.  A similarly equipped lab is also available at the Muck Crops
Agricultural Research Station in Celeryville.

*What to Do*:  Potato and tomato growers should protect plants with
appropriate fungicides as long as favorable environmental conditions
persist.  Heirloom and conventional tomato varieties alike are susceptible
to late blight.

*Home gardeners*: Destroy plants already infected – pull out the entire
plant(s), immediately bag it (them) in a plastic bag, and dispose of the
closed (tied or knotted) bag in the garbage.  Do not put the plants on a
compost pile or in a composter, or leave them lying about.  Live plant
tissues serve as a source of inoculum, and uprooted plants may support
active spores of the pathogen for some time.   Healthy-looking plants should
be protected with a fungicide containing chlorothalanil or copper; several
brands are available in garden centers and other retail outlets.
Chlorothalanil is more effective than copper in controlling late blight.

*Conventional farmers – open field*: Protect plants with chlorothalanil or
mancozeb (‘protectants’) before the disease appears.  Scout fields
intensively for late blight and destroy any infected plants.   Growers with
fields in which late blight has been found should also consider applying
Curzate, which has curative activity, plus a protectant fungicide.   Other
fungicides that can be used in a program that alternates products with
different modes of action include Gavel (already contains mancozeb),
Previcur Flex, Ranman, Tanos and Revus Top.  Previcur Flex, Ranman and Tanos
must be tank-mixed with a protectant fungicide.  If late blight has been a
problem in a potato field, vines should be killed 2-3 weeks prior to harvest
to minimize infection of tubers.   Destroy unmarketable potatoes – cull
piles serve as a source of inoculum for next season.

*High tunnel and greenhouse tomato farmers (conventional)*:  High relative
humidity and condensation (with water dripping onto plants) inside high
tunnels and greenhouses can be very favorable for late blight.  Prune
plants, raise side walls and /or use fans appropriately to improve airflow
through the canopy and minimize condensation.  Remove and destroy diseased
plants.  Ranman, Tanos, Previcur Flex, Revus Top and Gavel may be used in
greenhouses and high tunnels, but chlorothalanil formulations and Ranman may
not due to label restrictions.

*Organic growers*:  Follow management approaches described above for
conventional or home garden potatoes or tomatoes, except that only
copper-based fungicides may be used.  Several OMRI-approved copper-based
fungicides and formulations are available.

For more information, including more color photos, see our late blight fact
sheet on Ohioline:

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