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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 (http://oardc.osu.edu/sallymiller/Extension/index.htm) 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: http://ohioline.osu.edu/hyg-fact/3000/3102.html