TOMATOES DYING OF LATE BLIGHT
McGrath, Department of Plant Pathology and Plant-Microbe Biology, Cornell
University is our guest writer this week.
Dr. McGrath is raising awareness of a killer in our gardens - a spore
that decimates tomatoes, killing the whole plant in a matter of days. This disease, called late blight, also
attacks potatoes and is the organism responsible for the Irish Potato Famine in
the 1840's. It is imperative that
home gardeners recognize the symptoms, get a correct diagnosis, and responsibly
remove the infected plants so that the spores cannot travel. They are wind borne. The photos included in this article
were all taken by Dr. McGrath.
Visit her Blog at http://blogs.cornell.edu/hort/2009/06/26/late-blight-a-serious-disease-killing-tomatoes-and-potatoes-this-year/
Irish Potato Famine Disease affecting Gardens
and Farmers throughout the Greater Northeast
Revised by A. Wyenandt, NJAES, Rutgers
University and M.T McGrath, Cornell University
Original article by Thomas A. Zitter, Cornell
University, Ithaca, NY
Updated July 8, 2009 - Late blight, caused
by Phytophthora infestans, is a serious disease that affects tomatoes
and potatoes around the world. Late blight can become a serious problem because
it can quickly kill affected plants. Its spores are easily carried in wind currents to infect
other susceptible plants in even the most remote areas in our region.
Late blight occurs sporadically in the Northeast
in any given year because farmers diligently use methods to prevent the pathogen
from surviving overwinter. Since our summer thus far has been cool with
frequent rains, weather conditions have been very conducive for late blight development.
all tomato and potato plants grown in home gardens and in commercial fields are
susceptible to late blight! Commercial growers are able to respond to reports
of the disease by spraying fungicides to prevent its spread, which otherwise
would mean certain death of their entire crops.
Unfortunately, many homeowners may not be as aware
of this important disease, and if they take no corrective actions in a timely
manner, home gardens can provide a source of inoculums (i.e. spores) for their
neighbor's gardens and for commercial interests.
occurrence of late blight in 2009 is different compared to most seasons.
This is the earliest the disease has been reported over such a broad region of
the country. More tragic for the Northeast is that infected plants have
been found in large retail stores throughout the region (Ohio to Maine).
before has this been known to have occurred. The inoculum is exceptionally contagious
and can easily spread on air currents among tomato plants on garden center
shelves. Thus, a gardener purchasing a healthy appearing plant may
actually have one that just became infected.
Vegetable pathologists from throughout the
Northeast have spread the word of this impending disaster, and within a day of
the first sighting, a major supplier, working with the Department of
Agriculture in the affected states, had begun to remove affected plants from
What to do now? Many families have started vegetable
gardening, given the tough economic times, and tomato is the most important
crop in gardens. The organism is not seed borne (however, it is tuber borne in
potatoes), so that tomato plants started from seed locally in an area where
late blight was not present likely are free of the disease, at least for now.
Given the scenario, we must assume that many infected tomato plants have been
planted across the entire NE region.
Identification: The symptoms that develop on
tomato leaves, stems and fruit are quite dramatic, and are very obvious to the
naked eye. The leaf lesions are water-soaked, varying in size
from a nickel up to a quarter. They are water-soaked when the foliage has been exposed
to watering or heavy overnight dews. When these lesions dry out quickly,
they may appear lime-green in color or even become beige. The edge of the
water-soaked lesion, on either the top or the bottom leaf surface, will be
covered with white fungal growth that contains the spore inoculum (visible with
a hand lens). Spores are easily blown to surrounding areas, infecting plants and
even some weed species in the family Solanaceae (the black nightshade family). Brown
to almost black lesions develop on infected stems and the same lesions will
develop on infected fruit, either directly on the plant or a few days after
when they are sitting on a kitchen counter. It is not dangerous to humans. Most of the fruit can be used if the
affected area is removed.
examination and removal: Please inspect your tomato plants on a daily
basis! If symptoms are already appearing on plants in your garden, these plants
should be removed and put in a plastic bag for disposal. Do not put the removed
plants in a compost pile, as spores can still spread from the debris. Your
neighbors, not to mention commercial growers, will appreciate you taking this
Plant treatments: Commercial growers have a
number of fungicides that if applied early and often, can prevent infection and
slow late blight development. They would choose not to spray if they could, but
this destructive disease does not give them any other option. Homeowners do
have a few products that are registered for use; the most effective ones have
the common name of chlorothalonil, which will be on the label. These products
are only effective if used before the disease appears and should be reapplied every
5-7 days if cool, wet weather persists. Chlorothalonil is a protectant
fungicide, with no systemic movement in the plant, so thorough coverage is
necessary. Copper fungicides are not as effective as chlorothalonil.
Diagnostic Services: For those homeowners
interested in getting diagnostic testing for late blight on tomato or other
diseases on vegetables in their garden, please contact your state university's
plant diagnostic lab service. (These diagnostic services, in most states,
require a fee, which some are not charging for suspected late blight. Reach
them through your local County University Extension Service Office.) It is
critical to get potential late blight samples to the diagnostic clinic as
quickly as possible. Laboratories may be able to fax or e-mail results within