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Managing Late Blight in
Organically Produced Potato
Margaret Tuttle McGrath
Department of Plant Pathology and
Plant-Microbe Biology, Cornell University
Long Island Horticultural Research and
Extension Center
3059 Sound Avenue, Riverhead, NY 11901;
mtm3@cornell.edu
Late
blight is a potentially very destructive disease that fortunately occurs very
sporadically in most areas of the northeastern US most growing seasons. It can
destroy a crop if unmanaged. The pathogen is well named: ‘Phytophthora’ in Latin
means ‘plant destroyer’. Affected foliage tissue is quickly killed. Impact
is especially great when stems are infected because all tissue above this point
will die. This disease can be explosive especially under favorable conditions
because the pathogen can produce a lot of wind-dispersed spores and it can
cycle very quickly. With some strains of the pathogen under ideal conditions
it takes less than 3 days to go from spore landing on a leaf to visible spot,
and just one more day until spores are being produced on the spot! Others take
a little longer but still can progress from infection to spore production in 6
to 7 days. Many images of symptoms are available on the internet to assist
with identification. Mine are posted along with additional information at:
www.hort.cornell.edu/department/Facilities/lihrec/vegpath/photos/lateblight_potato.htm
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Steps for managing late blight in
organically produced potato:
1. Select less susceptible varieties
when possible. There are no varieties with a high level of resistance. Those
described as having some resistance include Elba, Kennebec, Allegany, Sebago,
Rosa, Defender, Jacqueline Lee, and Ozette. Elba is considered the most
resistant.
2. Use certified seed. Inspect them to
ensure none have symptoms of tuber blight. Infected tubers used as seed or not
destroyed from the previous crop are considered the primary source of initial
inoculum for late blight in the northeast.
3. Regularly inspect potato as well as
tomato crops, which are also susceptible, for symptoms of late blight. Local
extension office provides diagnostic service.
4. Check local extension newsletters
each week for information about late blight occurrence. Note that during
cloudy conditions spores of the late blight pathogen can survive being
dispersed in wind currents long distances (miles!) because they are protected
from the killing effects of UV radiation. Rain can bring these spores down on
to plants far from the affected plants that were their source.
5. When there is a risk of late blight
occurring and fungicide applications are going to be used as a component of
management, apply approved copper fungicides on a regular preventive schedule.
Late blight is difficult to control, especially when fungicides are not applied
before disease onset. Thorough spray coverage is critical since copper is a
contact fungicide. See section at end if any fungicide will be used.
6. If symptoms of late blight are found
in isolated areas in a planting, promptly pull up affected plants and cover them
in place with a tarp or dig a hole and bury them. Heat that develops from
sunlight hitting the tarp will quicken death of plant tissue and the pathogen.
It is best to pull plants in the middle of a sunny day after the leaves have
dried when there will be fewer spores and those dislodged in the process will
likely be exposed to UV radiation. Inspect plants daily thereafter for a week
in order to find any additional affected plants that develop symptoms. Apply
copper fungicides until vine kill. It is not possible to control late blight
by solely relying on removing affected tissue. Even when rain is not occurring,
dew over night can provide a sufficient leaf wetness period for infection.
Especially when conditions are favorable it may not be possible to control late
blight with copper. Monitor disease development and be prepared to jump to
step 8 below.
Tubers in an affected area could be
dug. They should be held in a dark, dry, warm (at least 65F) place for a week,
then inspected for symptoms of tuber blight before marketing.
Do not hill potatoes that remain in
the field in an effort to protect the tubers because the pathogen can be easily
spread on equipment, and the root pruning that will occur may stop plant growth
for several days.
7. Work in affected fields last. Clean
equipment between fields.
8. When late blight starts to become
severe the foliage should be destroyed to protect the tubers from infection and
to eliminate the planting being a source of spores for other tomato or potato
plantings on the farm or other farms. It is especially important to vine kill
when late blight is developing on stems because from this location spores can
more easily be washed down to tubers than from leaves. Destroy foliage in the
middle of a sunny, preferably calm day after the leaves have dried to minimize
the quantity of spores and also their likelihood of survival in the process.
Propane flamers are a good way to quickly kill foliage. Flail chopping is
another option.
Applying copper fungicide to protect
stems that remain from late blight is not recommended because conditions are
much less likely to be favorable for infection once all the foliage is
removed.
9. Harvest tubers after foliage has
died but before significant rainfall is predicted. Waiting two weeks to
harvest after vine kill is considered to provide an adequate time for spores to
die. Rain can wash spores down to tubers. And tubers should not be harvested
when wet. Infection is more likely to occur when soil temperatures are cool (below
54F). Avoid bruising and skinning while harvesting. Harvest separately and
last any areas that are low or had more severe symptoms of late blight. As
described above under 6, tubers from an affected field should not be marketed
until checked for blight. Prompt marketing is recommended. If stored, cool
down quickly and provide good ventilation in storage. Check stored tubers
frequently for symptoms.
10. Destroy any affected tubers. This is
how the pathogen survives over winter. Recommended methods include chopping,
burial, burning, spreading on fields where they will freeze completely over
winter, or feeding to livestock.
11. Promptly destroy any volunteer
potatoes in subsequent years. These can be a source of the late blight
pathogen.
12. Inform customers that tubers should
be consumed soon as they could have a shortened shelf life and that any tubers
that rot need to be put in the trash rather than composted to avoid providing
the pathogen a means to survive over winter.
Additional Information
About Copper and Other Fungicides.
OMRI-listed fungicides labeled for late blight include Sporatec,
Serenade, Sonata, OxiDate, and copper. Regalia* and Companion meet NOP guidelines and are in review with OMRI
(*registration pending in NY and PA). Check
with local organic certifying agency to determine what products are approved.
There is limited data from replicated experiments on efficacy for late blight
of products approved for organic production. Copper has provided some control
where other products have failed. However, copper is not considered very
effective because it has provided poor control in efficacy experiments where
excellent control was achieved with conventional fungicides. Poor efficacy,
combined with the fact that established spots, being uncontrollable with
copper, will continue to produce spores, plus the explosive nature of late
blight, is why a preventive spray program is recommended including by organic
growers in areas where late blight occurs regularly.
Before
using any fungicides read the label. Note that the ‘signal word’ for copper
fungicides is ‘danger’. The signal word assigned to a pesticide is based on
how harmful it might be if swallowed, inhaled, or exposed to skin or eyes of
the person handling it. Danger is assigned when the pesticide is highly
hazardous by at least one of these routes of entry into a person. The other
signal words used for pesticides are ‘warning’ for moderately hazardous
chemicals and ‘caution’ for slightly hazardous chemicals. In the precautionary
statement on pesticide labels is a section on ‘hazards to humans’, which
explains how the product could affect someone exposed to it. This is followed
by the ‘personal protective equipment’ (PPE) that is needed when mixing and
applying the pesticide. Hazards for copper fungicides are: “Corrosive. Causes
irreversible eye damage. May cause skin sensitization reactions in certain
individuals. Do not get in eyes or on clothing. Harmful if swallowed or
absorbed through the skin. Avoid contact with skin.’ Also ‘avoid breathing
dust.’ for some formulations. PPE that applicators and other handlers must
wear when using copper is: long-sleeved shirt and long pants,
chemical-resistant and waterproof gloves, shoes plus socks, and protective
eyewear. First aid information is also provided on labels for accidental
exposure; know this in advance to avoid delay in treatment. There are also
important ‘Agricultural Use Requirements’ described on labels. This includes
the ‘restricted-entry interval’ (REI), which is 24 hours for copper, what PPE
is required for anyone who enters and will contact anything treated before the
end of this interval, which for copper is the same as for applicators, and what
precautions must be followed after an application, which for copper includes
having an eye flush container at the WPS decontamination site for workers
entering the field for 7 days after treatment. Note that fruit cannot be
harvested during the REI. EPA's Worker Protection Standard for Agricultural
Pesticides (WPS) is a regulation that must be complied with on farms where any
pesticide is used, including those approved for organic production. Under this
regulation, all agricultural workers on the farm must receive pesticide safety
training, decontamination supplies, notification of pesticide applications,
access in a central location to a log of pesticide applications plus
information about these pesticides, any required personal protective equipment,
and emergency medical assistance when needed. Restricted-entry intervals must
be adhered to. Also, pesticide safety posters must be displayed.
Labels
also specify how often the product can be applied. At this time, most copper
fungicides are labeled for use every 5 or 7 to 10 days or ‘as needed depending
on disease severity’. This is interpreted (by state pesticide regulators,
company regulators, and organic certifiers) to mean these fungicides can be
applied more frequently than every 5 or 7 days since these labels do not have
restrictive statements about a minimum interval or maximum amount that can be
applied to a crop. Copper fungicides are exempt from tolerances. However, more
frequent applications generally are not considered necessary, even following
rain, because these products are formulated with adjuvants that help keep them
on foliage. More explicit description with a defined minimum retreatment
interval of 5 days and maximum annual rate of 25 lbs metallic copper per acre for potato will be on future labels
following re-registration (these limits are specified in EPA Reregistration
Eligibility Decision (RED) for coppers). It is recommended that these limits be
followed now. Labels need to be checked on new product containers for changes
such as this before using. It is especially critical where copper is being applied
frequently to test soil regularly to ensure this is not resulting in an
unacceptable accumulation of copper. Before applying copper more frequently
than every 5 or 7 days it is advisable to confirm with the certifier that this
is permissible.
When
using any pesticide note that it is a violation of Federal law to use the
product in a manner inconsistent with its labeling.
Some of this information on management was provided by Dr. Steve
Johnson, University of Maine Cooperative Extension
Please Note: The specific directions on fungicide labels must be adhered
to -- they supersede these recommendations, if there is a conflict. Confirm
state registration and organic approval with certifier. Any reference to
commercial products, trade or brand names, is for information only; no
endorsement is intended.
