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Managing Late Blight in
Organically Produced Tomato
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.
Typically potato is the main crop affected because infested tubers are the main
source of initial inoculum. Infected tomato transplants are another potential
source. Late blight 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. Additionally fruit at any stage
are susceptible. 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, progressing from infection to new lesion (spot)
producing spores 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_tomato.htm
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Steps
for managing late blight in organically produced tomato:
1. Use transplants
produced in an area where late blight is not developing on plants inside or
near the greenhouse. Some strains of the late blight pathogen can infect
petunia and some solanaceous weeds such as bittersweet nightshade. Inspect
transplants carefully before planting to ensure none have symptoms of late
blight. The pathogen cannot survive on tomato seed.
2. Select varieties with
resistance. Mountain Magic is one of several varieties with resistance to
early blight and Septoria leaf spot as well as late blight. Seed is expected
to begin to be marketed perhaps as soon as 2010.
3. Control volunteer
tomato and potato plants as well as solanaceous weeds, in particular hairy
nightshade and bittersweet nightshade, which are also susceptible.
4. Regularly inspect
tomato as well as potato crops, which are also susceptible, for symptoms of
late blight. Most extension offices provide diagnostic services.
5. 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.
6. 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.
7. If symptoms of late
blight are found in isolated areas in a planting, it may be possible to save
the crop. Success depends on how early in disease development symptoms are
found, how many infections are present that have not yet resulted in symptoms
(spore germination to symptom takes about 1 week), how quickly and thoroughly
diseased tissue will be removed, environmental conditions, proximity to other
gardens or farms where late blight is developing, and what management steps
will be taken. Immediately remove affected plant tissue. It is best to do this
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. Put affected tissue in garbage bags, dig a hole and bury it, or put
it in a pile and cover with a tarp. Heat that develops from sunlight hitting
the tarp will quicken death of plant tissue and the pathogen. Inspect plants
daily thereafter for a week in order to find any additional affected plants
that develop symptoms, then return to inspecting at least once a week. Apply
copper fungicides every 7 days as indicated on the label until final harvest or
the crop is destroyed. 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 9
below.
8. Work in affected
fields last. Clean equipment between fields.
9. When late blight
starts to become severe the foliage should be destroyed to eliminate the
planting being a source of spores for other tomato or potato plantings on the
farm or other farms. Propane flamers are a good way to quickly kill foliage,
but are not suitable where tomatoes are grown with straw or plastic mulch or
trellised. This is an obligate pathogen that needs living host tissue to
survive. To initiate plant death with trellised tomatoes, go through the
planting and cut all main stems at the base, then come back through and cut
stems further up in the canopy. Disturb foliage as little as possible to
minimize the amount of spores dislodged. It is best to do this work in the
middle of a sunny, preferably calm day after any moisture on leaves has dried
to minimize the quantity of spores and also their likelihood of survival in the
process. Next remove trellising line and stakes, then flail chop. Bagging
affected tissue or burying is recommended where feasible with small plantings.
The late blight
pathogen is not thought to be able to survive in plant debris, therefore it is
not necessary to physically remove affected plant tissue from a field.
The late blight
pathogen cannot survive on stakes, therefore it is not necessary to trash or
even disinfect the stakes to manage this disease. Stakes should be disinfected
however, especially if bacterial diseases also developed in the planting.
10. Fruit from an affected
field can develop symptoms after harvest and thus should be inspected just
before marketing. Customers should be aware of the potential that fruit could
have a shortened shelf life. It may be wise to recommend that any fruit that
rot be put in the trash rather than on a compost pile since there is a
possibility that the pathogen could produce spores before the fruit completely
rotted.
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 3 days and maximum annual rate of 17.4 lbs metallic copper per acre for tomato 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.
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.
Prepared: 08-10-09
