Abstract
Potatoes infected with late blight are purplish and shrunken
on the outside, corky and rotted inside.
Photo by Scott Bauer © ARS 2004
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New strains of late blight have emerged in recent years, making
potato production especially challenging. Several nonchemical options
are available for managing this disease, including cultural practices,
some varietal resistance, and alternative sprays that discourage
disease development.
Table of Contents
Introduction
Late blight (Phytophthora infestans) is a fungal disease
that attacks the leaves, stems, and tubers of potato plants. In
the 1840s, P. infestans caused the Irish potato famine,
when a million people starved and another million and a half emigrated
out of Ireland. (1) In recent years, highly
aggressive strains of this disease—many insensitive to popular
synthetic fungicides—have surfaced and created new challenges
for potato and tomato producers. (2)
P. infestans reproduces both sexually and asexually. Sexual
reproduction results in oospores—thick-walled spores that
can survive for several years in the soil. When oospores germinate,
they produce asexual spores called sporangia. Sporangia only survive
in living host tissue, such as cull potatoes. These are often the
original source of infection that initiates a major outbreak of
the disease. Once released, sporangia can easily be carried for
yards by rain splash and miles by wind. (3)
Wet conditions favor the disease. High humidity (greater than 90%)
favors sporangia development; they also germinate readily on wet
leaves. During moist weather, whole plants may be killed in a short
time. Late blight is one of the few plant diseases that can absolutely
destroy a crop, producing a 100% loss. (1)
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Disease Management
Tools available for organic management of late blight include forecasting
and monitoring techniques, cultural techniques, genetic resistance,
and alternative spray materials.
Forecasting and Monitoring
Several states have blight forecasting and reporting programs
to aid growers in managing this serious disease. Farmers should
contact their local Cooperative Extension office to determine whether
their area is being served by such programs and how to participate
in them. Cooperative Extension can also provide good publications
that aid in identifying the disease under field conditions. Several
states have plant pathology laboratories to evaluate tissue samples
and make disease identification.
Field scouting is important to late blight management. Catching
an outbreak in its earliest stages can reduce losses and increase
options for control. Growers should check fields twice a week. It
is important to look at leaves and stems under the canopy,
as this is where the disease gets established first. The first sign
of infected tissue is a water-soaked appearance of the leaves, which,
in dry weather, quickly turn dark brown and brittle. Infected areas
may be surrounded by a halo of chlorotic, or yellowed, tissue. During
moist weather, a white cottony growth will develop on the underside
of the leaves. Infected stems and petioles will turn dark brown
or black.
Symptoms first show up around low-lying areas, ponds or creeks,
near center-pivot irrigation rigs, and in places protected from
wind. Early-planted fields are likely to be affected first. (4)
Also note that the ideal conditions for an epidemic of late blight
are when night temperatures are 50 to 60° F, along with fog,
heavy dew, rain, or overhead irrigation, accompanied by daytime
temperatures of 60 to 70° F. Four to five continual days of
such weather are an open invitation for an outbreak.
Tubers going into storage should also be inspected and diseased
tubers removed. Since it is difficult to identify the disease at
this stage—especially if the tubers are dirty—it is
often advisable to send samples to a plant pathology lab. (5)
Cultural Control
Sanitation is the first line of defense against late blight. Avoid
piling and leaving culls. Culled potatoes should be disked, buried,
composted, or otherwise disposed of before the new crop emerges.
It goes without saying that however culls are dealt with, the further
removed from new production fields, the better. Volunteer potatoes,
solanaceous (potato family) weeds, and any infected plants should
be destroyed as soon as they occur. Growers who have the option
of planting several small, separated fields may have an advantage
in containing outbreaks. (6) Crop rotation also
helps—especially where volunteer potatoes are a problem—but
its efficacy is limited against this highly mobile organism. (6,
7) Tomato, pepper, and eggplant are all hosts
to late blight and should be avoided in rotation and as neighboring
crops.
Excessive viney growth—caused by over-fertilization with
nitrogen—invites late blight infection. (5)
While excessive nitrogen is seldom a problem in organic production,
it can be, especially under circumstances where high-nitrogen manures
are used.
Use of certified seed can reduce the amount of infestation from
infected seed pieces, but it will not prevent foliar infection from
other sources, such as neighboring fields. Careful attention should
be paid to seed potatoes. Any tubers that are discolored or show
signs of phytophthora infection should be culled and destroyed.
Do not mix seed lots. It may be worthwhile to investigate the severity
of late blight in the area from which seed potatoes have been ordered.
Planting should take place when soil temperatures are 50° F
and rising. Planting for an early harvest also helps to avoid infection.
(6)
Late blight spores may also be spread during seed piece cutting
and planting. The AireCup® planter, which uses vacuum pressure
instead of picks or cups, may help prevent early infection of seed
pieces. According to its manufacturer, this planter uses a vacuum
to "singulate" seed pieces, and air pressure to place
the seed pieces in the potato rows. It is capable of planting accurately
at a speed equal to or greater than pick planters. The manufacturer,
which also makes pick planters, notes that the AireCup® planter
is more accurate and makes fewer skips and doubles than pick planters.
For more information about the AireCup®Planter, contact:
Crary Company of Terra Marc Industries
Lockwood Product Line
237 NW 12th Street
West Fargo, ND 58078
800-488-8085
701-282-5520
Seed planting depth and hilling operations should be carefully
monitored. Shallow planting can expose tubers to late blight spores
washing down from leaves, creating problems later on during storage
and sale.
© 2004 SDSU
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When late blight appears in isolated sections of fields, spread
of the disease can be slowed considerably by quickly destroying
infected plants. (8) Killing the living potato
tissue halts further spore production. Organic growers might try
certifier-approved contact herbicides such as some of the vinegar
or citric-acid based organic herbicides (Bioganic,™ Burnout,™
AllDown Green Chemistry Herbicide,®), generic vinegar and/or
citric acid formulations, or a flame weeder. Tillage to bury the
infected plants should also be effective. Kill all susceptible plants
within 15 feet of the infection site. (6)
The duration of leaf wetness is a critical factor in late blight
infection. (9) Therefore, sprinkler irrigation
should be carefully scheduled, or minimized, particularly late in
the season when the closed potato canopy provides ideal conditions
for late blight development. If possible, rows should be oriented
parallel with prevailing winds to encourage better air circulation
and foliage drying. Studies in Israel noted that late blight infection
was greater on morning-irrigated potatoes than on potatoes irrigated
at midday or evening. (10) A rule of thumb:
if rainfall or irrigation water exceeds 1.2 inches in a 10-day period,
good conditions for late blight exist.
While vines can be allowed to die naturally prior to harvest, thorough
destruction of green vines limits late blight infection—especially
tuber infection. In conventional systems, vine desiccation is commonly
accomplished through chemical sprays. In organic potato production
the products for burning down vines are limited to certifier-approved
contact herbicides, vinegar or citric-acid based organic herbicides
such as Bioganic,™ Burnout,™ AllDown Green Chemistry
Herbicide®, generic vinegar formulations, flame weeding, or
other mechanical means. Mechanical flails are considered less effective
at limiting tuber infection than spraying. Flailing is also slower,
since fewer rows can be covered in a single pass. Flame desiccation
appears (initially) to be a faster and more viable alternative,
but this is not certain.
Leave tubers in the ground for about two weeks after the vines
have been destroyed, if possible. This allows blighted tubers to
rot so they can easily be left in the field. Following harvest,
it's appropriate to till in all residues and plant a cover crop.
(6)
Harvest should be managed to minimize damage to tubers and avoid
wet conditions. Infected tubers will continue to deteriorate and
spread the disease in storage. Post-harvest losses of up to 100%
can occur under some conditions. (1) Regulate
the flow of air through storage piles for minimal humidity, and
keep the tubers as dry as is reasonable. (5)
Varietal Resistance
Currently, no potato varieties are fully resistant to late blight.
A few cultivars, like Kennebec, Elba, Onaway, Rosa, and Sebago,
however, demonstrate a degree of resistance (11,
12) and are under serious study in breeding
programs.
Biotechnology is also being employed in the pursuit of late blight
resistance. Fully resistant genetically engineered commercial strains
are expected soon. (13) Genetically engineered
plants, however, are not acceptable for organic production. (14)
Alternative Sprays
Copper sprays can be used as a preventative to avoid the spread
of late blight. Several approved commercial copper products are
available, including Britz Copper Sulfur 15-25 dust, Champion WP,
Clean Crop COCS 15 sulfur 25 dust, and Cueva Fungicide Ready-To-Use.
As of August 2003, these copper products were listed with the Organic
Materials Review Institute (OMRI*) as "regulated,"
meaning that they could be used, but a plan must be in place that
indicates cultural controls are also being used and that ensures
copper does not build to toxic levels in the soil. Some copper products
may not be acceptable for certified production. Check with your
certifying agent.
The frequency of copper application may be quite high and exceed
the 9 to 15 sprays reported with conventional fungicides in some
parts of the country. This raises the issue of eventual copper toxicity,
as this element accumulates in soils—a definite concern for
sustainable production.
Nine to 15 sprays of copper as Bordeaux mix would probably deliver
anywhere from 2 to 6.5 lbs of elemental copper per acre to a field
in a single season. (This assumes a 6:8 ratio of CuSO4; to lime in
Bordeaux; 25% Cu in CuSO4 ; 2 to 4 lbs Bordeaux applied per spray.)
This would result in the addition, that season, of 1 to 3.35 ppm
copper to the six-inch plow layer of the soil where most nutrient
extraction occurs. If potatoes are grown in a five-year rotation—as
recommended to dodge soil-borne diseases—with crops requiring
little or no additional copper fungicides, an average addition of
approximately .2 to .7 ppm copper is made per year. The estimated
removal by crops on a practical rotation of small grains, potatoes,
and alfalfa might average .0225 ppm annually—only about 3%
to 11% of that applied.
The actual potential for toxic buildup of copper in the soil is
dependent on a host of additional factors:
- native levels of soil copper
- copper content of fertilizers and manures applied
- soil pH and buffering capacity
- leaching from rainfall and irrigation
- copper content of spray materials used
On many soils and in many farm situations, sustained copper spraying
might continue for decades—even centuries—at the frequencies
described, before toxic buildup would become a concern. However,
an additional problem with copper sprays is their impact on soil
organisms. At applied rates, copper fungicides are directly toxic
to several beneficial organisms, particularly earthworms and some
soil microbes such as blue-green algae—an important nitrogen-fixer
in many soils.
As a result of these concerns, organic growers and others using
copper sprays are encouraged to adopt an integrated approach to
late blight management that does not rely solely on copper.
Growers should also monitor soil copper levels through regular
soil testing when these sprays are regularly used and the conditions
warrant. Always apply all commercial pesticides according to label
instructions.
Compost tea, applied as a foliar spray, is also reported to suppress
late blight. In a German study (16), compost
teas made from either horse manure or cow manure were sprayed on
potato foliage as a control measure against late blight. These teas
were used either alone or with additional microorganisms added to
the mix. The tea treatments were compared to three fungicides or
to a water control. The compost tea alone was applied seven times
per week. Compost tea with additional microbes was applied 11 times
weekly. Fungicides were applied five times during the growing season.
Results from the experiment can be seen in Table
1. As you can see, compost + microbes was equal to Ridomil MZ
fungicide in reducing diseased leaf area and produced similar high
yields, as did two other fungicides. Ciluan and control produced
the lowest yields. Results comparing compost tea alone and compost
tea with added microorganisms, compared to a water control, are
shown in Table 2. The addition of microbes
to the compost tea was very beneficial, bringing yields from the
mixture up to double that of the tea alone or the control.
Table
1. Effect of compost teas with additional microorganisms compared
to three fungicides on potato late blight. ( 16) |
Treatment |
% Disease leaf area |
Yield (T/ac) |
Control |
96a** |
11a |
Compost + microbes |
11c |
15c |
Ridomil MZ |
8c |
15c |
Brestan 60 |
19b |
15c |
Ciluan |
18b |
13b |
**
Numbers followed by the same letter are not statistically
different. |
Table
2. Effect of compost teas with and without additional microorganisms
on an organic farm. ( 16) |
Treatment |
% Disease leaf area |
Yield (T/ac) |
Control |
93a** |
8a |
Compost tea alone |
90a |
9a |
Compost tea + microbes |
17b |
18b |
**
Numbers followed by the same letter are not statistically
different. |
Compost teas work by inoculating the leaf and stem surfaces with
microorganisms that serve as antagonists to invading pathogens such
as Phytophthora infestans by occupying the leaf surface,
making it difficult for the pathogen to get started. The beneficial
bacteria also induce resistance in the plants. Additional microbes
were added to the tea to enhance the antagonistic effect.
Currently, the use of compost tea is regulated in organic production
due to concerns about possible microbial contamination. Consult
your certifier before using compost tea on any crop harvested for
human consumption.
Organic farmer Jim
Gerritsen of Bridgewater, Maine, has never had a problem with
late blight on his organic potato crop. Since he supplies
organic seed potatoes to all 50 states, he has an extra incentive
to produce a clean crop. Gerritsen produces more than 300
cubic yards of compost from manure and bedding each year.
Most of this compost is applied to his fields. A small amount
is put into burlap bags and submerged in 55-gallon drums of
water to brew compost tea. The burlap bag acts as a filter
that catches larger particles that would clog the sprayer.
Compost tea is sprayed onto the crop at full strength at a
rate of 70 gallons per acre each week. The potatoes are sprayed
with tea about 10 times each season, starting when the plants
are six inches tall. (16) ATTRA has a
detailed publication on the preparation, use, and efficacy
of compost teas titled Notes
on Compost Teas.
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Serenade™ biofungicide is a wettable powder formulation of
Bacillus subtilis, QST-713 strain. B. subtilis
is applied as a preventative fungicide and works as an antagonist
against many pathogens, including P. infestans, that cause
late blight. When applied to the foliage, Serenade inhibits attachment
of the pathogen, stops it from growing, and induces an acquired
resistance in the plant. (17) Serenade was
discovered and commercially introduced by AgraQuest, Inc. and is
approved for organic production by OMRI. Rates range from 2 to 4
pounds per acre. Copper sulfate can be added to the mix. At the
2-pound rate, the cost is about $5.50 per acre. For information
on rates, formulation, and spray frequency for Serenade, contact
AgraQuest.
AgraQuest, Inc.
1530 Drew Avenue
Davis, CA 95616-1272
530-750-0150
530-750-0153 FAX
info@agraquest.com
©2004 manitoba AFRI
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Storox is a hydrogen-dioxide based pesticide that is OMRI approved.
It's a class-one danger, meaning the applier needs full personal
protective equipment, due to Storox's corrosive properties. Once
Storox dries, you can safely reenter the treated area. On potatoes
it can be used as a curative and preventative. Check the product Web site and MSDS Label (PDF / 57 K).
Foliar feeding has been associated with disease resistance. Two
materials that have acquired such a reputation are kelp-based products
and the Biodynamic™ preparation #508—made from the primitive
plant horsetail (Equisetum arvense). ATTRA has additional
information on foliar feeding, kelp, and Biodynamics™ available
on request.
In all instances where sprays are used, complete coverage of foliage
and stems is important. High-volume boom sprayers and air-assist
sprayers are usually most effective. Flood jet nozzles have been
shown inferior to hollow cone and flat fan designs. For aerial applications,
a minimum of 5 gallons of water per acre is recommended. (4)
Aerial application is expensive and not as effective as ground application,
but when you can't get into the field, to some growers it's better
than nothing.
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Summary
The emergence of new strains of late blight has created a serious
challenge for potato growers in recent years. Several organic options
for management are available, including cultural techniques, tolerant
varieties, and alternative spray materials. To be successful and
to avoid environmental consequences, these options should be evaluated
and adopted using an integrated approach.
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References
-
Mercure, Pam. 1998. Early Blight
and Late Blight of Potato. University of Connecticut, Integrated
Pest Management. 2 p. www.hort.uconn.edu/IPM/VEG/HTMS/BLTPOT.HTM
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Powelson, Mary, and Debra Ann Inglis.
1998. Potato Late Blight: Live on the Internet. American Phytopathological
Society, St. Paul, MN. www.apsnet.org/online/feature/lateblit/
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Williams, Greg, and Pat Williams.
1994. More on late blight of potatoes. HortIdeas. September. p.
103.
-
Mulrooney, Bob, and Joanne Whalen.
1998. Late Blight Control Update—1998. University of Delaware,
Newark, DE. www.rec.udel.edu/Update98/issue20.html
-
Strausbaugh, Carl, and Jim Hughes.
1996. Potato Late Blight. University of Idaho, Moscow, ID. www.uidaho.edu/ag/plantdisease
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Caldwell, Brian. 1998. Late Blight.
Organic Farms, Folks & Foods. January-February. p. 9.
-
Saling, Travis. 1998. Late Blight.
The Edible Garden.
-
Franc, Gary D. 1996. Potato late
blight fact sheet. Spudman. March. p. 49-50.
-
Stevenson, W.R. 1993. Management
of Early Blight and Late Blight. p. 141-147. In: Randall C. Rowe
(ed.) Potato Health Management. APS Press, St. Paul, MN. 178 p.
-
Carlson, H. 1994. Potato Pest
Management Guidelines. University of California Statewide IPM
Project. www.ipm.ucdavis.edu/PMG/r607101211.html
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Anon. No date. Cultural Cultivars.
Oregon State University, Corvallis, OR.
-
Williams, Greg, and Pat Williams.
1994. Watch out for late blight on potatoes. HortIdeas. August.
p. 95.
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Williams, Greg, and Pat Williams.
1994. Still more on late blight of potatoes. HortIdeas. October.
p. 111.
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Shapiro, Laura, Mary Hager,
Karen Springen, and Thomas Hayden. 1998. Is organic better? Newsweek.
June 1. p. 54-57.
-
Weltzien, H. 1991. Biocontrol
of foliar fungal diseases with compost extracts. p. 430-450. In:
J.H. Andrews and S.S. Hirano (eds.). Microbial Ecology of Leaves.
Springer-Verlag, New York.
-
Farrell, Molly. 1997. Applying
compost tea to prevent potato blight. BioCycle. May. p. 53.
-
Quarles, Bill. 2001. Serenade
biofungicide. IPM Practitioner. February. p. 10.
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Web Site Resources
www.cropinfo.net/Potatoblight.htm
Oregon State University's late blight forecast service. This
site contains much practical information on the disease, color photos
of infected plants and tubers, late blight hotline phone numbers
in Oregon, Washington, and Idaho, and management recommendations.
www.ipm.ucdavis.edu/DISEASE/DATABASE/potatolateblight.html
The University of California has county-by-county, Web-accessible
information that will automatically plot degree days for several
insect and disease pests.
www.potatonews.com
Has a world of information on late blight as well as other potato
information, and many links to other Web sites dealing with late
blight.
www.uidaho.edu/ag/plantdisease/
The University of Idaho's late blight Web site. Contains information
on cultural and chemical controls, forecasts, reporting fields,
and a blight update.
*OMRI
is a non-profit organization that publishes and disseminates lists
of generic and brand-name materials allowed and prohibited in organic
production. Contact:
OMRI
Box 11558
Eugene, OR 97440
541-343-7600
541-343-8971 FAX
Organic Alternatives for Late Blight Control in Potatoes
By George Kuepper and Preston Sullivan
NCAT Agriculture Specialists
Cole Loeffler, HTML Production
IP 131
Slot 115
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