Abstract
This publication discusses cultural controls and alternative pesticides that can be used in control of downy mildew, a destructive disease of cucurbits that is most destructive to cucumber and cantaloupe.
Table of Contents
Introduction
Downy mildew, caused by the fungal organism Pseudoperonospora cubensis,
is most destructive to cucumber and cantaloupe, though all cucurbits
are susceptible. Symptoms first appear as pale green areas on the
upper leaf surfaces. These change to yellow angular spots. A fine
white-to-grayish downy growth soon appears on the lower leaf surface.
Infected leaves generally die but may remain erect while the edges
of the leaf blades curl inward. Usually, the leaves near the center
of a hill or row are infected first. The infected area spreads outward,
causing defoliation, stunted growth, and poor fruit development.
The entire plant may eventually be killed. (1,
2)
The fungus is easily carried by wind currents, rain splash, farm
implements, or the hands and clothes of farm workers. It is favored
by cool to moderately warm temperatures, but tolerates hot days,
although long periods of dry hot weather can stifle the spread of
the disease. (3) Unlike powdery mildew, it requires
humidity to flourish. Therefore, downy mildew is most aggressive
when heavy dews, fog, and frequent rains occur. (1,
2)
Downy mildew does not overwinter beyond Mexico and the southernmost
tier of U.S. states, where it survives on cultivated and wild cucurbit
plants. Spores are blown northward each season as favorable seasonal
conditions advance. As a result, the disease is most common on late
summer plantings and is infrequently seen on spring cucurbits. (4)
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Monitoring
Keeping abreast of when, and how severely, downy mildew is occurring
in your area can help you determine the proper time to treat it.
The North American
Plant Disease Forecast Center is an online forecasting network
that tracks outbreaks of downy mildew from March through the end
of the growing season. Data is posted twice weekly. Growers can
use the website to identify areas where an outbreak is reported,
as well as spore movement in that area. The site also offers information
on control measures, photos, and more. Because the website relies
on growers and others to report the outbreaks, it isn't comprehensive
or foolproof. It is, however, a useful monitoring tool.
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Resistant Varieties
One of the principal means of managing downy mildew in cantaloupe
and cucumber is the use of genetically resistant cultivars. Resistance
has not been developed in other cucurbits, though some squash varieties
like Super Select and Zucchini Select are considered to be tolerant
(5), as are cucumber varieties like Poinsett
and Galaxie. (6) The Virginia Extension publication
Downy Mildew of Cucurbits (3) identifies other
resistant cucumber cultivars. Growers are advised to contact Cooperative
Extension and local seed suppliers for assistance in selecting resistant
varieties that also perform well in their location.
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Cultural Controls
Because this disease is carried to most fields on light winds,
cultural practices like crop rotation and sanitation have a limited
effect on the incidence of downy mildew. Still, there are several
things that growers can do to suppress the disease. Growing vigorous
plants, capable of withstanding or repelling disease onslaughts,
is the first step. This involves careful irrigation and soil fertility
management. ATTRA's publications on soil management and related
matters include Sustainable
Soil Management, Manures
for Organic Crop Production, Alternative
Soil Amendments, and Sources
of Organic Fertilizers and Amendments.
Good soil fertility management can often be backed up with foliar
fertilization, which some growers believe can assist in pest resistance.
For additional information see the ATTRA publication Foliar
Fertilization.
Further cultural considerations include selecting growing sites
with good air drainage, full sunlight, and low humidity. Using drip
irrigation, or scheduling overhead irrigation to avoid excessive
leaf wetness, will also reduce disease incidence. When detected
early, disease spread might be slowed somewhat by removing and destroying
infected plants, and by taking care not to transport the disease
by hand or on infected tools and equipment.
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Alternative Pesticides
Along with resistant varieties, fungicides are considered the principal
means of downy mildew control in cucurbits. There are several alternatives
to synthetic fungicides. Be certain to use all pesticides, synthetic
or natural, according to label instructions.
Copper
Copper-based fungicides have traditionally been recommended for
suppressing downy mildew in organic production systems. Caution
is advised, however, as copper can be phytotoxic to cucurbits. Crop
damage appears to be most common during periods of cool wet weather—precisely
the conditions in which downy mildew thrives. (7)
As a result, it is suggested that the most dilute application recommended
for each product be followed. (5)
The use of copper fungicides in organic production is somewhat
controversial. Copper is a regulated material in organic production.
Though an essential plant nutrient in small amounts, fungicidal
levels of copper are directly toxic to some beneficial organisms
such as earthworms and several soil microbes such as blue-green
algae—an important nitrogen-fixer in many soils. Excessive
use can also result in the buildup to toxic (crop-damaging) levels
in the soil—particularly in climates where little to no leaching
occurs. Thus, growers who use these sprays frequently must monitor
soil copper levels through regular soil testing.
Neem Oil
Neem oil is a botanical pesticide derived from the tree species
Azadirachta indica. It is a multi-purpose insecticide, miticide,
and fungicide labeled for control of both downy and powdery mildews
on cucurbits. (8)
Neem products, once considered largely benign to beneficial insects,
have demonstrated some negative impacts. Washington State research
has found neem to be toxic to ladybeetles, especially in their early
larval stages. (9) Being an oil formulation,
neem can also harm bees and should be applied when they are not
active in the field. (10) Therefore, while
neem oil is suitable for organic production, it should not be used
without clear need and plenty of caution.
For information on and sources of Trilogy 90EC™, a formulation
of neem oil, contact Certis USA, LLC (11).
Biofungicides
Serenade™, a relatively new fungicide based on the biocontrol
agent Bacillus subtilis, is available in a wettable powder formulation
that can be used for downy mildew control on vegetables. (12)
According to its manufacturer, Serenade turns on the plant's natural
immune system. It is also said to:
"…stop plant pathogen spores from germinating, disrupt
the germ tubes and mycelial growth and inhibit attachment of the
plant pathogen to the leaf by producing a zone of inhibition restricting
the growth of…disease causing pathogens." (13)
For sources of the product, contact the manufacturer, AgraQuest
Inc. (14)
Peroxides
Organic growers and others in alternative agriculture have often
mentioned hydrogen peroxide (H2O2) as a disease preventive for crops.
(15) While documentation on the use of food-
and/or pharmacy-grade peroxide in managing plant diseases is sketchy,
BioSafe Systems has recently released a peroxigen formulation under
the name of OxiDate™, which is labeled as a broad-spectrum
bactericide and fungicide. Downy and powdery mildews of cucurbits
are among the diseases it is said to control. Among the listed benefits
are biodegradability, little to no phytotoxicity, and the ability
to kill fungal spores on contact. (16)
Although the Organic Materials Review Institute (OMRI) had previously
approved OxiDate for organic production, it removed the product
from its listing in spring 2002 because of non-compliance with federal
regulations. If reformulated, it may be approved again in the future.
Contact BioSafe Systems (17) for additional
details.
Bicarbonates
In 1998, Church & Dwight Co. (18)—the
manufacturer of Arm & Hammer Baking Soda™—received
EPA registration for Armicarb 100™, a potassium bicarbonate
formulation, for use against downy and powdery mildews, botrytis,
and alternaria leaf-spot. (19) This product
is the direct result of research done at Cornell and funded by Church
& Dwight. Armicarb 100 is now available from Helena Chemical
Company (20) and Agri-Turf Supplies. (21)
Similar products are FirstStep™ by the Cleary Chemical Corp.
(22), Kaligreen™ by Monterey Chemical
(23), and Remedy™ by Bonide Products
Inc. (24) For additional information on the
use of bicarbonates in plant disease management, see ATTRA's Use
of Baking Soda as a Fungicide.
Compost Tea
Though still somewhat experimental, compost teas have proved successful
in managing a number of plant diseases. For details, please see
the ATTRA publication Notes
on Compost Teas.
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References
- Motes, Jim et al. 1986. Cucurbit
Production and Pest Management. Circular E-853. Cooperative Extension,
Oklahoma State University, Stillwater, OK. 40 p.
- Bernhardt, Elizabeth et al. 1988.
Cucurbit Diseases: A Practical Guide for Seedsmen, Growers &
Agricultural Advisors. Petoseed Co., Saticoy, CA. 48 p.
- Hansen, Mary Ann. 2000. Downy Mildew
of Cucurbits. Publication Number 450-707. Virginia Cooperative
Extension. www.ext.vt.edu/pubs/plantdiseasefs/450-707/450-707.html.
- McDaniel, M.C. No date. Control
Diseases of Cucurbits. Cooperative Extension, University of Arkansas,
Fayetteville, AR. 6 p.
- Ellis, Barbara W. and Fern Marshall
Bradley. 1992. The Organic Gardener’s Handbook of Natural
Insect and Disease Control. Rodale Press, Emmaus, PA. 534 p.
- Kucharek, Tom. No date. Downy Mildew
of Cucurbits. University of Florida Cooperative Extension Service.
http://plantpath.ifas.ufl.edu/takextpub/FactSheets/pp0002.pdf.
(PDF / 180KB)
- Cowall, Matt and May Hausbeck.
1995. Copper fungicide disease control options. Vegetable Update.
Cornell Cooperative Extension of Oswego County, NY. July 25. p.
11.
- Meister, Richard et al. (editors).
1999. The All-Crop, Quick Reference Insect and Disease Control
Guide. Meister Publishing Co., Willoughby, OH. p. 58.
- Banken, Julie and John Stark.
1998. Multiple routes of pesticide exposure and the risk of pesticides
to biological controls: A study of neem and the sevenspotted lady
beetle (Coleoptera: Coccinellidae). Journal of Economic Entomology.
Vol. 91, No. 1. p. 1-6.
- Anon. 1999. 1999 Main Catalog.
Peaceful Valley Farm Supply, Grass Valley, CA. p. 91.
- Certis USA
LLC 9145 Guilford Rd., Ste. 175
Columbia, MD 21046
Tel: 800-847-5620
Fax: 301-604-7015
www.certisusa.com
- Fravel, Deborah. 1999. Commercial
Biocontrol Products for UseAgainst Soilborne Crop Diseases. January
1. www.barc.usda.gov/psi/bpdl/bpdlprod/bioprod.html. (page no
longer active)
- AgraQuest Web site. No date.
www.agraquest.com/products/rhapsody/products-solutions/product-line.html.
- AgraQuest,
Inc.
1530 Drew Ave.
Davis, CA 95616-1272
Tel: 530-750-0150
Fax: 530-750-0153
E-mail: info@agraquest.com
- For example, market gardeners
Ward Sinclair and Cass Peterson made claims for successful disease
control using hydrogen peroxide on tomatoes and peaches. These
claims are cited in: Hofstetter, Bob. 1993. Homemade pesticides.
The New Farm. February. p. 14-16.
- BioSafe Systems. No Date. OxiDate
Product Literature. BioSafe Systems, Glastonbury, CT.
- BioSafe
Systems
36 Commerce St.
Glastonbury, CT 06033
Tel: 888-273-3088
Fax: 860-657-3388
E-mail: info@biosafesystems.com
- Church & Dwight Co.,
Inc.
469 N. Harrison St.
Princeton, NJ 08543-5297
Tel: 609-683-5900
- Anon. 1998. EPA approves reduced-risk
fungicide. The Grower. December. p. 8.
- Helena Chemical Co.
Tel: 901-748-3200
www.helenachemical.com/sales.html
- Agri-Turf Supplies Inc.
2257 Las Positas Rd.
Santa Barbara, CA 93105
Tel: 800-922-9634
Fax: 805-569-0073
E-mail: info@rosecare.com
http://store.yahoo.com/rosecare1/pea20.html
- Cleary
Chemical Corp.
178 Ridge Road
Dayton, NJ 08810
Tel: 800-524-1662
- Monterey
Chemical Co.
P.O. Box 35000
Fresno, CA 93745-5000
Tel: 559-499-2100
Fax: 559-499-1015
E-mail: info@montereychemical.com
- Bonide
Products Inc.
6301 Sutliff Rd.
Oriskany, NY 13424
Tel: 315-736-8231
Fax: 315-736-7582
E-mail: customerservice@bonide.com
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Additional Resources
What do Organic Gardeners do about Mildew?
Irish Organic Farmers and Growers Association
Explains mildew types, resistant varieties, and control measures.
Downy Mildew of Cucurbits
University of Illinois Extension
Provides a good overview of downy mildew, including information
on symptoms, disease cycle, and control.
Downy Mildew Control in Cucurbits
By George Kuepper
NCAT Agriculture Specialist
Cole Loeffler, HTML Production
CT 111
Slot 107
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