Pepper Diseases
Recognizing and Controlling Phytophthora Blight in Peppers
The Tomato Magazine
June 2005
By F.J. Louws, G.J. Holmes and J.B. Ristaino
North Carolina State University
Phytophthora blight, caused by Phytophthora capsici,
is one of the most destructive pepper diseases in the eastern
United States. The disease usually starts on plants in low, poorly drained
areas of the field especially after excessive rains. The pathogen can
be dispersed in soil, with surface water following drainage patterns,
and via splash dispersal from soil to foliage or from plant to plant.
During heavy rain and high wind, the pathogen can be distributed over
entire fields and cause extensive losses within a few days.
The disease initially occurs as a crown rot characterized by a black
lesion just above the soil line. Affected plants wilt and progressively
die. Diseased plants allow the pathogen to produce more inoculum that
is splash-dispersed by wind-driven rain to the upper parts of neighboring
plants including stems, leaves and fruit. Large black lesions form along
stems, especially at branching points where spores are more likely to
lodge. On fruit, the disease causes a soft rot, and fungus will often
sporulate on the surface, taking on a powdery, or whitish, cast. Lesions
can also occur on leaves, but this is less common. These foliar lesions
are circular with a necrotic center and tan-brown margin.
No Complete Management Program
There is no complete management program to reliably control Phytophthora
blight in peppers. If inoculum is present in the soil and a susceptible
pepper cultivar is grown, the disease can spread rapidly under conditions
of heavy rainfall despite growers’ best management practices.
To reduce risk of Phytophthora blight, a complete integrated management
program should be adopted. Such a program should emphasize water management
above all else.
The following practices should be considered:
• Rotate away from susceptible crops for a minimum of two years, preferably
four years. Grain crops are most suitable (e.g., corn and small grains),
but beans and crucifers are also good options. Potatoes, although related
to peppers, are not susceptible to this pathogen. This disease is particularly
severe in pepper fields following a cucurbit crop.
• Avoid poorly drained soils and low lying areas.
• Do not allow soil build-up at the headlands of fields, but create drainage
ditches to ensure maximum soil-surface drainage from furrows.
• Break up hardpans and plow-pans by subsoiling to increase soil drainage.
• Plant on dome-shaped ridges or beds that are as high as possible; during
transplanting, do not allow planting depressions to remain as water will
puddle here and create conditions favorable for disease development.
• Avoid excessive overhead irrigation.
• Enter infested fields last and clean equipment when moving from an
infested field to other fields.
• Immediately rogue infected plants to limit further spread.
• Maintain sufficient surface crop residue through the use of no-till
production or application of small-grain mulches to the bed and furrows
to limit splash dispersal and surface water movement. Note: No-till production
in early spring can increase the incidence of the root and crown rot
phase of the disease if soilborne inoculum is present but will limit
the blight phase (secondary spread).
• Assay irrigation water to ensure it is not contaminated with the pathogen,
and avoid drainage of water from invested fields back into irrigation
water sources.
• Plant Phytophthora tolerant pepper cultivars (e.g., Paladin, Aristotle).
• Use fumigation for fresh-market peppers, when necessary.
• Use fungicides as labeled, when necessary.
Should I Use a Fungicide?
The decision to use fungicides can be a difficult one and depends on
the history of the field and personal management decisions. Grower
experiences have shown that under heavy disease pressure, fungicides
have not proven effective at limiting economic losses. This could be
due to two problems: (1) The fungicide was not present in sufficient
concentrations during the onset of the disease due to soil/weather
conditions or due to timing and placement of application, and (2) Phytophthora
strains resistant to the fungicide may be present in the field.
During a 1997 survey, infected plants were collected from 12 fields in
North Carolina and one field in New Jersey. Three quarters of the fields
sampled contained isolates that were resistant to Ridomil Gold, and insensitivity
ranged from 11 to 80 percent within the fields. A total of 161 isolates
were evaluated and 57 percent were found to be resistant to Ridomil Gold.
This dramatic shift in populations of P. capsici from sensitivity to
resistance occurred over a short, three-year period.
Should a fungicide program be implemented? On farms without a history
of disease and practicing good crop rotation, a fungicide program may
not be necessary. Farms that have a history of the disease, that can
employ rotation and do not have a resistant population of the pathogen
will likely benefit from a fungicide program. If rotation is not practiced
in fields with a history of disease of if resistant populations are known
to be present, the benefit of a fungicide program is uncertain. Some
research evidence suggests Ridomil Gold provides benefit where resistant
populations are present. In all cases, the focus should be on water management
to minimize conditions conducive to the initiation and spread of this
destructive disease.
Editor’s note: The information here was presented during the Great Lakes Fruit, Vegetable and Farm Market Expo, held Dec. 7-9, 2004, in Grand Rapids, Mich. It is modified from: Louws, F.J., Holmes, G.J. and Ristaino, J.B. 2002. Phytophthora Blight of Peppers and Cucurbits. NC State University, Vegetable Disease Information Note 27. http://www.ces.ncsu.edu/depts/pp/notes/Vegetable/vdin027/vdin027.htm. Dr. Holmes can be contacted by phone at (919) 515-9779 or by e-mail at gerald_holmes@ncsu.edu.
© 2005 Columbia Publishing
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