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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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