Departments of Plant Pathology and Entomology
North Carolina Cooperative Extension Service
North Carolina State University

FRUIT
AND
VEGETABLES

North Carolina Pest News
Volume 13, Number 1, April 24, 1998
Stephen J. Toth, Jr. and Thomas A. Melton, editors

The information and recommendations in this newsletter are applicable to North Carolina
and may not apply in other areas.

From: Frank Louws, Gerald Holmes and Jean Ristaino, Plant Pathologists

Phytophthora Blight in Peppers and Cucurbits

Phytophthora blight caused by Phytophthora capsici is one of the most destructive pepper and cucurbit diseases in North Carolina and many other regions of the U. S. The disease was recorded in North Carolina as early as 1954, but has become a serious concern over the last 10 years. The disease usually starts 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. During conditions of heavy wind and rain, the pathogen can be distributed over entire fields and cause extensive losses within a few days.

The pathogen has a wide host range including pepper, tomato, eggplant and most cucurbits (e.g., cucumber, squash, pumpkins, watermelon and muskmelons). The pathogen can persist in soils for years if both mating types of the fungus occur in the same field.

Symptoms on Peppers

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 enable the pathogen to produce more inoculum that is splashed dispersed by wind-driven rain to the upper parts of neighboring plants including stems, leaves and fruit. Large black lesions form along stems, circular yellow lesions on the leaves, and white mycelial (fluffy) growth forms on affected fruit.

Symptoms on Cucurbits

In summer squashes the disease appears as a crown rot, often moving from the top of the crown down. However, in most cucurbits the fruit rot phase of white mycelial growth is the most common symptom. Crown rot has not been observed in watermelon and is uncommon in cucumbers, muskmelon and winter squashes. Highly susceptible crops include summer and zucchini squashes; pumpkins are moderately susceptible and cucumbers, muskmelon, winter squashes and watermelon are least susceptible.

There is no complete management program to reliably control this disease in peppers and cucurbits. If inoculum is present in the soil and a susceptible pepper/cucurbit variety 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 needs to be adopted with an emphasis on water management. The following practices should be considered:

1. 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 and tobacco are also not susceptible to this pathogen.);

2. avoid poorly drained soils and low lying areas;

3. do not allow soil build-up at the headlands of fields but create drainage ditches to ensure maximum soil-surface drainage from furrows and at the end of the fields;

4. break up hardpans and plow-pans by subsoiling to increase soil drainage;

5. always plant peppers/cucurbits on dome-shaped ridges or beds that are as high as possible and do not allow planting depressions to form near plants;

6. avoid excessive overhead irrigation;

7. enter infested fields last and clean equipment when moving from an infested field to other fields;

8. immediately rogue infected plants to limit further spread;

9. 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;

10. evaluate irrigation water to ensure it is not contaminated with the pathogen (a bioassay is available and further information can be obtained from the authors) and avoid drainage of water from fields back into irrigation water sources;

11. consider evaluating resistant pepper varieties (e.g., Paladin available from Rogers Seed);

12. use fungicides when necessary.

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 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 (G. Para and J. Ristaino), 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. Resistant isolates grew in the presence of both metalaxyl (Ridomil) and mefenoxam (Ridomil Gold). This dramatic shift in populations of P. capsici from sensitivity to resistance has occurred in pepper fields in the last 3 years in North Carolina.

Ridomil Gold EC is registered for pre-plant and early post-plant application to control Phytophthora blight in peppers. Ridomil Gold EC is labeled for use on cucurbits to control Pythium damping- off, but not Phytophthora blight. Thus, the manufacturer makes no claim on efficacy against this disease in cucurbits. Specific recommendations of rates and timing for Ridomil Gold can be obtained from the label. Placement of Ridomil Gold in the root zone is extremely important. Ridomil Gold applications should be incorporated or moved into the zone by irrigation. Ridomil Gold/Copper or maneb plus copper can be used as foliar sprays to limit the foliar phase of the disease. The maneb plus copper combination is a protectant spray and must be applied before conditions conducive to Phytophthora prevail. Growers may want to consider up to three maneb/copper applications on 7-10 day intervals starting 2 to 3 weeks after the last Ridomil Gold EC application.

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 or if resistant populations are known to be present, the benefit of a fungicide program is doubtful. In all cases, the focus should be on water management, to minimize conditions conducive to the initiation and spread of this destructive pathogen.

Web page last updated on April 27, 1998 by Stephen J. Toth, Jr..

This Web version is a cooperative effort between the North Carolina Cooperative Extension Service and the Center for Integrated Pest Management