Phytophthora blight, crown and root rot, and fruit rot are serious problems on cucurbit and fruiting (solanaceous) vegetables in the eastern half of the United States. Short vegetable crop rotations, center-pivot irrigation, and heavy rainfall create conditions favorable for Phytophthora diseases.
In South Carolina, Phytophthora diseases occur on vegetables in all parts of the state in clay, loam, and sandy soils. The disease occurs most often on summer squash and pepper, two of the most susceptible crops. Fruit rot on watermelon and pumpkin has been serious in some fields.
On summer squash and pumpkin, the first symptom is usually a soft rot in the crown of the plant that quickly leads to wilting and collapse (figure 1). Root rot is usually not seen. Fruit are very susceptible to a soft rot that starts as large, circular, tan, beige, or salmon spots. Fruit rot may develop post-harvest.
On peppers, Phytophthora blight often begins with a dry, corky canker on the main stem at the soil line (figure 2). Roots may turn chocolate brown and rot. Stem branch points turn black. Diseased fruit are covered with a thin, dense layer of white mold.
All cucurbits (squash, pumpkin, cucumber, melon, and watermelon) and solanaceous vegetables (pepper, tomato, and eggplant) are susceptible. Lima bean and snap bean also may be attacked by this pathogen when grown in infested soil during wet weather.
The Disease Cycle
The water mold that causes Phytophthora blight is Phytophthora capsici, named after pepper (Capsicum), the first host on which it was found. This organism is actually a type of algae that has adapted to living in and on plants.
The main source of the pathogen is infested soil. The pathogen survives in contaminated soil for many years. Other sources are surface water (ponds and streams) and infested crop debris, particularly cull piles of rotten fruit.
Phytophthora produces three types of spores: oospores, sporangia, and zoospores. Oospores are long-lived spores that form on fruit and survive in soil. They move with soil, run-off, and drainage water. Sporangia are short-lived spores that are spread by water.1 Zoospores are swimming spores that only form in water.
Phytophthora is active only in waterlogged soils. Wet soil triggers oospores to germinate and produce sporangia that release zoospores within thirty minutes. Zoospores are attracted to roots and infect root hairs. Sporangia produced above ground are splashed onto fruit and to other plants in the field. When sporangia land on plant tissues, disease starts quickly.
Five options help reduce the occurrence and severity of Phytophthora diseases on vegetables: water management, crop rotation, soil management, resistance (in pepper only), and fungicides. Combine as many of these practices as possible for the best control.
Because Phytophthora capsici needs wet soil to be active, use every technique available to improve soil drainage.
- Subsoil and use raised beds for all crops. Cut water furrows across plastic-mulched raised beds.
- Do not plant in low areas in fields, because disease will start there. Consider leveling low areas but avoid moving soil in fields already infested with the pathogen.
- Use caution with water-wheel transplanters so planting holes are not punched too deeply. Deep holes create areas where water will collect and provide ideal conditions for disease to start.
- Use irrigation water from wells, not ponds, streams, or rivers. The pathogen can be introduced to noninfested (“clean”) fields by irrigating with contaminated water.2
- Always rotate to a nonhost crop after cropping any susceptible vegetable. Crop rotation will not eliminate Phytophthora from infested soil, but it will keep the pathogen from increasing.
- Do not plant a fall susceptible crop in the same field after a spring susceptible crop.
- Enter noninfested fields before entering infested fields.
- Power-wash soil from machinery after working in infested fields.
- Use plastic or organic mulches to prevent infested soil splashing onto fruit.
- On small acreages, remove diseased and rotting fruit to prevent build-up of Phytophthora.
Bell pepper cultivars Vanguard, Paladin, Revolution, and Declaration are moderately resistant to Phytophthora blight. When these cultivars are grown, other management practices should still be used to slow disease and lengthen the harvest period. Under certain growing conditions, these cultivars may show “silver skin,” a blistering of the skin of the fruit that can lower fruit quality. Resistant cultivars are not available for other vegetable crops.
Fungicides are not a “cure-all” for Phytophthora diseases. They do not work well once symptoms are noticed. Fungicides should be used preventively, but disease still may occur in treated fields. Foliar sprays will be needed to manage fruit rots, as the fungicide may not get to the fruit from applications made through drip irrigation.
Ridomil Gold is no longer registered against Phytophthora, because the pathogen becomes resistant to this fungicide relatively quickly. Resistant isolates have been found in South Carolina and many other states. Resistance to Ranman has been found in the southeastern United States, so this fungicide also is not recommended.
To prevent resistance to fungicides, rotate at least two recommended fungicides from the table below (table 1), observing the maximum number of applications on the label. Tank mix with another fungicide when required.
For more information on fungicides, see the current edition of the Southeastern U.S. Vegetable Crop Handbook.3
Table 1. Recommended fungicides to manage vegetable diseases caused by Phytophthora capsici
|Fungicide||Application Type||Tank Mix||Maximum Number of Applications|
|Orondis Gold*||Drip||Not required||4|
|Orondis Ultra*||Foliar||Copper fungicide||4|
|Zampro||Drench, drip, foliar||Not required||4|
*Do not use both Orondis products on the same crop.
**Do not rotate Orondis Ultra and Revus.
Recommendations for integrating the above five options are listed in table 2 to help reduce the occurrence and severity of Phytophthora diseases on vegetables. Increasing levels of infestation call for combining more practices.
Table 2. Summary recommendations for integrated management of Phytophthora diseases on vegetable crops
|If you have:||Water management||Crop rotation||Soil management||Resistant cultivars||Fungicides|
|Noninfested fields on infested farm||x||x||x||x||x|
- Granke L, Quesada-Ocampo L, Lamour K, Hausbeck M. Advances in research on Phytophthora capsici on vegetable crops in the United States. Plant Dis. 2012 Oct; 95(11): 1588-1600. https://doi.org/10.1094/PDIS-02-12-0211-FE.
- Gevens A, Donahoo R, Lamour K, Hausbeck M. Characterization of Phytophthora capsici from Michigan surface irrigation water. Phytopathology. 2007 Mar; 97(4): 421-428. https://doi.org/10.1094/PHYTO-97-4-0421.
- Kemble JM, senior editor. Meadows IM, Jennings KM, Walgenbach JF, editors. Vegetable crop handbook for the southeastern US. Willoughby (OH): MeisterMedia Worldwide. 2019. Growing Produce. https://www.growingproduce.com/southeasternvegetablecrophandbook/.