Phytophthora blight / crown rot is a water mold or oomycete disease of several fruiting vegetable crops including cucurbits (cucumber, melon, squash), solanceous plants (tomato, eggplant, pepper), and to a lesser extent, legumes (snap beans, lima beans) caused by Phytophthora capsici. It causes large, irregular brown spots on leaves, which expand and coalesce in warm and wet conditions. On vines, water-soaked, dark lesions can girdle the stem and cause whole-plant wilt and collapse. Phytophthora capsici also causes damping-off in cucurbits, rotting the crown and root tissues and most often resulting in plant death. Fruit rot often occurs on the side of the fruit touching the soil, beginning as a water-soaked lesion that expands and is covered with white sporulating pathogen growth.
Primary Source: Infested soil and infected plant debris
Spread: Sporangia spread via very local wind currents and workers/equipment, while swimming zoospores spread via saturated soil and splashing water; the spores (sporangia) cannot travel long distances
Favorable Conditions: Warm temperatures and high humidity, high leaf wetness, high precipitation and irrigation, poorly draining soil
Infection & Disease Cycle
Phytophthora capsici can overwinter in the soil as oospores, surviving for several years (>20 years). Warmer temperatures and splashing precipitation/irrigation allow spores in the soil to come into contact with nearby plants. These spores can also travel longer distances when infested soil is transported by workers and equipment. After initial infection, lesions on the plant can bear another aerial spore type known as a sporangium. In saturated soil and warm temperatures, P. capsici can also release swimming zoospores. These zoospores infect the roots and crown of the plant, as well as lower leaves and fruit by splashing water. The pathogen can infect crops several times through a growing season, and remain in the soil after harvest. The disease can also continue to be active post-harvest causing breakdown of cucurbit or solanaceous crop fruits. Moisture and airflow management is critical to maintain healthy produce.
Cultural Control
Scouting regularly allows early identification of disease before significant spread and damage. The following practices can also help prevent disease development:
- Rotate away from susceptible crops ( >3 years)
- Plant resistant varieties when possible
- Plant in areas with well-draining soil
- Disinfest tools and equipment
- Maintain proper soil moisture
- Mulch with straw or dropped cover crops to reduce splash
- Destroy infected plant debris
- Do not irrigate from retention ponds that may receive run-off from infested fields (swimming spores can be present in the water and inoculate fields)
Chemical Control
Phytophthora capsici populations are known for developing resistance to fungicides when used over time. In particular, many populations have resistance to phenylamides such as metalaxyl or mefenoxam. For Wisconsin-specific fungicide information, refer to the Commercial Vegetable Production in Wisconsin (A3422), a guide available through the UW Extension Learning Store website. Or, for home garden fungicide recommendations, see Home Vegetable Garden Fungicides (D0062), a fact sheet available through the UW Plant Disease Diagnostic Clinic website. Always follow label directions carefully.
Resources
- Commercial Vegetable Production in Wisconsin (A3422) from the UW Extension Learning Store. This guide offers the latest recommendations for disease, insect, and weed management in Wisconsin’s most common commercial vegetable crops. Also included are lime and fertilizer recommendations as well as insect identification information and keys.
- UW Plant Disease Diagnostics Clinic. The University of Wisconsin-Madison/Extension Plant Disease Diagnostics Clinic (PDDC) provides assistance in identifying plant diseases and provides educational information on plant diseases and their control.
References
- Scheufele, S., and G. Higgins. 2015. “Phytophthora Blight.” Text. Center for Agriculture, Food, and the Environment. January 6, 2015. https://ag.umass.edu/vegetable/fact-sheets/phytophthora-blight.
- Schuh, Marissa, and Michelle Grabowski. 2022. “Phytophthora in Vegetable Crops.” 2022. https://extension.umn.edu/disease-management/phytophthora.
Written by Amanda Gevens, Ariana Abbrescia, Russell Groves, and Ben Bradford. Last updated Nov 2023
