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Phytophthora in vegetable crops

Quick facts

  • Phytophthora (Phytophthora capisci) can infect all cucurbits (pumpkins, squash, melons, etc.) as well as peppers, tomatoes, eggplants and beans. This is a different type of phytophthora than the kind that infects raspberries and strawberries.
  • Phytophthora is long-lived in the soil and driven by moisture.
  • Disease can infect all parts of the plant tissue, and the most apparent symptom is wilting and white, powdered sugar-like spores on infected plant tissue.
  • Disease is very hard to manage once present in a field, so focus on prevention.
  • Phytophthora travels through water. Plant in well-drained fields and use raised beds to improve drainage and avoid working in impacted fields that are wet.
  • Rotate out of cucurbit and solanaceous crops for a minimum of three years.
An orange squash with a brown and white lesion.
Fruit infection starting at the soil line and moving up a squash

How to tell phytophthora from other vegetable issues

A row of collapsed crookneck squash plants and fruits with water soaked lesions starting at the soil line.
Wilting summer squash plants due to phytophthora infection
  • Infections are often first noticed as a cluster of wilting and totally collapsed plants.
  • Plants pull up easily from the soil due to root loss. Infected roots and crown are black and water-soaked in appearance.
  • White growth, similar to powdered sugar, covers infected fruit and crowns.
  • Stem and leaf petiole lesions are light to dark brown, water-soaked, and irregular. Severe infections result in plants with large irregular brown spots forming on leaves before plants collapse.
  • Fruit develops soft, water-soaked rot. Infection may start where the fruit contacts the soil, where the stem connects to the fruit, or as a random circular spot. Infected fruit are soft, easily punctured and often collapse.
  • In some cases, infection on the fruit can show up after harvest. 
  • Because phytophthora is long-lived and requires intensive management, suspected infections should be sent to the UMN Plant Disease Clinic.

Biology

A pepper leaf with a vaguely circular brown lesion.
Phytophthora on pepper leaf
  • Phytophthora capisci is a  pathogen that can infect all cucurbit crops as well as peppers, tomatoes, eggplants and beans. Infection is most common in zucchini, squash, pumpkin, and pepper. 
  • Phytophthora infects every part of the plant including roots, crowns, leaves, vines and fruit. 
  • Phytophthora thrives in warm (75-85° F), wet conditions. Disease is more common in low-lying, poorly drained areas of the field, but can spread throughout the field if environmental conditions are right.
  • Spores can travel on soil stuck to equipment used in an infested area and on windblown raindrops.
  • Phytophthora overwinters in soil and plant debris. 
  • Phytophthora capsici is an oomycete, also known as a water mold. Oomycetes are not true fungi but are close relatives to certain kinds of algae. 
  • Phytophthora has swimming spores known as zoospores that can swim through films of water and saturated soils to locate a new host plant. As a result, new infections often appear in the direction in which water drainage occurs.
  • There are two different mating types of Phytophthora capsici. If only one mating type is present in a field, the pathogen can survive for two years. If both mating types are present, the fungus will create oospores, a hard-walled resting structure that can survive five or more years. It is unknown if both mating types occur in Minnesota.

Managing Phytophthora in home gardens

A pepper plants with water soaked sesion on the stems, and deflated pepper fruit covered in white spores. Photo: Don Ferrin, Louisiana State University Agricultural Center, Bugwood.org.
Stem and fruit symptoms in bell peppers

Correctly identify vegetable issues

Phytophthora is one of many soilborne pathogens and is not common in Minnesota gardens. You can use Ask a Master Gardener for help figuring out what is going on in your garden.

Cultural controls

Rotate different vegetable families into new areas of the garden on at least a three-year schedule.

Managing phytophthora on farm

A large field of peppers with a large area of brown, collapsed plants.
Pepper plants wilting in a low spot infected with phytophthora
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Authors: Marissa Schuh, horticultural IPM Extension educator, and Michelle Grabowski

Reviewed by Annie Klodd and Natalie Hoidal, Extension educators

Reviewed in 2022

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