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Phytophthora blight

Quick facts

  • Phytophthora can infect all cucurbit crops as well as peppers, tomatoes, eggplants and sometimes beans. Infection is most common in squash and pumpkin.
  • Leaves wilt and the entire plant may collapse if root and crown rot occurs. Infected roots and crowns are black. Plants pull up easily from the soil due to root loss.
  • Fruit develop water-soaked rot. Infected fruit are soft, easily punctured and often collapse.
  • Phytophthora overwinters in soil and plant debris.
  • Plant in well-drained fields. Use raised beds to improve drainage. Do not work in fields when soils are wet to avoid compacting soil.
  • Rotate out of cucurbit and solanaceous crops for a minimum of three years.

Phytophthora capsici causes phytophthora blight. This pathogen can infect all cucurbit crops as well as peppers, tomatoes, eggplants and sometimes beans. Infection is most common in squash and pumpkin. Phytophthora capsici infects every part of the plant including roots, crowns, leaves, vines and fruit. There are reports of phytophthora in only a few fields in Minnesota.

Identifying phytophthora blight symptoms

  • Large irregular brown spots form on leaves.
  • Stem and leaf petiole lesions are light to dark brown, water soaked and irregular.
  • Leaves wilt and the entire plant may collapse if root and crown rot occurs. Infected roots and crowns are black. Plants pull up easily from the soil due to root loss.
  • Fruit develop 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.
  • White fungal growth, similar to powder sugar, covers infected fruit.

What causes phytophthora blight

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.

Phytophthora thrives in warm (75-85° F), wet conditions. Disease is 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 windblown rain or in soil stuck to equipment used in an infested area.

Phytophthora overwinters in soil and plant debris. 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.

Preventing and managing the disease

  • Plant in well-drained fields. Use raised beds to improve drainage. Do not work in fields when soils are wet to avoid compacting soil.
  • Avoid planting susceptible crops in fields with a history of disease.
  • Rotate out of cucurbit and solanaceous crops for a minimum of three years.
  • Remove and destroy infected fruit and vines in small gardens.
  • In large fields, till in a small area of infected plants and a border of healthy plants. Thoroughly lean equipment afterwards.
  • Clean tools after working with phytophthora-infected plants.
  • Fungicide sprays can help prevent disease. Commercial growers should refer to the Midwest Vegetable Production Guide for specific fungicide recommendations.

CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Remember, the label is the law.

Michelle Grabowski

Reviewed in 2018

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