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Phytophthora blight is caused by Phytophthora capsici. This pathogen can infect all cucurbit crops as well as peppers, tomatoes, eggplants, and infrequently beans. Infection is most common in squash and pumpkin. Phytophthora capsici infects every part of the plant including roots, crowns, leaves, vines and fruit. Phytophthora has only been reported in a few fields in Minnesota.
- 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.
- Infected fruit are covered with white powder sugar like fungal growth.
Phytophthora capsici is an oomycete, often referred to as a water mold. Oomycetes are not true fungi, but are more closely related 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 (75-85F) wet conditions. Disease is commonly seen first in low lying poorly drained areas of the field, but can spread throughout the field if environmental conditions are right. Spores can be blown on windblown rain or carried in soil stuck to equipment that was 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 2 years. If both mating types are present, the fungus will create oospores, a hard walled resting structure that can survive 5 or more years. It is unknown if both mating types occur in Minnesota.
- Plant in well drained fields. Used 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 3 years.
- Remove and destroy infected fruit and vines in small gardens.
- In large fields, till in a small area of infected plants plus a border of healthy plants. Clean equipment thoroughly afterwards.
- Clean tools after working with Phytophthora infected plants.
- Fungicide sprays can help to prevent disease. Commercial growers should refer to the Midwest Vegetable Production Guide for specific fungicide recommendations.