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Phytophthora blight

Michelle Grabowski

plant with wilting leaves

G. Homes, CA Poly Tech State University,

close up of blackened plant stems

G. Homes, CA Poly Tech State University,

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.


cucumber with large white spot next to ruler

P. Bachi, KY State University,

Important biology

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.


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