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Phytophthora Blight in Cucurbit Crops

Phytophthora blight has been the most destructive disease of cucurbit crops on Long Island, and the most challenging disease to manage effectively.  Information about this disease and its management are posted at the VegetableMD website.

Crown rot is one of the common symptoms of Phytophthora blight.  Affected tissue often looks water-soaked and collapsed, especially with young plants.  Symptoms often extend up the stem from the crown rather extending down into the taproot as occurs routinely with Fusarium crown and root rot.  Affected plants wilt.

seedlings

crown

whole plant

The causal pathogen, Phytophthora capsici, sometimes can be seen on affected tissue upon close examination.  It is the white yeast-like growth in the photograph below.  This growth is mostly spores of the pathogen, which are moved by splashing water.  When not present, putting the stem on wet paper towel in a sealed ziplock bag overnight can provide conditions favorable for spores to form.

Phytophthora capsici

Vine and stem tissues are also susceptible.  Affected areas are initially water-soaked and then turn brown.

Phytophthora capsici vine

Phytophthora capsici stem

Plants with infected crown or vine tissue are killed.

killed plants

Fruit rot is a common symptom.  Sometimes spots develop on the top and sides of fruit resulting from splash dispersal.  Initially spots appear water-soaked.

infected pumpkin

infected pumpkin

More commonly fruit become affected where in contact with soil.

infected fruit

infected fruit

infected fruit

Fruit can become mostly to completely covered with pathogen spores which have a yeast-like appearance.

infected pumpkin

infected acorn squash

But affected fruit may have no spores.

fruit no spores

Sometimes affected tissue on a fruit is centered around a rotten stem, suggesting the pathogen moved into the fruit from affected vine tissue.

stem

Eventually the whole fruit collapses.

collapsed fruit

Fruit rot occurs in all cucurbits, but they vary in susceptibility with cantaloupe and pumpkin varieties that produce hard rinds being among the less susceptible.

cucumbers

cucumbers

summer squash

watermelons

watermelon

cantaloupe

Symptoms have been less commonly seen on leaves.  Spots are brown, often round, and can become large.  Sometimes there is a wilted, yellow border.  They resemble leaf spots due to late blight, which is caused by a related pathogen.  Leaf tissue in contact with soil can also be affected.

leaf

leaf

leaf

leaf

leaf

leaf

Petiole tissue is also susceptible.

petiole

petiole

petiole

Growing tip collapses when it is infected by Phytophthora capsici.

tip

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