Phytophthora Blight

Phytophthora blight, also known as late blight, is a fungal disease caused by several species of the genus Phytophthora.  Affected plants include:

The fungi involved flourish in wet, moderately warm weather.  They spend the winter in infected tubers and plant matter.  The disease is spread by soil and moisture.


  • Seedling death.
  • Large dark brown spots on leaves of cucurbits.
  • Wet spots on lower leaves in nightshades.
  • White, downy spots on undersides of lower leaves of nightshades.
  • Leaf drop.
  • Brown spots on stems of tomatoes.
  • Brown, collapsing vines in cucurbits.
  • Dark, sometimes sunken blotches in flesh of tubers.
  • Soft, sunken or water-soaked spots on fruit.
  • Fruit rot.
  • White yeastlike growth on fruit after harvest.
  • Generalized rotting and collapse.
  • Plant death.


Destroy all plants affected with phytophthora blight.  Do not use infected potatoes for seed.


The first step in preventing phytophthora blight is to avoid introducing it from outside.  Start by choosing blight-resistant fruit and vegetable varieties.  Buy disease-free seeds and seed potatoes from reputable sources.

Next, practice good garden sanitation:

  • Rotate garden crops, preferably with plants that do not get phytophthora blight.
  • Avoid using sprinklers to water the garden; keep the foliage dry.
  • Wait for rain or dew to dry before working in the garden.
  • Keep the weeds in check.
  • Use nitrogen in moderation to avoid rapid growth of soft, disease-prone tissue.
  • Keep plants trained off the ground with cages or trellises.
  • Prune climbing plants regularly to remove damaged tissue and keep growth under control.
  • Hill potatoes well to reduce their exposure to pathogens.

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Garden & Orchard Diseases

Garden & Orchard Diseases