Late Blight

Susceptible plants

Tomato and potato


  • Bluish gray, water-soaked patches develop on the leaves, especially lower leaves.
  • A downy white mold may develop on the lower leaf surfaces in humid weather.
  • Leaves eventually dry out, shrivel and turn brown
  • Watery spots may also develop on the stem
  • Spots develop on tomato fruit. Initially these are grayish-green, later turning wrinkled and cork-like.
  • The disease is most active in wet weather with cool nights and warm days.


Space plants out well to improve air circulation. Avoid watering in the evenings; water in the morning so that the plants have time to dry off. Water at soil surface whenever possible, rather than sprinkling. A mulch of grass clippings, wood chips, etc. will prevent the spores of the disease from splashing up onto the leaves with rainwater. (Make sure that no herbicides have been used on the grass.)

When damage is noticed, remove as many of the affected leaves as possible. Do not compost these leaves; the spores will survive. Applying a sulphur or copper fungicide at regular intervals will help protect the healthy leaves.

In the fall, remove all plant material from the garden. To help prevent the return of the disease, clear plastic can be laid over the soil, secured with bricks and soil, and left for a time. The sun’s rays will heat the soil, killing many of the spores.

Next year, try to avoid planting your potatoes or tomatoes in the same area of the garden.

Dr. Bill Paton and Shauna Peters