Persistent wet weather puts tomatoes at risk for late blight

Vijai Pandian
Late blight can be devastating to tomato growers.

As gardeners around Wisconsin eagerly anticipate the tomato harvest, the state's first confirmed report of late blight in 2017 is bringing disappointment to some growers around the state.

A highly destructive fungal disease caused by the mold Phytophthora infestans, late blight can wipe out an entire tomato or potato crop in a short period of time. The fungal spores of late blight can spread quickly by wind, and once infected, plants can show symptoms within one week. It is one of the few plant diseases that are difficult to control, and it can cause severe economic loss to growers.

Persistent wet weather over the first half of 2017 has created ideal conditions for late blight disease to spread more quickly than is typical. In Brown County, late blight is no stranger, with multiple reports from gardeners and growers in previous years.

Home gardeners are strongly encouraged to scout their tomato and potato plants every day and immediately take preventive measures against late blight.

In cases of late blight, patches turn blackish in color.

What to look for

  • Check your foliage for any sign of pale green or olive green patches along the margins or at the tip of the leaves
  • The patches can quickly enlarge and become a brown-blackish water-soaked lesion
  • During early-morning hours, the underside of the leaves develop a white-grayish fuzzy growth
  • Infected plant stems develop dark brown to black lesions
  • Infected tomato fruits have sunken spots with golden to chocolate brown lesion with a distinct ring around them
  • Potato tubers develop a dark brown to purple discoloration

What to spray?

Fungicides should be applied prior to infection or they won’t be effective in preventing the disease.

Fungicides containing copper or chlorothalonil as the active ingredient can be used as a preventive application. Schedule the fungicide application once in five to seven days, however under excessive wet conditions, fungicides can become ineffective. To learn more about late blight management, visit

Organic copper fungicide products include: Copper Fungicide, Copper Soap, Dragon Dust, Earth Tone, Garden Dust. Chlorothalonil fungicide products include: Daconil, Fung-onil, Ortho Max Disease Control.

Always read the product label and follow its direction for safety and rate of application.