How to Control Potato Late Blight by Using Fungicides Correctly

Using a new tool, small-scale farmers can now know what kind of fungicide to use and when and how often to apply it.

Wilmer Pérez (CIP)

The disease

Late blight is a potato disease that attacks leaves, stems, and tubers. It is caused by a microbe called Phytophthora infestans.

Wilmer Pérez (CIP)
Wilmer Pérez (CIP)
Wilmer Pérez (CIP)
Wilmer Pérez (CIP)
Wilmer Pérez (CIP)
Wilmer Pérez (CIP)
Hebert Torres (CIP)

The cause

Late blight is caused by Phytophthora infestans, a type of microbe similar to fungus. It can be introduced into a crop in two ways:

  • Phytophthora spores are carried by the wind from neighboring potato fields to yours
  • You planted seed tubers infected with Phytophthora.
  • Spores travel. If there is no rain, the spores fall from the leaves and are carried by air to other healthy plants, and the cycle starts again.
  • Spores spatter. In the presence of rain, spores are spread from a sick plant to a healthy one by rain spatter, again starting the cycle.
  • Spores are washed down to the tubers. In the presence of rain, spores are washed from the leaves to the tubers. If the soil is wet, spores penetrate tubers, causing blotches. If these tubers are planted, plants with late blight grow, and the cycle starts again.


Controlling late blight is achieved by planting late blight–resistant varieties and using fungicides.

Using resistant potato varieties

The best way to control late blight is to plant resistant potato varieties.

Using fungicides

To control potato late blight using fungicides, you must know the following concepts:

  • Fungicides: These are poisons that control plant diseases caused by fungi, like Phytophthora infestans.
  • Commercial name: This is the name under which a fungicide is sold.
  • Active ingredient: This is the name of the chemical that kills the fungus. Some fungicides have one active ingredient; others have two or more. There are fungicides with different trade names, but with the same active ingredient.
  • Mode of action: This is how a fungicide works on the plant.
  • Formulation: This is how fungicides are presented. These can be solids (powders and granules) or liquids.
  • Contact fungicides act outside the plant, like an ointment works on skin.
  • Systemic fungicides work inside the plant as a pill acts inside the body.

The tool

Late blight is a rapidly progressive disease. Therefore, you should visit your field at least once a week, especially when you have planted a susceptible variety of potato. These visits allow you to discover late blight and take the necessary control measures, which include the application of fungicides.

  • How resistant is the potato variety you’re growing
  • How many days it has rained in the last week
  • How many days have passed since you last applied a fungicide
  • Do not apply a fungicide
  • Apply a systemic fungicide
  • Apply a contact fungicide

Where to get the discs

The discs do not require the Internet or batteries to operate, but to get them for the first time, you must connect to the Internet and download them from this page.

  • The mayor of the town: Tell him what you need the disks for and why it would be good for the region if you used the tool.
  • The salesperson of the company that produces the fungicide you use: Those companies could help you because they would sell more fungicides. Ask the owner of the business where you regularly buy fungicide to contact his sales representative and explain what you need help with.
  • The agricultural technician: Ask the farm technician in your area to give you a hand. These institutes are there to help you.

Advantages of using the tool

Farmers who have followed the disc set’s recommendations get a better or equal harvest compared to farmers who did not use the disc set. Those who use the discs often use less fungicide, reducing their production costs.

If you have any questions about the tool, write to Wilmer Perez (CIP) at

A very special thanks to Gregory Forbes, CIP consultant, for the review and correction of this article.


Late blight is one of the most important diseases of potato cultivation worldwide. In the Andes, late blight is a severe disease to manage because potatoes are produced there year-round and because the climate is increasingly unstable. The disease usually affects plants before they flower. Because of that, farmers have to apply fungicides as soon as the plants' sprout, which causes other problems. One is that farmers don’t handle fungicides carefully, which can affect their health and that of their families. The other problem is that because farmers use fungicides incorrectly, late blight is not well controlled, and harvests are smaller. For all these reasons, the International Potato Center (CIP), in collaboration with other institutions in Ecuador and Peru, developed a low-cost tool to help farmers improve their use of fungicides. The tool helps them decide which fungicide to use, when to start applications, and how often to apply a fungicide.

About the author

Jorge Luis Alonso G. is a writer specializing in potato cultivation who writes marketing content for ag-tech companies. He has lived with his family in Canada since 2018.



Freelance B2B Technology Content Marketing Writer - Canada |

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