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)

Small-scale potato farmers in developing countries use fungicides to control late blight, increasing production costs and exposing their health by applying the toxins without protection. They are often unsure of which type of fungicide to use (systemic or contact) and the appropriate time to use it. A low-cost tool helps solve the problem by teaching them through a set of disks which fungicide to use, when, and how often to apply it. This article, written for farmers, offers fundamental knowledge of late blight, explains the advantages of using the wheel kit, and provides alternatives for producing the discs.

Spanish Version

By Jorge Luis Alonso G.

Ireland’s potato crop was wiped out in 1845 by a disease called late blight. Because the potato was the primary food source in that country, hundreds of thousands of people died of hunger, and others had to leave. Since then, all farmers who grow potatoes know the disease; it is present in the vast majority of potato areas in the world.

Scientists in Peru and Ecuador have been experimenting with a low-cost tool that allows small-scale farmers to make better use of fungicides to prevent late blight.

But to understand the tool you have to know the symptoms of the disease, the microbe that causes it, and the role of fungicides and resistant potato varieties.

Tables and graphs and parts of this article were adapted from Learning to Control Potato Late Blight — A Facilitator’s Guide, published in 2007 by the International Potato Center (CIP).

The disease

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

Late blight can be confused with other diseases that have similar symptoms, so it is crucial to know the symptoms of late blight.

Symptoms on leaves: Round, watery-looking, dark brown blotches. Sometimes these blotches are surrounded by a yellowish-green ring.

Wilmer Pérez (CIP)

Symptoms on stems: Dry, dark brown blotches. Stems become brittle and may break at the location of the blotch.

Wilmer Pérez (CIP)

Symptoms on tubers: Light brown blotches that are slightly sunken may appear on the tuber’s surface. Dry, light brown blotches may be seen when cutting the tubers. Tubers with late blight symptoms do not have a bad odor.

Wilmer Pérez (CIP)

Other diseases that have similar symptoms to late blight include frost, early blight, phoma leaf spot, and septoria leaf spot. So you have to know the difference between them.

Frost: Produces blotches of dry and torn appearance, generally on the upper leaves.

Wilmer Pérez (CIP)

Early blight: Produces blotches with brown-colored rings. The leaf may turn yellow.

Wilmer Pérez (CIP)

Phoma leaf spot: Produces blotches that can join and cover most of the leaf.

Wilmer Pérez (CIP)

Septoria leaf spot. Produces blotches with tiny black spots. These blotches may be surrounded by a yellow ring.

Hebert Torres (CIP)

White-colored fuzz often surrounds the blotches on leaves and stems affected by late blight. This fuzz is the best way to differentiate late blight from other potato diseases.

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.

To fully understand why Phytophthora appears, you must know the life cycle of late blight.

Spores travel from sources of Phytophthora. The life cycle starts with spores coming from potato crops, wild plants, and voluntary plants infected with late blight.

Spores penetrate leaves and healthy stems. In the presence of rain, spores reach a healthy plant and penetrate it, causing small brown blotches.

Blotches grow. If the weather is cold, blotches grow slowly. If the weather is warm, blotches multiply.

Fuzz forms. In the presence of rain, white-colored fuzz appears around the blotches.

Spores travel to other plants and tubers:

  • 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.

Why is rain a key issue? Because it has five effects: (1) It helps the spores enter the plant. (2) It helps produce fuzz on the plants. (3) It transports spores from the leaves to the tubers. (4) It carries the spores from a diseased plant to a healthy one by spattering. (5) It washes the fungicide from the surface of the leaves.


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.

Resistant varieties do not become infected with late blight quickly. They do not die during rainy weather (or they die later), and production is therefore better. In a resistant variety potato, few spores penetrate the plant, blotches grow slowly, and little fuzz is formed.

Susceptible varieties get a lot of late blight. In rainy weather, they may die, and production is low. Many spores penetrate the plant, blotches grow fast, and much fuzz is formed.

The benefit of planting resistant varieties is that less fungicide application is required. As a result, you spend less money and time, decrease the amount of environmental contamination, lower the health risk, and reduce the risk of losing crop yield during a rainy year.

Is the potato variety you are planting resistant to late blight? Is it susceptible? Is it moderately resistant? If you don’t know, ask an agricultural technician in your area.

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.

This graph shows the commercial name and active ingredient of different fungicides.

Of these five concepts, pay close attention to the mode of action, which can be contact or systemic.

  • 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.

Now that you know how these fungicides work, look at their characteristics:

Make sure you understand their differences because you’re going to need that knowledge to use the tool.

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.

The difficulty in controlling for late blight is knowing when to start the fungicide applications, which fungicide to use, and how often to apply the fungicide.

Researchers from Peru and Ecuador have developed a tool to answer these questions. The tool is called the Potato Late Blight Management Disc Set. It is made of cardboard and does not need access to the Internet or batteries.

The disc set considers three critical pieces of information:

  • 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

The discs are made of cardboard and covered with plastic. Each disc represents a category of variety: red for susceptible varieties, yellow for moderately resistant varieties, and green for resistant varieties.

Each disc includes two rotating circles, one for the number of days with rain in the last week and one for the number of days that have passed since you last applied fungicide.

By rotating the circles, you get different readings, which you can see as numbers. The sum of these numbers gives you a recommendation on the use of fungicides:

  • Do not apply a fungicide
  • Apply a systemic fungicide
  • Apply a contact fungicide

But seeing the tool in use is easier than explaining it, so watch the following video (English subtitles).

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.

As soon as you download them, print them and attach them to cardboard, laminate them in plastic, and center them so the circles rotate and you can see the recommendations.

If you have problems assembling the discs, you can ask several people to help you:

  • 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.

(Join your neighbors in getting discs; it’s easier to get help when several people ask.)

International Potato Center (CIP) has developed an application for smartphones. Thus, farmers don’t need to download the discs or put them on cardboard and laminate them. This app is available in Spanish and English (although the information included is about Peruvian potatoes varieties). Likewise, Instituto Nacional de Investigaciones Agropecuarias (INIAP) has developed a smartphone app (only in Spanish). The information included is about Ecuatorian potatoes varieties. This video (Spanish) shows you how the CIP app works.

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.

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Freelance B2B Technology Content Marketing Writer - Canada |

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