Agronomy •  2023-06-21

Proactive Strategies for Protecting Potatoes from Early & Late Blight

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late blight in potatoes

Protecting the yield and quality of your potato crop is always a top priority, but with every growing season, challenges arise. Two common diseases, early blight and late blight, can be found in most potato-growing regions across Canada, and when left untreated, can cause serious economic impacts.1 In fact, it is estimated that late blight alone is responsible for up to $10 billion in lost revenue due to potato crop damage worldwide each year.2 As a grower, understanding the difference between these two diseases, and having a proactive crop management plan in place, can help significantly reduce the risk posed to your fields. 

Impact of early & late blight on potatoes

Early blight is caused by the fungus Alternaria solani. It is an endemic disease in which the inoculant is always present, meaning that the disease can redevelop and emerge each growing season.3 Early blight affects the leaves, stems, and tubers of potatoes, and if not managed properly, can reduce the yield, size, and storability of the tubers, damaging the overall quality and marketability of the entire crop.4

Late blight is caused by the fungus Phytophthora infestans, which is the most damaging pathogen affecting potatoes. It is a highly aggressive disease that, if left uncontrolled, especially in weather conditions that are conducive to development (warm and humid), can destroy potato fields in as little to five to seven days.5 Late blight can survive for long periods of time by living on infected cull and seed tubers, tubers left in the field after harvest, and even slivers from seed-cutting operators.Post-harvest losses can also occur if infected tubers are stockpiled.

Recognizing signs & symptoms

As the names suggest, early blight and late blight reference the approximate time each disease begins to appear, although the timing for both can overlap.6

Early blight, which usually affects older, lower leaves first, thrives in warm temperatures with high humidity. Initial symptoms appear in the form of small brown or black lesions surrounded by a narrow light-green or yellow halo.3 If left untreated, these lesions will begin to appear on stems and upper leaves. Infected tubers will have dark lesions which may appear circular or irregular in shape and will have a texture similar to cork.4

Late blight often appears in the form of wet, light-to-dark green spots on the plant leaves, typically at the tips or in the margins. If favourable conditions for disease development continue––high humidity; moderate temperatures during daytime and cooler in the evenings––over time, these lesions will darken, and a noticeable white fungus may appear on the underside of the leaves7. As the disease continues to spread, plant stems may begin to droop, and leaves may die off. The tissue of infected tubers will feature a reddish-brown discolouration under the skin.

leaf blight  leaf blight

Managing early & late blight

Even a single blight-infected plant is enough to put your entire field at risk. The best solution for avoiding crop quality or yield losses is adopting a preventative system that includes seed selection, cultural practices and fungicide application.7

For both early blight and late blight, start with disease-free seed (resistant cultivars are also available and can help reduce risk).8

Maintaining good overall plant health via fertilization and reducing plant stress can help minimize the chances of early blight infection. Monitor fields, specifically edges and rows along adjacent fields that have hosted potato plants in the previous year.8 Start scouting once plants have reached 12" in height.

A clean operation is essential for preventing the infection and spread of late blight. Keep cull/compost piles away from growing areas and consider removing or destroying volunteer potatoes by hand or tillage. If scouting an infected crop, make sure to wear rubberized apparel, like hip waders and tall rubber boots, that can be thoroughly washed and rinsed afterward to minimize the risk of spreading the disease.7

Preventative fungicide application is an effective tool for both early blight and late blight and should be used even when resistant cultivars are grown. It’s best to be prepared to spray, if conditions are favourable for disease development, with subsequent applications as needed.3

Using Tanos™ for preventative blight control

Tanos™ fungicide protects your potato yields from both early blight and late blight by combining powerful control with resistance management––utilizing Group 11 and Group 27 active ingredients. Tanos rapidly penetrates leaf surfaces, providing both locally systemic and post-infection control of late blight. After application, a portion remains within the leaf’s waxy cuticle, where it avoids wash-off, resulting in exceptional control of early blight.

Apply Tanos following one to two applications of a preventative broad-spectrum fungicide, such as chlorothalonil or mancozeb, at a rate between 226 and 340g/a. To manage resistance, alternate Tanos with a different mode of action fungicide. Always read and follow label directions.

Protect your potato crops this season and beyond with Tanos fungicide. Contact our horticultural specialists to learn more.