Late Blight

Class: Oomycete

Common Name

Late Blight

Scientific name

Phytophthora infestans (from Greek: Phytophthora, a plant destroyer)

Potential Hosts

Tomatoes and potatoes

Who am I?

Late blight is responsible for the great 1847 Irish Famine and is perhaps the most notorious example for phytopathology. Late blight is a major threat to important crops around the world and is a driving force for new fungicide development.

Late blight is documented in all members of the Solanoideae plant family, but potatoes and tomatoes have higher tendencies of getting infected.

Late blight is often misrepresented as a fungus; however, it is an oomycete, a fungus-like organism similar to algae.

Cool weather (15-25 C) and extended periods of high humidity, as a consequence of rain and fog, can result in a late blight epidemic. The entire potato and tomato fields be severely damaged within a few days or even destroyed within a matter of 2 weeks.

Late blight symptoms include the spread of brown to black lesions on leaves surrounded by a thin halo of brighter pigmentations. Green fruits are more vulnerable to infection and could become marble-brown in appearance and the fruit becomes hard. In high humidity conditions, gray to white “fluff” will appear around the edges of darker lesions.

Control measures


Working with resistance cultivars: Not all tomato and potato varieties have the same late blight resistance.

The sooner the better: It’s easier and more cost effective to overcome infestation and successfully control late blight at its initial stage. Make it a routine to monitor the field regularly and search plants for the presence of late blight on a weekly basis.

As with any moisture-favoring disease, various agro technical measures can be taken during crop growth to reduce the chances of infection and spread:

Improve air circulation: Promote dry foliage and shorten the duration of wetting periods by introducing net curtain vented areas.

Proper soil drainage: The presence of standing water will promote the spread of late blight.

Cover the ground with polyethylene sheets to reduce evaporation from the soil.

Conventional (chemical)

When conditions appear favorable for late blight or such conditions are expected in the upcoming days, it may be recommended to spray applications of fungicides (based on one or more generic active ingredients) in 5 - 7 days intervals.

The following fungicides are used in one or more parts of the world: chlorothalonil, cymoxanil, dimethomorph, iprodione, azoxystrobin, propamocarb HCL, mandipropamid, copper hydroxide, and sulfate.

Never perform two consecutive iterations with the same active ingredients.


It is extremely difficult to manage late blight organically. Organic growers should know the risks they are taking and do all they can in the cultural aspect. Though, some copper-based fungicides are considered organic and can be utilized.

Caution and careful notice should be taken when using any plant protection products (insecticides, fungicides, and herbicides). It is the grower’s sole responsibility to keep track of the legal uses and permissions with respect to the laws in their country and destination markets. Always read the instructions written on labels, and in a case of contradiction, work in accordance to the product label. Keep in mind that information written on the label usually applies to local markets. Pest control products intended for organic farming are generally considered to be less effective in comparison to conventional products. When dealing with organic, biologic, and to some extent a small number of conventional chemical products, a complete eradication of a pest or disease will often require several iterations of a specific treatment or combination of treatments.

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