NC State Extension Publications

Pathogen

Tomato late blight is caused by the oomycete pathogen Phytophthora infestans (P. infestans). The pathogen is best known for causing the devastating Irish potato famine of the 1840s, which killed over a million people, and caused another million to leave the country.

Host Crops and Plants

Solanaceous plants such as potato, tomato, petunia and nightshade, but the disease is economically important mainly in potato and tomato.

Host Parts Affected

All above-ground portions of the plant, but the disease is usually first recognized by its foliar symptoms.

Identification

The first symptoms of late blight on tomato leaves are irregularly shaped, water-soaked lesions, often with a lighter halo or ring around them (Figure 1); these lesions are typically found on the younger, more succulent leaves in the top portion of the plant canopy. During high humidity, white cottony growth may be visible on underside of the leaf (Figure 2). Spots are visible on both sides of the leaves. As the disease progresses, lesions enlarge causing leaves to brown, shrivel and die. Late blight can also attack tomato fruit in all stages of development. Rotted fruit are typically firm with greasy spots that eventually become leathery and chocolate brown in color (Figure 3); these spots can enlarge to the point of encompassing the entire fruit.

Late blight on leaves

Figure 1. Brown to gray lesions on the surface of leaves.

Dr. Lina Quesada, NCSU Vegetable Pathology Lab

Late blight on leaves

Figure 2. Dark water soaked lesions on the underside of leaves covered with white pathogen spores and mycelia.

Dr. Lina Quesada, NCSU Vegetable Pathology Lab

Late blight on fruit

Figure 3. Late blight on tomato fruit.

Dr. Lina Quesada, NCSU Vegetable Pathology Lab

Favorable Environmental Conditions for the Disease

The pathogen is favored by cool, wet weather; clouds protect the spores from exposure to UV radiation by the sun, and wet conditions allow the spores to infect when they land on leaves. Nights in the 50s / 60s and days in the 80s accompanied by rain, fog or heavy dew are ideal for late blight infection. Under these conditions, lesions may appear on leaves within 3-5 days of infection, followed by white cottony growth soon thereafter (Figure 2). This white cottony growth is a sign of rampant spore production. Although spores may also be produced on tomato fruit, they are more commonly produced on leaves. Spores can spread readily by irrigation, equipment, wind and rain and can be blown into neighboring fields within 5-10 miles or more, thus beginning another cycle of disease.

General Disease Management

  • Plant early in the season to escape high disease pressure
  • Do not allow water to remain on leaves for long periods of time
  • Scout plants often and remove infected plants, infected fruit, volunteers and weeds
  • Clean tools and equipment after leaving a field
  • Sign up to recieve alerts at the USAblight website
  • Plant resistant varieties when possible. New varieties resistant to some strains of P. infestans have recently been developed. A plum tomato variety named ‘Plum Regal’, as well as a new campari-type called ‘Mountain Magic’ and the large-fruited variety ‘Mountain Merit’ have resistance to some strains of late blight, as well as the variety ‘Defiant’ from Johnny's seed company.
  • Protect the crop with fungicides

Disease Control for Conventional Growers

Before disease occurs, apply fungicides at 7-10 day intervals. After disease is detected in your area, apply fungicides at 5-7 day intervals. Alternate products and tank mix mancozeb or chlorothalonil to avoid generating fungicide-resistant strains. For the latest fungicide recommendations for tomato late blight see the Southeastern US Vegetable Crop Handbook. Fungicide labels are legal documents, always read and follow fungicide labels.


Example products for tomato late blight control:

Active ingredient Example product PHI (day) Group
Fluopicolide Presidio 4FL 2 43
Oxathiapiprolin + chlorothalonil Orondis Opti A + B 0 U15 + M
Oxathiapiprolin + mandidpropamid Orondis Ultra A + B 1 U15 + 40
Propamocarb Previcur Flex 14 28
Cyazofamid Ranman 3.6SC 0 21
Ametoctradin + dimethomorph Zampro 4 45 + 40
Mandipropamid + difenoconazole Revus Top 1 40 + 3
Dimethomorph Acrobat 50WP 0 40
Cymoxanil Curzate 60DF 14 27
Mefenoxam + chlorothalonil Ridomil Gold Bravo 14 4 + M
Mancozeb + zoxamide Gavel 75WG 5 22 + M
Mancozeb Dithane 5 M
Chlorothalonil Bravo 0 M

For example, before disease you can use: Acrobat + Bravo, or Curzate + Bravo, or Gavel, every 7 days in rotation. After disease you can use: Revus Top + Bravo, or Presidio + Bravo, or Ridomil Gold Bravo every 5 days in rotation.

Disease Control for Organic Growers

Organic growers have less chemical options that are effective; the only OMRI labeled active ingredients that have decent efficacy against late blight are fixed copper formulations.

Disease Control for Home Gardeners

Products containing the active ingredients copper or chlorothalonil (the trade name of one product with chlorothalonil is known as ‘Daconil’) are the best and only effective products available to home gardeners. In addition, home gardeners should grow varieties with resistance if they are worried about late blight in future years because most chemicals available to the home gardener are not sufficient to control late blight once it appears. Once plants are infected in a home garden, there is little that can be done to protect them besides weekly fungicide sprays.

Useful Resources

Acknowledgements

This factsheet was prepared by the NCSU Vegetable Pathology Lab.

Author:

Assistant Professor, Vegetable Pathology
Plant Pathology

Publication date: Dec. 6, 2017

Recommendations for the use of agricultural chemicals are included in this publication as a convenience to the reader. The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsement by North Carolina Cooperative Extension nor discrimination against similar products or services not mentioned. Individuals who use agricultural chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage regulations and examine a current product label before applying any chemical. For assistance, contact your county Cooperative Extension agent.

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