Early Blight of Tomato

Pest Management Fact Sheet #5087

Originally developed by Dr. Bruce Watt, Extension Plant Pathologist, University of Maine Cooperative Extension
Updated and revised by Dr. Alicyn Smart, Assistant Extension Professor & Extension Plant Pathologist and Katherine Ashley, Graduate Student, University of Maine.

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Other Name: Alternaria

Pathogen: Alternaria solani, Alternaria tomatophilia

Introduction

Early blight of tomato, caused by the fungus Alternaria solani, is perhaps the most common foliar disease of tomatoes in the Northeast and is also common on potatoes and other nightshades. This disease causes direct losses by the infection of fruits and indirect losses by infection of stems, leaves, and reduction of plant vigor.

Alternaria spores begin to germinate within 2 hours over a wide range of temperatures but at the optimal range of 80°F to 85°F it can only take a half hour. Another 3 to 12 hours are required for the fungus to penetrate the plant depending on the temperature. After penetration, lesions may form within 2-3 days, or the infection can remain dormant awaiting proper environmental conditions (  and extended periods of wetness). The optimal temperature range for sporulation is about coupled with abundant moisture (as provided by rain, mist, fog, dew, or irrigation) is present. Infections are most prevalent on poorly nourished or otherwise stressed plants.

If the infected plants are not removed at the end of the season, the fungus can spend the winter in infected plant debris in or on the soil where it can survive at least one or perhaps several years. New spores are produced the following season and can be transported by water, wind, insects, animals, and machinery. Once the initial infections have occurred, they become the most important source of new spore production and are responsible for rapid disease spread. Infected seed can also be the cause of diseased plants.

Host Plants

  • Most frequently on tomatoes and potatoes
  • Eggplant
  • Solanaceous weeds (black nightshade- Solanum pytcanthum, hairy nightshade- Solanum physalifolium)

Symptoms

Fruit: Fruit rot is a common symptom. Spots are black and can have concentric circles.

Stems: Symptoms include collar rot, stem cankers, and brown lesions with dark concentric circles.

Leaves: Early blight produces a wide range of symptoms at all stages of plant growth. It can cause damping-off, collar rot, stem cankers, leaf blight, and fruit rot. The classic symptoms occur on the leaves where circular lesions up to ½ ″ in diameter form. Within these lesions dark, concentric circles can be seen. The leaf blight phase usually begins on the lower, older leaves and progresses up the plant. Infected leaves eventually wither, die, and fall from the plant.

Control

  1. Use only clean seed saved from disease-free plants.
  2. Remove and destroy crop residue at the end of the season. Where this is not practical, plow residue into the soil to promote breakdown by soil microorganisms and to physically remove the spore source from the soil surface.
  3. Practice crop rotation to non-susceptible crops for 3 years. Be sure to control volunteers and susceptible weeds.
  4. Promote good air circulation by proper spacing of plants.
  5. Orient rows in the direction of prevailing winds, avoid shaded areas and avoid wind barriers.
  6. Irrigate early in the day to promote rapid drying of foliage.
  7. Healthy plants with adequate nutrition are less susceptible to the disease.
  8. Minimize plant injury and the spread of spores by controlling insect feeding.
  9. Hand-picking diseased foliage may slow the rate of disease spread but should not be relied on for control. Do not work in a wet garden.
  10. Use resistant or tolerant varieties such as Mountain Magic, Juliet, Defiant PhR, Verona, or Jasper. Check seed catalogs or packets for the “EB” designation, indicating resistance to early blight.
  11. The preventative fungicide chlorothalonil (Bravo) used on a seven to ten-day schedule gives effective control.

Fungicides for Early Blight Control

Fungicide Typical Application Interval Examples of Trade Names
azoxystrobin, pyraclostrobin 7 to 14 days Quadris, Amistar, Cabrio EG
Bacillus subtilis 5 to 7 days Seranade
chlorothalonil 7 to 14 days Daconil, Bravo, Echo, Fungonil and others
copper products 7 to 14 days Bordeaux Mixture, Kocide, Tenn-Cop,Liqui-cop,  Basicop, Camelot
hydrogen dioxide Commercial only, see label Oxidate
mancozeb and maneb 7 to 14 days Dithane, Penncozeb, Manex
potassium bicarbonate 5-14 days as needed Armicarb 100
ziram 7 to 14 days Ziram

 


When Using Pesticides

ALWAYS FOLLOW LABEL DIRECTIONS!

Pest Management Unit
Cooperative Extension Diagnostic and Research Laboratory
17 Godfrey Drive, Orono, ME 04473-1295
1.800.287.0279 (in Maine)


Information in this publication is provided purely for educational purposes. No responsibility is assumed for any problems associated with the use of products or services mentioned. No endorsement of products or companies is intended, nor is criticism of unnamed products or companies implied.

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