Early blight of tomato

Quick facts

  • Early blight is one of the most common tomato diseases, occurring nearly every season wherever tomatoes are grown.
  • It affects leaves, fruits and stems and can be severely yield limiting when susceptible cultivars are used and weather is favorable.
  • Severe defoliation can occur and result in sunscald on the fruit.
  • Early blight is common in both field and high tunnel tomato production in Minnesota.

Host and pathogen

Tomato leaf with spots that are round, brown and up to half inch in diameter. Larger spots have target like concentric rings and tissue around spots is yellow
Leaf spot symptoms of early blight on tomato

Early blight can be caused by two different closely related fungi, Alternaria tomatophila and Alternaria solani. Alternaria tomatophila is more virulent on tomato than A. solani, so in regions where A. tomatophila is found, it is the primary cause of early blight on tomato. However, if A. tomatophila is absent, A. solani will cause early blight on tomato. Both pathogens can also infect potato, although A. solani is more likely to cause potato early blight than A. tomatophila. Both pathogens can also infect eggplant and several Solanaceous weeds including black nightshade (Solanum ptycanthum), and hairy nightshade (Solanum physalifolium)

Identification

Signs and symptoms

Leaves

  • Initially small dark spots form on older foliage near the ground
  • Leaf spots are round, brown and can grow up to half inch in diameter.
  • Larger spots have target like concentric rings and tissue around spots often turns yellow
  • Severely infected leaves turn brown and fall off, or dead, dried leaves may cling to the stem

Stem

Stem with oval to irregular, dry brown areas with dark brown concentric rings
Early blight stem infection
  • Seedling stems are infected at or just above the soil line. The stem turns brown, sunken and dry (collar rot). If the infection girdles the stem, the seedling wilts and dies.
  • Stem infections on older plants are oval to irregular, dry brown areas with dark brown concentric rings.

Fruit

  • Fruit can be infected at any stage of maturity
  • Fruit spots are leathery, black, with raised concentric ridges and generally occur near the stem
  • Infected fruit may drop from the plant

Environment

  • Disease develops at moderate to warm (59 to 80 F) temperatures; 82 to 86 F optimum
  • Rainy weather or heavy dew, 90% humidity or greater

Biology and disease cycle

Tomato fruit with spots that are leathery, black, with raised concentric ridges near the stem
Early blight fruit rot symptoms

Alternaria tomatophila and A. solani overwinter in infected plant debris and soil in Minnesota. The pathogen also survives on tomato seed or may be introduced on tomato transplants. Lower leaves become infected when in contact with contaminated soil, either through direct contact or through rain-splashed soil. Spores can germinate between 47 and 90 F and need free water or humidity of 90% or greater. Spores infect plants and form leaf spots as small as 1/8 inch diameter and in as little as five days. Spores can be spread throughout a field by wind, human contact or equipment, resulting in many reinfection opportunities throughout a growing season.

Management

Resistant cultivars.

There are many resistant tomato cultivars available, often designated with an "EB" in seed catalogs. There is also an extensive list of resistant cultivars on Cornell University's vegetable pathology website. Resistant varieties are not immune to early blight. Rather moderate levels of resistance to either leaf infection, stem infection or both are present.

A few common cultivars with early blight resistance include:

  • Iron Lady
  • Mountain Supreme
  • Mountain Magic
  • Defiant PhR
  • Jasper
  • Juliet
  • Verona

Cultural control

  • Use pathogen-free seed, or collect seed only from disease-free plants.
  • Rotate out of tomatoes and related crops for at least two years.
  • Control susceptible weeds such as black nightshade and hairy nightshade, and volunteer tomato plants throughout the rotation.
  • Fertilize properly to maintain vigorous plant growth. Particularly, do not over-fertilize with potassium and maintain adequate levels of both nitrogen and phosphorus.
  • Avoid working in plants when they are wet from rain, irrigation, or dew.
  • Use drip irrigation instead of overhead irrigation to keep foliage dry.
  • Stake the plants to increase airflow around the plant and facilitate drying. Staking will also reduce contact between the leaves and spore-contaminated soil.
  • Apply plastic or organic mulch to reduce humidity and provide a barrier between contaminated soil and leaves.
  • In the fall, remove or bury infected plants to reduce the likelihood of the pathogen surviving to the following year.
  • For greenhouse production, early blight has been reduced by as much as 50% by covering houses with UV-absorbing vinyl film.

Chemical control

Below is a partial list of fungicides available for control of early blight on tomato. Applications should be made when environmental conditions favor disease to be the most effective and repeated according to label instructions.

It is important to alternate between different chemical families to avoid the development of pathogen insensitivity to particular active ingredients. Some insensitivity to the chemical family 11 has become more common in some areas, so particular care should be taken to rotate these with other chemical families. Also, if insensitivity is already present in a given field population of early blight, fungicides in chemical family 11 will not provide good control.

 

 

Common fungicides for control of early blight on tomato.

Active ingredient Common product names Chemical family Comments
Penthiopyrad Fontelis 7 Very good
Boscalid Endura, Lance WDG 7 Very good
Pyraclostrobin Cabrio 11 Very good, but insensitivity is becoming more common
Fenamidone Reason 11 Very good, but insensitivity is becoming more common
Azoxystrobin Quadris 11 Very good, but insensitivity is becoming more common
Cymoxanil and Famoxadone Tanos 27 and 11, respectively Good, but insensitivity is becoming more common
Fluxapyroxad and Pyraclostrobin Priaxor 7 and 11, respectively Good, but insensitivity is becoming more common
Pyrimethanil Scala 9 Good
Difenoconazole and Cyprodinil Inspire Super 3 and 9, respectively Good
Mancozeb Dithane, Manzate, Penncozeb M Good
Mancozeb and Zoxamide Gavel M and 22, respectively Good
Difenoconazole and Mandipropamid Revus Top 3 and 40, respectively Good
Cyprodinil and Fludioxonil Switch 9 and 12, respectively Good
Chlorothalonil Bravo, Echo, Equus M Fair
Copper (copper hydroxide, copper oxychloride, etc.) Kocide 2000, Champ Formula 2, Nu-Cop 50DF, C-O-C-S WDG M Fair. Some copper products OMRI listed � copper is best option for organic production

CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Remember, the label is the law.

When treating fruits or vegetables, make sure the plant you wish to treat is listed on the label of the pesticide you intend to use. Also be sure to observe the number of days between pesticide application and when you can harvest your crop.

Anna Johnson, Michelle Grabowski, Extension educator and Angela Orshinsky, Extension plant pathologist

Reviewed in 2018

Share this page:

© 2018 Regents of the University of Minnesota. All rights reserved. The University of Minnesota is an equal opportunity educator and employer.