Early blight of tomato

Host and pathogen

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. The tissue around spots often turns yellow.
• Severely infected leaves turn brown and fall off, or dead, dried leaves may cling to the stem.

Stem

• 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 and black, with raised concentric ridges. They 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

• 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 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.