Prevention And Treatment of Common Tomato Diseases

Tomato is a vegetable commonly grown by peasant farmers in Africa. The fruit called berry when ripe can be eaten raw, used for soup or stew preparation or in preparing vegetable salad and other food. Common tomato diseases;

READ : Growing tomatoes plant from a tomato slice

Tomato Diseases

  • Late Blight
  • Early Blight
  • Septoria leaf spot

Tomato Diseases – Late Blight

  • Caused by Phytophthora infestans

Survival of the late blight pathogen

  • Only survives in living host tissues
  • When the host dies, the late blight pathogen dies
  • How does it survive from one season to another?
  • In seed tubers, over-wintering volunteers, cull piles
  • Seed probably most important
  • Difficult to see; seed not washed
  • Late blight spreads during seed cutting

Late Blight Sources

  • Infected Potato, Tomato, Petunia, Hairy nightshade
  • Family: Solanaceae
  • Home gardens can be source of infective propagules that threaten other small and large plantings
  • In some parts of the world, the pathogen can survive outside of the host as oospores – special structures that survive in soil for long periods of time in Europe, Mexico
  • Not in US or Canada (at least, not that we know of!)

How the Pathogen Spreads

  • The late blight pathogen produces spores (infective propagules) during cool, wet weather
  • Spores are microscopic and lemon-shaped
  • Moved by wind, especially during thunderstorms
  • Requires 12 hrs leaf wetness to infect (dew, mist, fog, rain)
  • 5-7 days from infection to symptoms and production of new spores
  • Fragile, killed by hot dry weather and UV
  • Many cycles of spore production
  • Spores can fall to soil and produce swimming zoospores that infect tubers

Late blight history in US (and probably Canada)

  • Sporadic since 1840s
  • 1992 introduction of new strains from Mexico
  • In 1995, a more aggressive strain was identified
  • causes more tuber rot, can tolerate higher temperatures
  • Even more new strains identified in 2009 and 2010
  • The pathogen is extremely variable

What happened in 2009?

  • Huge epidemic of late blight in eastern US
  • Source was late-blight infested tomatoes that were distributed to big box retail stores
  • Weather was conducive
  • Late blight spread rapidly in eastern US
  • Both tomatoes and potatoes hit hard
  • Home gardens and organic growers
  • Later spread to the Midwest

Managing Late Blight

  • Eliminate initial inoculum
  • No cull piles
  • No volunteers
  • Use certified disease-free tuber seed, but also check the tubers for symptoms before planting
  • Purchase tomato seedlings from reputable local growers; and do not purchase any that appear unhealthy
  • Consider starting your favorite tomato varieties from seed
  • Keep foliage as dry as possible
  • Improve air flow by not planting too densely
  • Stake plants
  • Use mulch


  • Monitor gardens early and often for late blight
  • Remove and destroy infected plants as soon as they are observed during the growing season
  • Seal in plastic bags before discarding in trash, or
  • Burn the debris right away
  • Clean up the garden at the end of the season – remove all host plant parts and destroy
  • Do not compost late-bight infested plant material

Fungicides for Late Blight

  • Protect healthy plants – apply fungicides on a regular basis if late blight is in the area
  • Chlorothalonil (such as Bravo or Ortho® Max Garden Disease Control)
  • Copper-based fungicides (such as Bordeaux mixture)
  • Some are approved for use in organic production
  • Every 5-7 days during cool, wet weather
  • Must be applied before symptoms are observed or as soon after as possible (these are not curative)
  • Not all fungicides have activity against the late blight pathogen
  • Use only fungicides labeled for late blight control on edible crops

What About Using Resistant varieties?

  • Some resistant tomato varieties are listed in seed catalogs – these may provide some degree of resistance or tolerance, but may not be reliable (too many strains of the pathogen)
  • No commercial potato cultivars with good resistance are available
  • We can’t depend solely on resistance for control – the late blight pathogen is too variable

Tomato Diseases – Early Blight

  • Early blight (Alternaria) symptoms on tomato leaf
  • Note bullseye pattern of concentric rings –characteristic of early blight
  • While early blight is a damaging tomato disease, it is not as devastating as late blight
  • Control of early blight on tomatoes is essentially the same as for late blight

Tomato Diseases – Septoria Leaf Spot

  • Typically has small “shot” size necrotic areas
  • Not nearly as destructive as late blight
  • Controls are the same as for early and late blight


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