What are the differences in early and late blight of tomatoes? Can you cut off affected areas of one or both to prolong the life of the plant? What can one do to prevent this disease, what can one do to stop the infection, what can one do to prevent the disease the next season?


1 Answer 1


Here's the biggest difference: Late blight is caused by the pathogen Phytophthora infestans, while early blight is caused by Alternaria solani.

Phytophthora infestans is an oomycete, a fungus-like eukaryotic microorganism. It is similar to Alternaria solani (a fungus) in that it is a localized disease, and doesn't spread internally. P. infestans is a more aggressive disease, and spreads much faster than A. solani. It also kills all infected material, causing dark, watery spots to form. these eventually become hard and brown.

Control of P. infestans is very difficult. There is no cure for badly infected plants.

  • If you notice signs of infection, remove all affected parts and destroy them.

  • If the disease spreads anyway, destroy the entire plant.

You can prevent it to some extent by:

  • Rotating your crops every year, remembering that both tomatoes and potatoes are susceptible to P. infestans.

  • Destroying all affected crops as soon as possible. Remove them from the site, and burn them, or place in air-tight bags and send to the landfill. Do not compost.

  • Growing resistant varieties like 'Defiant', 'Plum Regal', 'Mountain Magic 'and 'Mountain Merit'

Control of A. solani is easier, but it's best to prevent the disease, or catch it early. From: Sudden Outbreak of Yellow Leaves on Tomato (and Other) Plants:

  • Remove all leaves showing signs of early blight (yellowing, dry margin, large to small round dead spots.)

  • Do not touch the unaffected leaves with the removed portions, or your hands until they are thoroughly washed.

  • Spray with a copper fungicide (like BONIDE© Liquid Copper Fungicide). Apply once every 5 days, and after any rain. Continue for 3-4 weeks, or until the plant stops developing the symptoms.

To prevent future attacks, you can try to:

  • Minimize soil-foliage contact

  • Make sure the plants get good air circulation (including proper spacing)

  • When you water, try not to wet the foliage, or splash soil onto the leaf undersides. The fungus spreads faster in wet conditions.

  • To go along with that, an organic mulch will help keep soil from splashing, and is also useful for many other reasons.

  • Make sure the plants have a support to climb on.

  • Rotate the plantings each year, to stop pathogens from inhabiting the soil. Remember that potatoes are also vulnerable to early blight, so plan accordingly.

  • Perfect answer! There's something I wanted to add but, can't think right now! Horses rampaging....
    – stormy
    Commented Aug 6, 2014 at 2:24
  • @stormy curious... what were you going to add?
    – J. Musser
    Commented Aug 6, 2014 at 19:44
  • I will add that the best protection I have found for my plants it to use mulch to prevent splashing of dirt up onto the lower leaves. I just use grass clippings and make sure to keep up on it; adding thin layers on top of each other week after week.
    – Evil Elf
    Commented Apr 12, 2020 at 13:08

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