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I was so proud of my tomato plants (grown from seeds), but now I'm worried that they have died...

Instead of having a usual French summer (with 30 C at least and dry weather) it is raining a lot and it is cold (18-20 C). Up to now (until last week or so) I was not worried because the tomatoes did well, a lot of flowers and some fruits were appearing. Then suddenly the stems became black, not only black dots and yellow leaves as described here but real areas. In addition the leaves have dried out.

Is there anything I can do? If not, what could I have done to avoid this situation beforehand? ("In gardening there is always a next year")

Here are some pictures (click to enlarge).

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My guess would be "late blight". –  Ed Staub Jul 12 '12 at 20:04
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2 Answers

up vote 3 down vote accepted

It's very sad that the list of things that attack tomatoes is so long. This could be the Didymella stem rot fungus.

It's likely you do not have access to the controlled fungicides that greenhouse growers use so control is either cutting off the diseased areas or lifting and destroying severely diseased plants.

Why your plants get infected is a longer term issue that is tied is with having a source of infection (usually dormant spores in the soil) and the right conditions (plant stress + cold + wet).

Some ideas for control:

  • use sterilized potting mix and plant in pots or containers (removes source of infection)
  • plant tomatoes in a different location every year (might help avoid a source of infection)
  • improve soil conditions with more organic matter
  • plant resistant varieties
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Removing the infected plants would mean to remove them all... :( . I think I will wait for the fungus to come close to the fruits, then take off the fruits and let them ripe inside in a dark place. –  Patrick B. Jul 13 '12 at 18:34
    
That will work! –  kevinsky Jul 13 '12 at 19:15
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Regardless of which species of fungus it is, a helpful preventative is to mulch your plants. Mulch reduces splash-up of fungal spores from the ground onto the stem and leaves, by absorbing the force of raindrops as they hit the ground.

Use mulch sparingly until plants are well-established - it can harbor insects that will go after seedlings.

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Maybe it was coincidence: I mulched them with oak-leaves (it is not yet leave-mold) I kept from last year to exactly do that. IIRC, only after doing so the stems went black. Could the fungus have come out of the leaves? –  Patrick B. Jul 13 '12 at 18:32
    
Yes, it could have. I generally avoid mulching with uncomposted leaves, because they suck up nitrogen out of the soil, they mat in a way that's good for fungus and bad for soil aeration, and there's usually a lot of splash-up from the intact dead leaf surface. –  Ed Staub Jul 14 '12 at 3:48
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