Why are my tomato stems going black?

by Rolf Budd
(Norwich, UK)

Q. Last year my tomatoes suddenly started to get black leaves. The black colour spread to the stems and finally, the green tomatoes started to turn a brown rusty colour. I pulled all the plants out and disposed of them at the local recycling center. All the young tomato plants which I gave to friends survived OK ... the problem was just mine.

This year, the same thing happened. All the plants I gave to friends have no problem. I have had to remove and dispose of all of mine. The only difference is this year I grew them in a newly acquired greenhouse. None were started in the same soil as the infected ones from last year.

I am at a loss to what I can do. What is this disease? and how can I stop it in the future?

Yours,
Rolf Budd, a very frustrated amateur gardener


A. Rolf,

We are so sorry for your tomato problem and completely understand your frustration.

The culprit could be a number of different fungi or bacteria that overwinter in your garden soil. Each of the following diseases exhibits dark discoloration on tomato leaves, stems, and fruit.

• Early blight
  
• Late blight
  
• Fusarium wilt
  
• Verticillium wilt
  
• Alternaria stem canker
  
• Tomato bacterial canker
  
• Buckeye rot
  
• Bacterial pith necrosis
  

What to do
You have already done a terrific job of disposing of infected plants. There are at least 3 additional precautions you can take in order to have healthy tomato plants next year.

1. Rotate your crops. Avoid planting tomatoes in the same location as you have for the past 2 years. The fungi or bacteria that cause the above diseases overwinter in the soil. That's why they continue to bother your plants.

2. Solarize your soil. Treat infected soil to destroy as many of these organisms as possible. One way is to make your own solarizing tarp. The tarp spread out over the selected garden plot collects heat from the summer sun, raising soil temperature high enough to kill disease-spreading organisms. It is most effective when put into place during the hottest part of the summer. First till the selected garden area. Dig a low trench around its circumference. Soak the area thoroughly with a sprinkler. Cover the area with a sheet of clear plastic 1-4 mm thick, allowing plastic edges to extend into perimeter trench. Back fill the trench to secure plastic. Leave the plastic in place for 6-8 weeks, making sure to remove debris and excess water that pools on top so that the sun’s rays can penetrate the tarp. The sun heats to soil to destroy bacteria and other organisms. Solarizing infected soil can make it usable for the next season’s crop even though it means the selected area cannot be cultivated during the peak of the current summer.

3. Burn the affected area. During the fall, gather leaves, sticks, and straw over your tomato plot. Take legal steps to secure permission to burn this garden residue on your property. Have a garden hose handy in case sparks fly. On a still day (little wind), burn the pile. The heat can destroy or cripple fungi and bacteria in the soil, while the remnants of the fire add trace minerals to the soil.

One other route to take ...
If the above steps don't work, or if you'd like to take an aggressive step right away, consider installing a raised bed for your tomato plants. They are not expensive. In doing so, you can grow tomatoes in clean top soil, manure, and compost.

Finally ...
Make sure you use pure seed when you start your tomato seeds!

Good luck and happy gardening!
Your friends at Tomato Dirt