Tomato stems, leaves, tops of fruit are going black

by Alan Smith
(Essex, UK)

Q. About 2 weeks ago, my tomato plants' stems and leaves started turning black. The fruit themselves is also turning black -- not the bottoms, but the tops. Plants are getting rapidly worse.

A. 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
Dispose 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