Black Tomato Stems: Treatment of Tomato Stem Diseases in the Garden

One day your tomato plants are healthy and robust and the next day they are full of black spots on the stems of the tomato plants. What causes the black stems on tomatoes? If your tomato plant has black stems, don’t panic, it is most likely the result of a fungal disease of the tomato stems that can easily be treated with a fungicide.

Help, the stalk is turning black on my tomatoes!

There are a number of fungal diseases that cause tomato stalks to turn black. Among them is Alternaria stem canker , which is caused by the fungus Alternaria alternata . This fungus is already living in the soil where the spores landed on the tomato plant when the remains of old infected tomatoes were disturbed. Brown to black lesions develop on the soil line. These cankers eventually enlarge, causing the plant to die. In the case of Alternaria stem canker, there is unfortunately no treatment available. However, there are varieties of tomatoes that are resistant to Alternaria.
Bacterial canker is another tomato stem disease that causes black spots on the stems of tomato plants. It is easily visible on older plants
like brown stripes and dark lesions. Lesions can appear anywhere on the plant. The bacterium Clavibacter michiganensis is the culprit here and survives indefinitely in the plant tissue. To prevent infection, disinfect the equipment with a chlorine solution and soak the seeds in water at 130 degrees F. (54 C.) for 25 minutes before planting. Cultivate areas of the garden where tomatoes have been grown thoroughly to decompose and accelerate the decomposition of old plants.
Black stems on tomatoes can also be the result of late blight. Alternaria solani is the fungus responsible for this disease and spreads in cool, wet weather, often after a period of rain. The fungus grows in soil where infected tomatoes, potatoes or parasols have been grown. Symptoms include small black or brown spots less than half an inch wide. They may be found on the leaves or fruit, but more commonly on the stems. In this case, a topical application of copper fungicide or Bacillus subtilis should eliminate the infection and allow for future crop rotation.
Late blight is another fungal disease that thrives in humid climates. It usually appears in early summer when humidity rises to 90% and temperatures are around 15-25 C. Within 10 hours, purplish-brown to black lesions begin to splash on the leaves and spread along the stems. Fungicides are useful to manage the spread of this disease and use resistant plants whenever possible.

Prevention of tomato stem diseases

If your tomato plant has black stems, it may be too late or a simple application of fungus can remedy the problem. Ideally, the best plan is to plant hardy tomatoes, practice crop rotation, disinfect all equipment and avoid overcrowding to prevent disease from getting into your tomatoes.
It may also be helpful to remove the lower branches and leave the stem bare until the first set of flowers, then mulch around the plant after removing foliage to that point. The mulch can act as a barrier and remove the lower leaves so that spores sprayed by rain cannot infect the plant. In addition, water in the morning to give the foliage time to dry out and remove diseased leaves immediately.