Stems Turning Black on Tomato Plants

Gardening is a joyful and maddening business for those involved. In a day, your beautifully healthy tomato plants can develop problems that look frightening, such as stems that suddenly turn black. Often, black stems are caused by fungus and can be treated, but the appearance of such discoloration can be unnerving to say the least. Before panicking, however, take a sample of the plant to your university extension to determine the cause of the problem -- a simple fungicide may be all that's required, but different fungi respond to different types of fungicide. Common causes of these symptoms include Alternaria stem canker, bacterial canker and both early and late blight.

Alternaria Stem Canker

Alternaria alternata, a fungus which invades tomatoes that have been planted in infected soil, causes Alternaria stem canker. The fungal spores land on the plant when old tomato debris is disturbed or works its way up from the soil itself. Dark brown to black cankers develop at or just above the soil line, eventually enlarging and girdling the plant, causing its death. Disease is often spread when rainwater or overhead irrigation upsets spores. There is no treatment for Alternaria stem canker, but Alternaria-resistant varieties of tomatoes are available to the home gardener.

Bacterial Canker

Bacterial canker can attack any part of a tomato plant, but it is not obvious on young plants. Older plants show variable signs of invasion by the bacteria Clavibacter michiganensis, depending on where the bacteria has invaded. Systemic infections tend to cause brown streaking and brittleness throughout the plant; however, localized infections result in dark brown to black lesions. These lesions can appear anywhere on the plant, including the stem, though leaves are the most common location. C. michiganensis can survive indefinitely in plant tissues, but thorough composting will destroy their feeding grounds. If you start your own seedlings, disinfect greenhouse equipment with a bleach solution and soak seeds in 130-degree Fahrenheit water for 25 minutes before planting. Make sure that tomato gardens are tilled thoroughly after harvest and again before planting in the spring to break up decaying plants.

Early Blight

Alternaria solani, a fungus that appears in the beginning of the growing season, causes early blight. It tends to spread in cool, humid weather after a prolonged rain. The fungus lies in wait in soils where infected tomatoes, potatoes or nightshades have been planted -- crop rotation is a useful tool in the battle against this organism. Early blight is characterized by small black or brown spots, no more than a half inch across located primarily on stems, leaves and fruits of older plants.It can be treated using topical applications of copper fungicide or Bacillus subtilis, a biological fungicide, according to package directions.

Late Blight

Late blight is a fungal disease common to tomato plants living in humid environments. It often appears in the early summer as conditions become ideal for the growth and spread of the fungus Phytophthora infestans -- humidity upwards of 90 percent and temperatures between 60 and 78 degrees can initiate infection in as little as 10 hours. Generally, purple-brown to black lesions form on leaves from small water-soaked spots and spread downward to stems. Fungicides should be sprayed before plants show signs for best management, but they can still be effective if applied once symptoms are present. Many strains of P. infestans are now resistant to mefenoxam, but other fungicides such as dimethomorph, mancozeb and pyraclostrobin still appear to be effective. Use resistant plants where they are available to reduce the need for these fungicides (and the potential that the fungus will develop a resistance).