Tomato stem canker is caused by the fungus Didymella lycopersici. Spores attack the plants via wounds, causing the stem to rot. Affected plants wilt and die, so it’s important to identify the problem quickly and remove infected plants before the problem can spread.
If the stems of your tomato plants (and not the leaves) are going mouldy, then the cause may be a fungal disease called stem canker. This is often the result of fungal spores entering a wound in the stem, possibly where side shoots have been removed.
Tomato plants with canker develop dark brown sunken spots that spread to girdle the stem. Usually found on the lower part of stems, as the disease progresses, secondary cankers will appear higher up. Eventually the plant withers, the fruits shrivel and turn black, and the plant dies.
The best way to treat tomato stem canker is to remove and dispose of the entire plant immediately. Don’t add it to your compost as the fungus can survive for a year in soil and affected plant debris.
Fungal problems tend to be exacerbated by humid conditions, like those found in closely stocked greenhouses. A good preventative measure would be to increase the ventilation by leaving the door and windows open and watering in the evening rather than the morning to reduce humidity.
Careful watering at the base of plants will prevent splashes dispersing spores to other plants. And always buy good quality seeds from a reputable source. Head to our tomatoes hub page for more growing and care information.