Tricounty News

What causes blossom-end rot in tomatoes?

By Janelle Kuechle,

U of M Extension

While we are all anticipating the first tomatoes to ripen in the garden, some are having problems with tomato blossom-end rot.Ê Affected fruit is easily spotted and will have a tan to black spot at the blossom end of the fruit.

Blossom-end rot usually begins as a small water-soaked area at the blossom end of the fruit. This may appear while the fruit is green or during ripening. As this lesion develops, it enlarges, becomes sunken and turns black and leathery. In severe cases, the entire lower half of the fruit may become flat or concave. Secondary decay bacteria often invade this lesion resulting in rotting of the fruit rendering it unusable. Blossom-end rot can appear at any stage of development in the fruit, but it is most commonly seen in the first fruit of the season.

Blossom-end rot is not a disease, but is a physiological disorder associated with a low concentration of calcium in the fruit. Calcium is required in relatively large concentrations for normal cell growth. When a rapidly growing fruit is deprived of necessary calcium, the tissue breaks down, leaving the characteristic dry, sunken lesion at the blossom end.Ê In most cases, blossom-end rot is normally not caused by a lack of calcium in the soil, but rather by some factor that makes the calcium unavailable to the plant. These factors include drought stress, fluctuations in moisture and temperature, heavy applications of nitrogen fertilizer and root pruning due to cultivation. Blossom-end rot is also very common in container grown plants. This is because these plants have a restricted root area and are therefore more vulnerable to stress caused by rapid changes in temperature and moisture. They tend to dry out much more easily than plants in the ground and require a careful uniform supply of moisture.

Maintaining a uniform supply of moisture through regular watering can minimize blossom-end rot. Plants generally need about one inch of moisture a week for proper growth and development and it is important that the soil does not become excessively dry between watering. Applying two to three inches of organic mulch will help keep the soil temperature and moisture level uniform. Avoid heavy applications of nitrogen fertilizer as rapid and luxuriant growth also predisposes the fruit to blossom-end rot, especially during periods of dry, hot weather. Ammonium-based nitrogen may increase blossom-end rot as excess ammonium ions reduce calcium uptake. Avoid root pruning caused by deep cultivation within a foot of the plants. The soil around tomato plants should never be hoed or cultivated deeper than one inch to avoid root injury. Keeping all of these tips in mind will help to ensure a plentiful crop of tomatoes each year.