By Beth Berlin
University of Minnesota Extension
It was a bit of a tough year for tomatoes. The early part of the growing season was quite wet. Although this meant gardeners didn’t have to take time to water, the rain was in excess at some points and created conditions that caused problems for tomatoes. Blossom-End Rot is one of the issues some gardeners are seeing this year.
Blossom-end rot symptoms include tan or black spots at the blossom end of the tomato. It usually starts as a small water-soaked area at the blossom end that may appear while the fruit is green or during ripening. Eventually the lesion enlarges and becomes sunken in, turns black in color, and even has a leathery appearance. In severe cases, the entire lower half of the fruit may become flat or concave or the fruit may not reach full size due to the rot. This may then lead to secondary decay bacteria, resulting in rotting of the fruit and rendering it unusable. The most common time to see blossom-end rot is on the first fruit of the season; however it can appear at any stage of development in the fruit.
Contrary to what many would believe, blossom-end rot is not a disease, but instead 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. In most situations there actually isn’t a lack of calcium in the soil; instead the calcium is unavailable for uptake by the plant. This may be caused by several factors, including heavy application of nitrogen, drought stress, fluctuations in moisture and temperature, and root damage due to localized cultivation.
Container gardeners often experience blossom-end rot because these plants have a restricted root area and are more vulnerable to stress caused by rapid changes in temperature and moisture. Container plants tend to dry out more easily than plants in the ground and require a careful uniform supply of moisture.
Minimizing blossom-end rot can be done by maintaining a uniform supply of moisture through regular watering. In general, plants need about one inch of moisture a week for proper growth and development, and yet it is critical to not overwater. Gardeners can apply two to three inches of organic mulch to help keep the soil temperature and moisture level uniform. Avoid heavy applications of nitrogen fertilizer; this will lead to lush, rapid growth, yet makes the fruit more susceptible 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 damaging the tomato’s roots from deep cultivation within one foot of the plant; to minimize this, simply cultivate to only one inch depth near the plant.
For more information on blossom-end rot or other tomato issues visit http://www.extension.umn.edu/garden/diagnose/plant/vegetable/tomato/.