Low calcium causes rot in tomatoes

KENNEWICK -- A friend mentioned that she was putting her extra milk on the tomatoes in her garden to prevent blossom-end-rot.

She was aware that blossom-end-rot, contrary to its name is not a parasitic disease caused by a fungus or bacteria. It is a physiological disorder caused by a lack of adequate calcium in the developing fruit.

Tissues in developing fruit need greater quantities of calcium than other tissues. If there are insufficient quantities of calcium available, the tissues will break down creating a leathery spot on the blossom end of the fruit. Logically it would seem fertilizing with calcium would solve the problem, but in our region blossom-end-rot is seldom caused by a lack of calcium in the soil. It most often is caused by saturated soil conditions, drought stress or extreme fluctuations in soil moisture all of which impair the plant's ability to absorb calcium from the soil.

Blossom-end-rot also may be related to underdeveloped root systems early in the season, especially if there has been an abrupt change from cool spring weather to warmer summer conditions. Plants with growth stimulated by heavy applications of nitrogen also are prone to blossom-end-rot. Damage to roots from enthusiastic cultivation are another cause of blossom-end-rot.

Blossom-end-rot typically is noticed when the fruit is about one half to one third of its full size, starting out as a small water-soaked lesion on the bottom (the blossom end). As the fruit matures, the "rot" spot may stay relatively small or can become considerable in size. The lesions dry out and become a flat tan to black leathery area on the bottom of the fruit. Fruit with blossom-end-rot are safe to eat unless invaded by decay organisms. However, fruit with blossom-end-rot usually ripen early and don't have good flavor.

Knowing the causes of blossom-end-rot gives clues to avoid it:

-- First and foremost, the best way to avoid blossom-end-rot is to maintain even soil moisture, not letting plants fluctuate from very dry soil to very wet conditions. A good mulch can help you maintain soil moisture and suppress weed growth.

-- Don't plant your tomatoes until the soil warms up in the spring. Tomato roots don't grow well in cold soil and may not be able to absorb the needed amounts of calcium until after the limited root system develops. Early planting can result in "rot" on the first tomatoes of the season.

-- Don't promote excessive growth by over-fertilizing. Fertilize your plants judiciously.

-- While some sources will recommend foliar applications of calcium, these are seldom effective in mitigating blossom-end-rot problems. Other sources recommend adding lime or gypsum to the soil. This is helpful in areas where the soils are acidic or lack adequate calcium, but most soils in our region are alkaline and contain more than enough calcium.

By the way, milk is not a source of calcium for tomatoes.

* Marianne C. Ophardt is a horticulturist for the Washington State University Extension Office in Benton County.

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