Blossom End Rot on Tomato (photo credit- Dr. Steven Bost, Univ. of Tennessee Plant Pathologist)

The bottom side of your tomato fruits appears to be rotting? The problem is not a disease, but a calcium disorder called “blossom end rot of tomatoes”. This nutritional malady also affects squash, pepper, eggplant, melon, and cucumber.

There are 3 causes for calcium deficiency: 1.) a deficiency of available calcium in your garden soil, 2.) inadequate soil moisture, or 3.) over-feeding plants with nitrogen fertilizer.  One, two, or all three causes may in play at the same time.

Watering during periods of extreme drought will feed more soil calcium to the plant.  Irrigate to make up for natural rainfall deficits. up to 1 ½ inches per week.  Mulching around vegetable plants with grass clippings, straw, or wood chips also helps. Be certain that lawn clippings have not been sprayed over the past month with herbicides which control dandelions and other broadleaf weeds.

Lime is a good source of calcium, but it takes several weeks for quick forms of lime to breakdown and become easily absorbed by roots. Dolomitic (gray) and hydrated lime may take 3-4 months to activate in a soil and are best applied over the winter months. Test your garden soil in autumn to determine when and how much lime to add. Use the correct amount as lime will raise the soil pH, making it less acidic.

High nitrogen levels may promote rapid vegetative growth, but the plants don’t absorb enough calcium to compensate, resulting in localized calcium deficiency in the fruit (or vegetable).

Finally, plant roots amy become injured from flooded soil or mechanical injury from tillage equipment such as hoes and rototillers. Root loss from poor aeration means the plant is not taking up enough water and nutrients.