Can you tell me what to do about dry rot on my tomatoes? In years past, we have gotten it occasionally on our green tomatoes. This year, we are seeing it on our tomatoes once they have turned orange. We have lost many tomatoes thus far. We have been told many things — lack of water, too much water or a calcium deficiency. The person who said it's a calcium deficiency said to sprinkle epsom salts on the ground around the tomatoes. What is the cause? What can we do to prevent it? Any help you can give us would be greatly appreciated.

I imagine you're describing the condition known as blossom-end rot. This starts as a black or brown spot at the blossom end of the fruit and slowly enlarges into a sunken, leathery lesion. It can affect fruits at any stage of development. Blossom-end rot looks like a disease but is actually a physiological disorder caused by a calcium deficiency within the fruit. This deficiency may result from many conditions, but the most frequent cause is due to uneven levels of soil moisture, which result in water stress. Once the damage occurs, secondary decay organisms may infect the lesion and cause rotting.

The easiest way to overcome the problem is to provide plants with a uniform supply of water. One deep soaking per week for a plant mulched with 2 or 3 inches of organic matter will usually be sufficient. Unmulched plants will need to be watered more frequently, especially during extended hot, dry weather. The goal is to moisten the root zone to a depth of 6 to 8 inches with each watering, and to allow the top 3 or 4 inches of soil to dry before watering again. Just as lack of water may cause blossom end rot, so can too much water. Always grow tomatoes in well-aerated soils, and don't water so frequently that the soil stays waterlogged.

Another factor that may play a role in blossom-end rot is acidic soil conditions. Low pH limits the amount of calcium available to plants. Tomatoes grow best in soils having a pH between 6.5 to 6.8. Adding lime will correct acidic conditions, but a soil test should be made first. Any adjustments made at this time will be too late to affect this year's crop. Avoid excessive levels of nitrogen fertilizer, which aggravates this condition. Plants that are staked and pruned are more susceptible to this condition than those grown in cages without pruning or allowed to sprawl.

Pick off and discard any fruits with blossom-end rot so that unaffected fruits will have a better chance of developing normally. A light side dressing of calcium nitrate fertilizer, or a foliar spray with a calcium-rich fertilizer, may benefit unaffected fruits, though studies testing the efficacy of these treatments have produced mixed results. Be sure to follow the manufacturer's recommendations. Avoid cultivating deeply near the roots of your plants. This can damage roots and limit the plants' ability to absorb water, calcium and other nutrients.

Epsom salt is magnesium sulphate and is a source of magnesium, not calcium. The best natural sources for calcium are limestone, wood ashes, rock phosphate and bone meal. Limestone and wood ashes neutralize soil acidity. These will help if your soil is too acidic but could be harmful if your soil is alkaline. This is why a soil test is so important. If you have alkaline soil and a calcium deficiency, apply gypsum instead of lime.

Write to Chip Tynan at chip.tynan@mobot.org or Horticultural Answer Service, Department PD, P.O. Box 299, St. Louis, Mo. 63166. Check his blog at mobot.org/gardeninghelp/hilight.asp