Q. Why do my tomatoes have a single brown spot on the bottom of the fruit? — Donna F., Atascadero

A. Your tomatoes most likely have blossom-end rot. Blossom-end rot starts out as a small light brown or black spot at the blossomend of immature fruit. The affected area gradually expands into a sunken brown or black lesion as the fruit ripens. A pathogen or pest does not cause this disease, nor does it spread from plant to plant in your garden.

Blossom-end rot results from a low level of calcium in the fruit and water balance in the plant. It is aggravated by high salt content or low soil moisture and is more common in sandier soils. It is especially prevalent when rapidly growing plants are exposed suddenly to a period of drought.

Amending your soil with compost, when planting, is the best way to improve your soil structure for holding oxygen, water and nutrients. To reduce the chance of rot, monitor soil moisture to make sure that the root zone neither dries out nor remains saturated. A good drip system, watering your vegetables in the mornings, will help to ensure adequate soil moisture levels.

Mulching around your plants will also help with moisture retention. Follow recommended rates for fertilizers as over fertilizing can cause elevated salt levels in the soil. Use of fertilizers low in nitrogen, but high in phosphorus, such as 4-12-4 or 5-10-10, will help correct the problem of blossom-end rot.

Although differences exist among varieties with respect to susceptibility to blossomend rot, there are no varieties developed as of yet to have shown a useful resistance.

There is no substitute for proper treatment of the soil, to maintain adequate supplies of water and calcium. If your fruits have blossom-end rot, pick, discard in your compost pile and wait for the next fruit to set. With good moisture control and proper fertilization, you will soon be enjoying the fruits of your labor — a sweet, ripe, juicy tomato!