Ask the gardeners

We have been raising disease-resistant tomatoes in our garden plot for the past 8 years. The plants grow nicely for a while, but then one plant develops wilt, turns lavender-brown, and we pull it up and toss it away. Then the other plants get the same thing. Also, many of our plants produce tomatoes that have a black patch formed on the bottoms of the fruit, thus ruining them. What's wrong?

Although many tomatoes are listed in catalogs as disease-resistant, they are not 100 percent so. Nonetheless, they still help ensure a good crop.

Rotate your crops so you don't grow tomatoes in the same spot each year.

Verticillium wilt remains in the soil. If you have too small a plot to rotate , you can try growing your tomatoes plants in half barrels, tubs, or other large containers, and then change the soil each year.

The black leathery patch on the bottoms of the fruit is called ''blossom end rot'' and is associated either with a lack of water while the tomatoes are forming fruit or a shortage of calcium.

Keep the plants watered and mulched to trap moisture. Also, add about a handful of limestone around each plant to supplement the supply of calcium.

I have some blue grapes, but last year some of them stayed red instead of getting a nice blue color. Was this caused by the weather or something I did wrong?

If grapes have a balance between leaf surface for good growth, and sunlight for ripening, they will usually color up normally. If you prune them too lightly , resulting in excessive leaf growth and number of bunches, they will not color up.

Too much rain or cloudy weather will also affect the color. Injury from powdery or downy mildew or insects can cause off-color fruit.

I've been told to add some extra potassium to the soil where I plan to grow carrots. I do not have a source of wood ashes, nor do I know where to get some greensand locally. What do you suggest I do to get extra potassium into the soil?

Carrots are not necessarily potash lovers. Any balanced fertilizer containing the big three nutrients - nitrogen, phosphorus, and potash (the percent indicated by numbers on the container in that order, N-P-K) is fine.

More important than potash (potassium, or the symbol K) is a good supply of organic matter in the soil and adequate water during the growing season.

Decomposed table scraps, grass clippings, sawdust, leaves, manure, and the like can be used in a compost pile to build up your organic ''soil bank.''

Greensand is an iron-potassium silicate that gives a green color to the minerals in which it occurs. It contains about 6 percent potash and is used by organic gardeners as a source of this nutrient.

If you have a question about your garden, inside or out, send it to the gardening page, The Christian Science Monitor, One Norway Street, Boston, Mass. 02115. Doc and Katy Abraham are nationally known horticulturists, authors of several books on gardening, and greenhouse operators for the past 25 years.