Q. Believe it or not, I have small tomatoes growing on my potato vines. Do you want to come and see them?

A.They are potato seed pods, not tomatoes. After the potato plant blooms, the seed pods form. The cool, damp weather early this summer favored seed-pod development.

Q. My crape myrtle looks awful: The leaves are stunted, the flower buds do not develop normally and most fall off before opening. There is a whitish substance on the leaves.

A. Many varieties of crape myrtle, except some of the newer ones, are susceptible to a fungus disease called powdery mildew. It has been bad this year. Once the plant is badly infected, no amount of spraying will restore it to normal during the season.

When the leaf buds open in the spring, check on them for signs of the disease. If you find them, spray weekly with ActiDione PM, or very two weeks with Benlate. Directions should be followed closely. The strain of mildew that affects crape myrtle does not attack roses, lilacs or plants other than crape myrtle.

Q. I sprayed my string beans with Black Leaf 40 on account of beetles, and the next day the plants were badly wilted. What could have happened? Last spring I used the sprayer to kill poison ivy but washed it out thoroughly two or three times.

A. After using a sprayer for weed control, it is difficult, very difficult, to clean it out sufficiently to make it safe to use except for weeds.

To make the sprayer safe, clean it with a solution of ammonia, one pint per ten gallons of hot water. Then try it experimentally on one or two plants before using it generally.

Q. The leaves of my vinca are turning brown and the stems are rotting. Can you tell me the cause and cure?

A. It's due to a fungus disease which has been worse than usual because of the cool, wet spring weather. The first symptom is when young shoot tops turn dark brown, followed by wilting and die-back of the stems to the surface of the soil. Most of the affected stems are blackened and soft.

Remove badly infested plants and then spray with fermate of maneb. Make three applications at 10-day intervals. Directions on the label should be followed closely. Hot, dry summer weather usually lessens the need for spraying.

Q. Many of the leaves of my tulip tree are turning yellow and dropping. The same thing happened last year and the year before that. Can you tell me what to do bout it?

A. During hot, dry summer weather some of the leaves of the tulip tree (Liriodendron tulipifera) turn yellow and drop. It's believed that this is due to a moisture deficiency in the soil made worse by the hot sun. It's a climatic and not a fungus condition.

Q. Can impatiens be grown indoors in winter as house plants?

A. It can be done, but they're not very good plants for that purpose. To bloom, they require a much higher humidity than can be found in the average home. If you want to try them anyway, take cuttings in August and September and root them. Leave the potted cuttings outdoors in semi-shade until late summer.

Q. I planted healthy red geraniums outdoors in the ground in early May. They were beautiful, full of blooms, until two weeks ago when the leaves started to turn yellow, and now they're decaying at the roots.

A. Most likely it's due to too much water or poor drainage. Geraniums can be seriously damaged if their roots stand in water for long. It prevents free movement of air through the soil and the roots don't get the oxygen they require to function. On the other hand, the soil in which they are growing should not be permitted to dry out completely. When watering, apply water to the soil and try to keep it off the foliage and flowers. This is especially important during hot and humid weather.

It's important not to sprinkle them lightly with the garden hose. When you water, do it thoroughly, applying enough to moisten the soil to a depth of six inches. Then wait until the soil is dry before watering again.

Q. I have a dogwood I started from seed five years ago. It has grown beautifully but has never bloomed. Other dogwoods nearby, much smaller than mine, bloom every year. Is it because it's a male and can't bloom?

A. Like animals, during their life cycle, plants pass through the stages of juvenile development, maturity and old age. They do not produce flowers or seed while in the juvenile stage. Even after reaching sexual maturity, they may pass through a period of adolescence before settling down to producing seed for reproduction.

The flowering dogwood bears perfect flowers. Each flower has both stamens (male) and pistel (female). Generous fertilization to speed growth usually has a tendency to retard flowering.