HERE ARE some answers to questions about fruit and vegetables plants readers have been writing in about:

Q: How should blueberries be pruned?

A: The blueberry needs no pruning until the third year after planting. In the third year, remove only the small spindly and twiggy growth near the base of the plant. Each year after the third year, remove dead and injured branches, fruiting branches close to the ground, spindly, bushy twigs on mature branches and old stems or parts low in vigor. Blueberry bushes which are pruned moderately each year produce larger berries but smaller yields than unpruned bushes. The heavier the pruning, the bigger the berries and the small the yield. Prune anytime during the dormant season, after most of the leaves have dropped in the fall.

Q: I want to plant three or four apple trees, and I want the kind that don't have to be sprayed all the time.Can you name a few?

A: At this time there is little hope of getting a good crop of apples without regular spraying.Miss one timely application and your whole crop can be lost to diseases or insects.

Q: What do you look for in determining when to harvest sweet corn?

A: The color of the silk, mainly. Harvest corn as soon as the silks have dried and turned brown. Also make sure the ear has a good tight shuck cover and feels firm near the tip. Sweet corn usually reaches maturity 15 to 20 days after tassling and silking.

Q: Can a gardener usually tell what insects are on his vegetables by the type of damage they do?

A: A Ragged or frayed leaves usually mean a worm or beetle is present. Honey dew, a sticky substance, indicates aphids. Vegetables that appear drought-stressed when soil moisture is ample may have spider mites hiding on the underside of the leaves. Marginal leaf burn and twisted buds indicate thrips in the folded leaves. Recognizing these types of damage can be important because often the gardener only sees the damage and not the insect.

Q: I have several kinds of vegetable seed left over. Can they be stored for use next spring?

A: The two most important factors shortening seed life are high moisture and high temperatures. A Mason jar with a new lid is an excellent moisture-proof container. Cloth and paper bags (even if plastic coated) and thin plastic bags are not moisture-proof but can be used to keep the seed separate when placed in a moisture-proof container. The best place to store them is in a refrigerator, next best a cool cellar.

Q: Neither my tomato or bell pepper plants had as much fruit on them this year as last. Any thoughts on what might cause this?

A: It probably was due to high temperatures in early June and early July. Temperatures of 95 degrees and above will severely limit fruit set on tomatoes, peppers and lima beans, and reduce the yields of snap beans and squash.

Q: What do I need to do grow good white potatoes? I've heard they have a lot of troubles.

A: Many diseases and several insects cause trouble with white potatoes. By using certified seed potatoes and resistant varieties and by spraying or dusting at 10 to 14 day intervals with zineb or maneb and Sevin most insects and diseases can be controlled. Harvest the potatoes after the vines have died. Store them where the temperature will be 35 to 40 degrees F. At lower temperatures they become sweet and at higher temperatures they will sprout.

Q: My snap bean leaves often turn yellow and dry up. It looks like dry weather, but they get plenty of water. Any ideas?

A: Sounds like spider mites. Corn, eggplants, strawberries, cucumbers and tomatoes are favorite hosts of spider mites. Hot dry weather together with repeated sprays of Sevin favor mite build up. A thorough spray of the miticide Kelthan to the leaves is suggested. Or, turn the garden hose on them and wash them off.