USDA Agricultural Research Service (ARS) has developed a new white potato which, it says, out yeilds Russet Burbank, currently the top potato in the United States.

Named Lemhi Russet and created by ARS geneticist Joseph J. Pavek and plant pathologist Dennis L. Corsini of the University of Idaho Research and Extension Center, it is higher in vitamin C and more resistant to malformation. Pavek and Corsini also developed the popular Butte potato released in 1977.

The new variety produces attractive, russet-skinned tubers that are uniformly oblong in shape and blockier and larger than Russet Burbank potatoes. Test yields have been 34 percent higher in producing No. 1 tubers than for Russet Burbank. Lemhi Russet is also 6 percent higher in solids, 50 percent lower in sugars that build up in storage, and 22 percent higher in vitamin C. It has excellent resistance to both necrosis and common scab, and is equal to Russet Burbank in resistance to early blight and Verticillium wilt.

A new potato, named Explorer, that can be grown from seed is being introduced by Stokes Seeds of Buffalo, N.Y., for 1982. It has been possible to grow potatoes from seed but the seeds were unreliable, had poor germination and would only produce at 50 to 60 percent true to type.

After five years of intensive work, seed germination has been improved to 80 percent or better, according to Stokes. Explorer is the first open-pollinated potato from seed to be virtually free from wide variations, they say. Open-pollinated, seed-grown potatoes, such as Explorer, will mature at varying intervals, allowing the home gardener a continuous harvest of potatoes of one size or one harvest with potatoes in a variety of sizes. Explorer is an excellent choice for gardeners but a poor choice for commercial growers who need one size potato at harvest time, according to Stokes.

Potatoes are cultivated on more than 50 million acres throughout the world and yield more than 300 million tons of tubers yearly, according to Agricultural Research, a publication of USDA Science and Education Administration (SEA).

The United States. annually produces about 15 million tons of potatoes on 1.5 million acres with a crop value of over $1 billion. From its humble origin and despite two serious diseases, early and late blight, the potato has made its way to the forefront in the world market as a major food crop.

Potato breeders trying to develop new, improved varieties are hampered by the plant's obscure background, according to specialists. A better knowledge of the parentage of today's potato would help breeders increase its resistance to insects, diseases, nematodes and even drought and cold.

Late blight, the same disease that resulted in the Irish potato famine more than a century ago, is a major problem with growing potatoes. Research has shown that the disease can develop under the right combination of temperature and humidity conditions. For example, at a certain temperature range, if the humidity exceeds 90 percent for so many hours, it provides the right environment for development of the fungus.

In the past, the potatoes were sprayed every seven to 10 days beginning in late June until the end of August or into early September.

Now it is possible to eliminate two and sometimes three of the sprays and still control the disease successfully. It is done by spraying according to the conditions that are expected to develop rather than regularly every week or 10 days.

Q. How should onions from my garden be stored?

A. They will last indefinitely at around 33 degrees F in a dry area. Dig the onions when the tops fall over, let them dry for a few days and then store them.

Q. I picked my cucumbers while they were small this summer, but about one in three was so bitter it could only be eaten by slicing and salting. My soil is acid, was that the cause?

A. Usually fluctuations in temperature cause the bitter taste of cucumbers. Cucumber mosaic disease also can cause bitterness. The disease overwinters on pokeweed, milkweed and other weeds and in the spring is spread by aphids and other insects. Plant a variety resistant to the disease, such as Burpee's M&M hybrid.

Q. I've tried several times to grow parsley from seeds and the seeds never come up. What is the secret of getting them started?

A. Parsley seeds are slow to germinate. Plant them off to one side and keep them moist but not soggy wet. They can also be planted indoors in Jiffy-7s and when they sprout, plant pot and all outdoors. It should be possible to buy small plants from a garden center if you continue to be unsuccessful in starting them yourself.

Q. Should I cut off the tops of my asparagus or leave them on through the winter?

A. It is a good idea to leave the tops on all winter, cut them off in the spring and mix them with the soil through shallow cultivation.

Q. I dug a persimmon tree and planted it in my back yard three years ago. It has grown nicely but there is no sign of persimmons. What could be wrong?

A. Your tree may be a male or it could be a female without a male tree to provide pollen. Both bloom but only the female bears fruit, and then only after pollination.