Two good books of considerable interest and importance to gardeners have been published -- one about plant diseases and what to do about them and the other about how they grow tomatoes in England.

For over 25 years, "Westcott's Plant Disease Handbook" has been the authoritative guide in its field. The Fourth Edition, revised by R. Kenneth Horst, PhD, (published by Van Nostrand Reinhold, 832 pages, well illustrated, $32.50), is now available.

Using the original format, the handbook offers easily accessible data on hundreds of trees, shrubs, vines, flowers, and vegetables likely to be grown in home gardens.

It includes research findings about viruses and/or virus-like agents, all recently discovered diseases and those known diseases now found on new hosts.

Each plant entry shows the type of disease to which the plant is susceptible, the scientific name of the organisms causing these diseases and the states where they have been reported. There is a specific description of each disease and specific control measures for each disease and the side effects.

For many years, Dr. Cynthia Westcott counseled home gardeners and studied home gardening problems in nearly every state. Dr. Horst is an associate professor of plant pathology at Cornell University. He has researched ornamental plant diseases and his research activities have resulted in more than 60 publications.

The chief hazard any garden plant has to endure is its owner or gardener, says Westcott. "I make this statement unequivocally, after nearly 50 years of visiting home gardens across the country, doctoring gardens in East, and answering countless inquiries.

"Jumping to conclusions is as dangerous to plants as to humans. A spotted or yellowed rose does not necessarily mean rose blackspot. More than half the specimens sent to me as blackspot are examples of mite injury or spray injury or reaction to weather conditions; yet gardeners blithely go on increasing the spray dosage, confident that more and stronger chemicals will lick the disease and seldom noticing they are nearly killing the patient in the process.

"All chemicals used as sprays or dusts are injurious to plants under some conditions, the injury varying with the chemical and the dosage, with the species and even the variety of plant, with temperature, soil moisture and many other factors.

"So please, don't jump to conclusions. Don't do anything in a hurry because the plants are getting sick fast and there is no time for a proper diagonsis. Always read all of the label carefully.

"You have all the time in the world for proper identification, since, by the time the disease is serious enough for you to notice, it is probably too late for protective spraying this season anyway."

The luscious, red, juicy, sweet-flavored home grown tomatoes available to gardeners during the summer are a far cry from the tough-skinned, plastic-looking-and-tasting objects sold as tomatoes by supermarkets about eight months out of the year.

A lot of research has been done in England toward growing good tomatoes for the market in greenhouses during the winter months. "The U.K. Tomato Manual" (published by Grower Books, London, England, distributed in U.S. by ISBS, Inc., 2130 Pacific Ave., Forest Grove, Ore. 97116, 230 pages, illustrated, $25) is mostly a report on that research.

Edited by H. G. Kingham, ADAS regional glasshouse adviser, with 20 contributing authors all recognized in their own field and most of them well known to British growers, the book was first published in 1973 and is now available with supplements covering a number of changes in production techniques for tomatoes.

The short dull days of winter are, in every respect, unfavorable for tomato growth and development, says A. Calvert, one of the contributors, a member of the physiology department at Glasshouse Crops Research Institute, whose work on the effects of temperature and carbon dioxide on tomato crops is of international repute.

"Not only does the poor light limit photosynthesis but the long nights are also unfavorable because they deplete the organic matter within the plant. The energy required for plant respiration comes from the breakdown of some of the carbohydrates produced by photosynthesis.

"Since respiration continues throughout the night it follows that the longer the night, the greater will be the depletion of carbohydrate.

"On the other hand very short night-periods can also be adverse for the tomato. If the day is more than 18 hours, giving a night of less than six hours duration, tomato leaves may become interveinally chlorotic due to a breakdown of the chloroplasts in the leaves.

"It is, however, interesting that the trouble can be avoided if the night temperature is considerably lower than the day."

Satisfactory seed germination is achieved at temperatures around 68-70 degrees F., Calvert says. Lower temperatures give slow, uncertain germination, while at higher levels it is likely to be erratic.

Research on temperature requirements of young tomato plants has shown that the rate of vegetative growth is highest when both day and night temperatures are approximately 77 degrees F., he says. This is true even in the poor light conditions of winter. It does not follow, however, that this is the best temperature to use in the production of commercial crops, it merely indicates the response for one aspect of plant performance.

It is a fact that higher night temperatures induce a large leaf area so that in the following light periods the plant can absorb more of the available light energy. Nor is it altogether true that high night temperatures give rise to tall, thin plants. Plants tend to be tall and thin chiefly because the light is poor.

Tom Stevenson's answers to garden questions appear in the Weekend section of The Washington Post each Friday.