GARDENING : VALLEY'S SPRING HOT SPELLS RETARD LEAF FUNGUS.

Byline: Joshua Siskin

This has been a strange spring in its unrelenting heat and the virtual absence of overcast days. The results have been pleasantly surprising - at least in the leaf fungus department.

With summer only a week away, three famous spring fungus diseases - sycamore anthracnose, peach leaf curl and crepe myrtle mildew - have hardly been noticeable. The many hot spells we experienced during the last three months appear to have retarded the development of these blights.

In general, leaf fungus is not as much of a problem in Southern California as it is in more humid parts of the country. The aridity here is a natural preventative to airborne fungus disease, which requires moist leaf and stem surfaces to thrive. The advice against watering lawns at night, for instance, is followed religiously back East; in that region of the country, excessive humidity - when combined with wet grass during the night - inevitably leads to the outbreak of lawn diseases from New York to Florida. In Los Angeles, however, nighttime watering seldom results in lawn fungus problems.

One fungus that you will see this year is powdery mildew on roses. There is probably a powdery mildew fungus for every plant on earth, and roses are especially attractive to powdery mildew spores; if a rose leaf is moist for more than a few hours, white fungal strands will start to grow on its upper side. Downy mildew, which is much rarer, attacks the lower side of the leaf.

It has been demonstrated that fine horticultural oils, such as Sun Spray and Neem, are effective in combating powdery mildew. Although designed to deter soft-bodied, plant-sucking insects - such as aphids, mealy bugs and white flies - these preparations also have anti-fungal properties.

At a recent pest-control seminar, John Karlik, University of California Cooperative Extension agent in Kern County, distributed a list of landscape roses, which are recommended both for their beauty and for their pest resistance. He is especially fond of the Meidiland types. He also made the point that the popular ``Iceberg'' white rose - the most popular landscaping cultivar - holds its flower petals on the stem long after they turn brown (an undesirable trait) while the comparable ``Simplicity,'' whether in white or pink, sheds its petals as soon as they fade.

The fact that Kern County has a university-trained rose expert is no accident. Bakersfield, you see, is where two-thirds of the roses in the United States are propagated. All the major rose growers, including Jackson and Perkins, do their growing in Bakersfield. You wonder if the hot, dry summers there - which must effectively stifle powdery mildew - have something to do with this. Karlik waters the roses in his Bakersfield back yard twice a week with a drip system. Each rose gets a total of four gallons of water per week, or two gallons per watering.

Book bargains in bloom: If you are looking for books with lots of full-color pictures of plants, but don't want to pay full price, visit Bargain Books at 14426 Friar St., Van Nuys. This incredible store has used plant books in mint condition for half price. The store is located one block south of Victory Boulevard and a half block east of Van Nuys Boulevard.

People just getting to know plants often are frustrated by the lack of inexpensive books with color pictures. The Sunset Western Garden Book has black and white and green drawings that don't do the novice much good. I have found the HP series of paperbound books - on ground covers, shrubs, trees, dry climate plants, annual flowers, bulbs and vegetables - to be most satisfying not only for their color photos, but for their always interesting accompanying text.

Tip: If you have pop-up spray sprinklers in your lawn, turn them on for five minutes a day, seven days a week, during warm weather. Ideally, you should water in the early morning. Plants start their daily photosynthesis at this time, which is when they are most efficient at taking up water from the soil.

MEMO: Joshua Siskin's column appears every Saturday. He welcomes questions from readers. Write to him in care of the Daily News Features Department, P.O. Box 4200, Woodland Hills, Calif. 91365-4200. You can also reach him through this on-line mailbox: JoshSiskinaol.com. Topics of general interest will be discussed in the column.