Ask the gardeners

I have a lilac bush with a grayish white material on the leaves. I have sprayed with Malathion but it doesn't seem to help. Many leaves have dropped prematurely.

The grayish white coloration on leaves indicates mildew caused by a fungus. It is common on lilacs and many other plants, including roses, phlox, begonias, etc. It is more prevalent during periods of high humidity and low air circulation.

Heavy infestation causes leaf drop. Malathion is an insecticide and not a fungicide; therefore it has no effect on mildew. Benomyl is a satisfactory fungicide, but most folks don't bother spraying, since lilacs are pretty tough and usually remain sturdy in spite of mildew.

It helps to rake up the leaves and seal them in plastic bags for disposal. This removes a large number of the spores.

I have a bed of daffodils which have not flowered for the past four years, even though I have been careful to let the leaves ripen. Can I make them flower next spring by applying some bone meal or some other fertilizer or should I replace them?

All leaves and no blooms on either tulips or daffodils indicates they are overcrowded. You can separate the bulbs and put them in a spot where they will grow back to blooming size, but when they have been crowded for that long, it will take a while.

If you want blooms next spring, plant new bulbs in their place. It's a good idea to add some bone meal or superphosphate when you plant the new ones (about a handful per square foot), along with a sprinkling of balanced fertilizer (liquid or dry). Work some compost or leaf mold down into the soil about a foot if soil is either clay-like or very sandy.

In the spring of 1980 we planted two cherry trees. Within a few months one of them died. The other tree appeared promising but now a thick gummy exudation appears near the crotch of the trunk. The leaves started yellowing early and have all fallen off. Does the thick, rosin-like material have something to do with this?

The material is called a ''gumboil'' and is telltale evidence that borers have been at work. They are white grubs with brown heads and work on stone fruits, tunneling beneath the bark.

Dig off the gumboil and try to poke a wire up inside the tunnel, or you may be able to make small lengthwise cuts, removing a small portion of the bark, so the borer is exposed.

If they can't be reached without a major operation, get some borer paste sold at garden stores and squirt it into the holes. Then seal the holes with putty or bubblegum. The paste gives off a gas which will kill the borers.

Keeping trees fed and watered helps, because moths that lay the eggs pick out the weakest trees. Putting Fels-Naptha soap cakes in pieces of nylon stocking and tying them above the crotch repels the moths.

If you have a question about your garden, inside or out, send it to the Gardening page, The Christian Science Monitor, One Norway Street, Boston, Mass. 02115.