Leaf Miners Make Home In A Whole Range Of Plants

Today we continue "The 10 Most Wanted" list of common pests and problems that assail Puget Sound area gardeners during the garden season. Every Friday and Sunday through the Green Gardening Programs' June 26-27 garden tour we are discussing least-toxic control of another of "The 10 Most Wanted." To obtain a free tour map by mail, call 547-7561 by noon Monday.

Pest No. 8: Leaf miner

Do your columbine leaves look as if someone had squiggled an irregular doodle on them with a gray-yellow pen? Does your lilac have blotched leaves, maybe with some that are rolled up? A whole range of landscape and vegetable plants provide housing for leaf miners of various species.

These insects hatch from eggs laid underneath leaves. The newly hatched larva of these small moths and flies burrows into the leaf and settles down to make a home between the upper and lower leaf surfaces. When mature enough, the larva drops out of the leaf and hides in the soil to pupate.

When gardeners observe the damage, they see the "mining" the larva does by eating out the leaf from the inside. Sometimes you will be able to observe the larvae by peeling off the surface of the blotched area and looking for them. Vegetable gardeners get particularly disgruntled by attacks on spinach, chard and beet greens.

Floating row cover is a nonchemical way to protect your garden from leaf miners. This is a lightweight spun-bonded material laid loosely over crops - newly seeded rows or newly set plants.

Light and rain can penetrate, but row cover prevents the flying stage of the leaf miner from laying eggs on the leaves. If the cover is well-protected at the edges (no gaping holes) it is very effective.

Fall-sown vegetable crops may be less susceptible to attacks.

For larger plants such as lilac, hand-pick affected leaves. Clean up any fallen leaves under the plants. Encourage beneficial predators such as lacewings and spiders. (Do not spray wide-spectrum insecticides that also kill beneficial insects.)

Remember that mature, healthy plants can lose up to 25 percent of their leaves without suffering damage. Keep the plants well watered if they are losing leaves to leaf miner attacks.

Q. I have a rusty looking blight on my raspberries. The leaves are withering and dying. What is it?

A. Raspberries are subject to several blight diseases as well as a yellow rust. The rust fungus causes yellowish spotting of the foliage and leaf drop, but doesn't really cause withering or blighting of new tissue.

Raspberries and many other plant species were particularly hard hit this spring by Botrytis, which does cause tissue blighting. Powdery mildew can also cause similar symptoms. Pseudomonas, a bacterial disease, has also been virulent this season on many plant species.

Leaf and shoot death of infected plants is common. To make it even more complicated, another fungus organism, verticilium, can cause a wilting and dieback of new canes.

To accurately diagnose your raspberries is probably not possible from this vantage point. You may wish to bring a sample of your afflicted raspberry to your nearest WSU Master Gardener plant-problem clinic.

For a schedule of clinics held weekly in King County, send a self-addressed, stamped, business-size envelope to WSU Cooperative Extension, 506 Second Ave., Room 612, Seattle, WA 98104-2394.

Gardening runs Friday in Scene and Sunday in Home/Real Estate. It is prepared by George Pinyuh and Holly Kennell, Washington State University/King County Cooperative Extension agents, Mary Robson, Master Gardener program assistant, and volunteer Master Gardeners.