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Fielding Questions: Powdery mildew is unsightly, but won’t kill peonies

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Fielding Questions: Powdery mildew is unsightly, but won’t kill peonies
Fargo ND 101 5th Street North 58102

Q. My peonies have a white residue on the leaves and seed heads. I sprayed a fungicide on July 26. We sprinkle with city water.

What is causing the peony problem, and how should I cure it? Now the leaves are drying up.


– Christine Marshall, Valley City, N.D.

A. The white coating is powdery mildew, which is a fungus commonly attacking peonies, lilacs and other shrubs and perennial flowers. It develops more severely in humid weather, or where overhead sprinkling keeps the foliage wet.

The good news is that it won’t kill your peonies. But it does look unsightly, especially as it progressively causes the foliage to dry up.

Fungicides containing chlorothalonil (indicated on the label in fine print for the active ingredient) are useful, but must be applied to healthy green foliage as a protection and preventative before the mildew appears or at the earliest signs. Fungicide sprays would need to be applied in June most years.

Watering from the base instead of overhead sprinkling helps prevent powdery mildew’s spread. Water in the morning so foliage dries more quickly, and avoid evening sprinkling.

Once the mildew has progressed, it isn’t possible to get affected leaves to return to normal appearance. Foliage should be cut back to ground level in September after a hard freeze. Completely remove and discard tops because the fungus overwinters on infected plant parts in and around soil.

Q. We replaced a white birch this summer, and it lost all of its leaves right away. Now it has started to bud, but the buds are turning into something other than actual leaves.

We love trees, but it sure seem to be having our share of issues.

– Judy Henderson, West Fargo

A. You’re not alone in your tree difficulties. We’ve had several springs that have been overly cool and wet followed by periods of heat with mid-summer dry spells. These extremes are especially difficult in areas of heavy clay soil. Going from too wet to too dry bakes the soil to brick, and tree roots suffer.

Your newly replaced white-barked birch seems to have suffered severe transplant shock. Such shock can be caused by root disturbance. Or if the weather happens to turn hot, limited roots can’t pump enough water upward to sustain leaves even though the soil may be moist.

Now your birch is budding with green “catkins” instead of leaves. These inch-long “pods” are flowers. Often when young trees are under deep stress they give an abnormal attempt to reproduce, an instinct to provide a future generation. The tree probably senses that its life is threatened.

Test for life in the twigs by scratching the outer bark with your thumbnail. A fresh green layer indicates life and hope. If the layer has turned brown, that portion is dead.

Wait-and-see is often the only choice, with replacement as needed. No fertilizers or other products would help.

Q. My son bought a house in north Fargo with raspberry plants on the southwest corner. This small planting has produced enormous fruits larger in size and quantity than my whole 8-by-20-foot patch.

Is this a variety issue or is it sunshine and soil fertility?

– James Morken, West Fargo

A. Small backyard raspberry plantings are often very productive because they are kept narrow due to space restraints. An ideal raspberry row is only 18 inches wide. The reduced competition between canes allows plants to bear heavily. Your son’s planting may be the ideal width along with good location.

Because raspberries “sucker” so freely, a patch can easily become very wide, causing plants to compete for fertilizer and water. Reducing each row to the recommended width helps production.

Recommended varieties include Boyne, Latham, Nova, Killarney, Redwing and Heritage.

If you have a gardening or lawn care question, email Don Kinzler at Questions with broad appeal may be published, so please include your name, city, and state for appropriate advice.