Downy Mildew on Roses

Beware the mildew scourge, 2005-06

PLANT CARE: Get disease-resistant plants to be safe

Kerry Moore

    The recent cycle of warm weather followed by rain and then more warm weather is an ideal climate for the creation of powdery mildew.

    You may see what looks like a dusting of flour on phlox, for example, but the good news is these bloomers can be cut down a foot or two and will come back with flowers in late summer. Not so with roses, which are also prone to powdery mildew. When contacted at his Select Roses operation in Langley, Brad Jalbert says powdery mildew, while annoying, can be handled. What has him worried is downy mildew.

    “Downy mildew has hit hard here in the last five years. It generally follows the path set by infestations of tomato blight.” (Tomato blight struck the Lower Mainland about 10 years ago.) Downy mildew makes purplish, irregular blotches on rose leaves, followed by leaf drop.

    “The climber that everyone loves, called New Dawn, is particularly susceptible. If this mildew is on it, it spreads to everything.”

    This mildew has its victims even among professional growers. Jalbert was very hopeful about one of his new roses called Lime Sublime.

    “Downy mildew hit it overnight and every leaf fell off. This is much worse than black spot.” Hot, dry weather will kill downy mildew, though, he says, as well as a spray that it sold “everywhere in the world” except in Canada. It has low toxicity and only takes one spray.”

    Until the government approves this wonder worker, “copper sprays will have some effect. In winter, spray with dormant oil mixed with copper rather than the usual sulphur.”

    As to powdery mildew, try this homemade spray: 1 tbsp. baking soda, 1 tbsp. summer oil (sold in garden stores) and a gallon of water, or try store-bought remedies. And be careful, Jalbert says, to keep roses watered. “If under stress from the roots drying out, powdery mildew will take advantage of the plant’s lower resistance.”

    When looking for a new rose, consider those with high resistance. Jalbert mentions the floribunda Pretty Lady, and Fellowship (also called Living Easy). Take a gander, too, he says, at Marilyn Monroe, a hybrid tea rose. “It’s a gorgeous apricot. The outer petals turn a light green in hot weather.”

    Jalbert advises it is best in Zone 8, as Marilyn needs special protection in winter cold and in harsh winds.

    For future consideration, Jalbert says watch for Lasting Love, “a fragrant, velvety-red rose with superior disease resistance and due out next year.

    “It’s important to understand that not all roses are disease prone. There are many great easy-to-grow roses. Cities plant them all over as do the shopping malls. Those roses get zero care. Just pick the right rose for your garden.”

    For more information look on his website, www.selectroses.ca. The nursery is at 22771 38th Ave., Langley; 604-530-5786.

kmoore@png.canwest.com A rose is a rose — and may be susceptible

 Powdery mildew: A white powdery substance that covers leaves.

    What: Linked with humidity and temperature. Powdery mildew spores do not need water on a leaf to germinate.

    How: Created when daytime temperatures rise with humidity between 45 and 75 per cent. If daytime temperatures rise to 29° C (85° F), it stops.

    Downy mildew: Greyish-white fuzz forms on the bottoms of the leaves, round purple blotches with yellow edges are on the tops. Leaves turn brittle and drop.

    What: Often confused with powdery mildew and black spot, downy mildew can defoliate a plant in one day.

    How: Appears after periods of cool, wet weather. It clears when the weather warms. It needs moist conditions to spread, so no overhead watering.

— From The Weather Resilient Garden by Charles W. G. Smith (Storey, $29.95) and Roses for Dummies by Lance Walheim (IDG Books, $24.99).

Brad Jalbert offers suggestions on rose care. Select Roses in Langley carries many varieties (right).