Hello – I have a lovely purple ninebark (could be diablo) and the last couple of years it’s suffered badly from powdery mildew which I didn’t detect until it was well advanced. I wondered whether (a) applying dormant spray now would prevent an outbreak this year, and (b) if so, is the spring too advanced? I’ve never used dormant oil but understand you have to apply it no later than a certain time in the spring — I’m just not sure when that is. Many thanks in advance for your help!
Thank you for contacting the Toronto Master Gardeners
Horticultural oils first became popular as a way to control pest problems on fruit trees. The primary way horticultural oil kills insects is by suffocating them. The oil blocks the spiracles through which insects breathe. It also disrupts the metabolism of insect eggs and the ability of some insects to feed, causing them to starve to death.
You are correct, application of horticultural/dormant oil is both time and temperature specific. It should not be applied if the temperature is below 4C or if the temperature approaches freezing soon after application of the dormant oil spray. It should be applied early in the season just before either leaves or flowers show signs of swelling or breaking open. You don’t want to apply it too early, since the insects need to be active and breathing, in order to be affected.
Since powdery mildew is a fungal disease that affects a wide range of plants applying dormant oil will not help. Powdery mildew is an air-borne fungi, and is one of a large group of the most common fungi that cause harm to garden plants. Powdery mildew appears as opaque and powdery grayish-white spots. According to the book, What’s Wrong With My Plant (And How Do I Fix It?) by plant pathologist David Deardorff, PhD, and naturalist Kathryn Wadsworth, the best approach is prevention by removing infected plant material (and putting it in the garbage, not the compost), mulching to prevent water from splashing around and spreading the fungal spores, providing air movement around the individual plant, and choosing resistant cultivars such as Little Devil or Amber Julilee. When you water your garden make sure to water only the roots of the plant, keeping the foliage dry and water early in the day so that the water has a chance to evaporate. If the infection is severe you might consider purchasing a fungicide from your local nursery. As always, remember to follow directions on the bottle. This article gives additional information on powdery mildew treatment.