In the past few weeks I’ve received a number of questions from friends (whether that be on Facebook or in person) about powdery mildew. Some people are quite educated about the stuff (like Tracy who knew the scientific name for lilac powdery mildew to be Microsphaera penicullata) and some are completely unsure of what they’re looking at.

What is it?
Powdery mildew is a disease that affects numerous plants and there are many types of fungal spores that are the cause. Microsphaera penicullata affects lilac trees but Erysiphe cichoracearum tends to find and damage cucurbits (the melon family that includes cucumbers and squash). In short, it looks like this:

You will see white spots or completely white leaves from a distance and when you get up close, you will notice it is fuzzy. It is, essentially, a mould. And like the mould that starts at one end of the bread loaf and travels between slices, so, too, will powdery mildew. The brown patches you see on this phlox leaf are older colonies of the bacteria.

How it spreads
Powdery mildew is easily spread by wind – that’s how it gets to your garden in the first place. Once there, it can be bounced around by rain or wind and will harbour amongst plant parts during the winter. It has no trouble surviving our Canadian winters.

Host plants are numerous but include: cucurbits, phlox, artichoke, beans, grapes, apples/pears/stonefruit, and lilac. Hot humid conditions are ideal but definitely not necessary; the spores will spread and grow even when nights go down to 10°C.

Control
For annual edibles, opt for resistant varieties. For garden perennials, if you are purchasing new, you can look for resistant varieties but I often find that most people who experience mildew problems are noticing it on older perennial plants they’ve had for years. In this case, cut off and destroy (or throw away) infected leaves and stems. Don’t compost (and that’s probably the only time you’ll ever hear me say that). Annual plants that are susceptible are far and few between but include zinnia, snapdragon, and verbena. Avoid these if you have had powdery mildew problems in the past.

Prevention
Why not try to stop it before it becomes a problem? Of course, nothing is 100% effective, but some of these products work quite well:

– Bordo Copper Spray
– Garden Sulphur
– 1tsp baking soda mixed with 1 litre (4 cups) warm water, sprayed onto entire plant

Some other helpful tips:
– Avoid getting leaves wet when watering (water from the base or at soil level)
– Keep crowding to a minimum to allow for good air flow
– Remove debris and dispose of debris that may be infected with mildew spores (a better use for those ‘leaf’ bags, if you ask me)

There you have it – I’m hoping this has cleared up some questions surrounding powdery mildew. Remember, that it’s not the same as downy mildew (the fungus affecting impatiens across North America). This is a whole other ball game.