This column originally appeared back in 2006 but I thought it was a good idea to reprint it, as conditions have been ideal for another outbreak of powdery mildew this past spring. If you’ve got white spots on the foliage of your garden plants, read on.
There are a few very basic “housekeeping” steps that one can take to help plants. Don’t give plants too much fertilizer, because the disease looks to establish on new growth first. You’ll often see this happening on roses.
Good air circulation around plants is vital. Don’t crowd plants, and if they become bunched together as the growing season progresses do some thinning out of stems and foliage to encourage air movement through the plant’s structure.
Pick off foliage that is covered with powdery mildew and dispose of it in the garbage. Rake up any leaves that fall at the base of the plant during the growing season, and do a thorough cleanup in the fall when all the leaves have come off. If the affected plant is an annual that will be coming out in the fall anyway, why not remove it now and reduce the likelihood of further infestations?
Hose off diseased plants with water. Huh? It seems that a strong blast of water keeps the organism in check somewhat. It doesn’t eliminate it, but is does help to reduce the problem, and you’ll probably also take out a few aphids while you’re at it.
Make notes on which plants were badly affected by powdery mildew, and resolve to use different varieties in the future. There are many disease resistant roses available, and work is ongoing to develop new resistant varieties of many susceptible plants, like summer phlox.
Did you know that your kitchen holds a couple of products that will help to control powdery mildew? A Brazilian scientist discovered in 1999 that milk sprayed onto cucumber leaves infected with powdery mildew helped to stop the spread of the spores. The original tests used a one part milk to one part water solution, but even rates as low as ten percent were found to work, and it didn’t matter what type of milk was used.
Changing the pH on the plant surface appears to lessen the risk of the disease developing. An application of potassium bicarbonate, or baking soda, can be very effective if you begin spraying even before the disease is present. Mix 15 ml of baking soda in 1 litre of water with 2.5 ml of dish soap and spray every three or four days.
Sulfur is probably the most effective fungicide product available in garden centres. It comes as a dust, or liquid formulation. It won’t eliminate the mildew that is already present, but it can help to control its spread if applied regularly.
A word of warning; those favourable conditions that led to the development of powdery mildew earlier in the spring? They’ll be back with a vengeance as we head into late summer. That’s right, the high day temperatures and cool night values that we enjoy so much in September and into October will likely accelerate the spread of mildew on affected plants. Take action now and you’ll be seeing a lot more green and a lot less white on your plants this fall.

Leave a Reply

Your email address will not be published. Required fields are marked *