By the time you see it, it might be too late

Is it hot enough for you? My zucchini don’t seem to mind; despite the heat, it looks like it will be a bumper harvest. Apart from a few nibbled leaves, they’re looking healthy enough. There haven’t been any serious ailments — so far.

Like others in the cucurbit family (cucumber, squash, pumpkin, etc.) zucchini are susceptible to powdery mildew, as are many other plants including phlox, crab apple, and lilac. On zucchini, it begins with a few splotches here and there and before you know it, all the leaves turn from green to an unsightly grey. Powdery mildew looks like white or grey dusty patches on leaf surfaces. Initially, it can be missed, as many in the squash family have silvery grey markings that occur naturally on the leaves.

When plants become infected, it’s too late to do anything as there isn’t a cure. Commercial producers have access to fungicides but home gardeners have limited options. Prevention and control is the best approach. And even that can be a challenge.

Mildew is a type of fungus, and there are many varieties — some mildews are plant specific and others don’t care what plant they infect. Fungal spores are formed when humidity is high and dispersed on the wind when humidity is low. Not all types of powdery mildew need moisture on the leaves for them to become infected. In fact, fresh spores on dry leaves can be rinsed off.

Cleaning up and disposing of all infected leaves in fall is a common suggestion but you won’t get every leaf, or every spore, and since the invisible spores travel on the wind, you’d need to clean up every single one in your neighbourhood.

For prevention and control, measures that have shown some positive though limited results involve spraying the leaves with a solution of baking soda (1 tsp/5 mL of baking soda in a quart/one litre of water) or milk (one part milk in nine parts water). Whether skim, two per cent or whole milk is best isn’t clear, but I’d guess the resulting fragrance will be the same. As for the baking soda, which is sodium bicarbonate, its cousin potassium bicarbonate was shown in tests to be 25 to 35 per cent more effective, and more effective than most other home remedies.

There are many suggested formulas online — including one using mouthwash — except few have been seriously tested for efficacy. And depending on what’s in the often-misinterpreted formulas, they can do more harm to plants and to important soil microbes.

One that’s frequently touted is the Cornell Formula Fungicide, presumably developed by Cornell University. Problem is, there is no such formula. Dr. Ken Horst, renowned plant pathologist from Cornell University, has stated, “The myth of the so-called ‘Cornell Formula’ continues to spread and I feel that the record needs to be set straight.”

It all began when information was cherry-picked from incomplete experiments and then the so-called Cornell Formula took on a life of its own, only to be cut, copied, modified and pasted by countless bloggers, so beware. One of the best ways to deal with mildew is to simply use plants that are resistant.

Mildew on plants is a nuisance and can be serious, as in the case of impatiens downy mildew, which had an enormous impact on impatiens production. I am happy to report that new strains of these popular plants with high resistance to IDM are being bred by the Syngenta company, and should be on the market in the next year or two.

At least with plants like zucchini, appearance is less of an issue, and as powdery mildew usually shows up later in the season, sometimes not until fall, it doesn’t necessarily affect the harvest.

As for my bumper crop, keep your car locked if you happen to visit.

To chat with local gardeners, share tips, pics, discuss powdery mildew, see Grand Gardeners on Facebook.

David Hobson gardens in Waterloo and is happy to answer garden questions, preferably by email: garden@gto.net. Reach him by mail c/o Etcetera, The Record, 160 King St. E. Kitchener, Ont. N2G 4E5