As sure as the autumn leaves turn orange and fall, you can be certain of this: the leaves of every courgette, every winter squash, every cucumber in your garden will be covered in a downy white powder right now.
Let’s get one thing straight: this is not (probably) your fault. Mildew strikes as soon as the air turns damp at the end of summer, yet the soil’s moisture is diminished by the demands of a long season’s growth. It is inevitable on any member of the cucurbit family – so that’s all the above, plus melons, other summer squashes like patty pans, gourds and luffas. It’s also quite likely on grapevines, and sometimes on roses and (earlier in the year) gooseberries, too.
On the plus side, these aren’t all the same disease: the mildew which affects your courgettes won’t infect your roses, and your rose mildew won’t attack any nearby grapevines. The bad news is that all powdery mildews behave in pretty much the same way: that is, by spreading rampantly from leaf to leaf wilting them to unsightly grey rags and eventually defoliating the plant altogether.
For courgettes and squash, if you leave mildew to rage as it will, this means a premature end to the harvest: not always a problem if you have spent summer, like I have, finding ingenious ways to include courgettes into every recipe you cook from curries to cakes.
Similarly I am quite relaxed with the winter squash as the fruits have usually set before mildew strikes, so losing the plant a few weeks sooner won’t necessarily affect the crop.
It’s not so great if you grow susceptible varieties of roses, or gooseberries: mildew looks very unsightly, sabotaging your beautiful rose bush, and gooseberry mildew will rot the fruit. Ditto grapevines.
Mildews are among the fungal diseases getting worse as climate change begins to bite (there’s nothing a mildew likes more than warm, damp autumns – and that’s exactly what we’ll increasingly be getting). Many varieties which could previously cope with a mild dose are no longer able to stay healthy. Last year classic roses like Munstead Wood and A Shropshire Lad were pulled from the market for exactly this reason.
So how’s a gardener to deal with the problem?
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