Mildew on Courgettes

Courgettes and the cucurbit family can be susceptible to powdery mildew. My courgettes have all succumbed to it this week (August 2021). Off I went to the library, and I looked to garden literature to compare expert advice to treat it.

Topics below include:

  • What is powdery mildew?
  • Whether it affects the fruit
  • Can it be treated?
  • What to do with the leaves
  • Future prevention

What is Powdery Mildew?

This is a fungal disease that covers the leaves, giving the entire leaf a white dust-like coat.

Sally Smith (2010): she described it on the BBC Dig In Blog (2010) as a disease which favours dry soil, when dry days are succeeded by cold nights (BBC Dig In blog, bbc.co.uk, 2010).

Alan Titchmarsh (2009): he says powdery mildew can appear after a dry summer when the evenings are getting cooler (How to Garden, BBC Books, 2009).

Guy Barter (2018): the chief RHS Horticulturist as at 2018 explains it is part of a group of fungal diseases, mainly separated into either powdery or downy mildew. The powdery one is the white covering which affects courgettes and he says they are not necessarily singular – the infection could be unique to your courgettes, for example (RHS Can Anything Stop Slugs?, Mitchell Beazley, 2018).

Mildew on my courgettes

My Courgettes in August

My own courgette plants, of the varieties of Astia, Romanesco, and De Nice, have all succumbed to mildew this week (August 2021). In July, only one of the plants, a De Nice courgette, gained powdery leaves during a heatwave. Meanwhile the rest (there are about a dozen plants in total) remained unaffected. They were watered daily and received a mulch of grass cuttings.

Fast forward to August and gale-force winds preceded a fortnight of persistent rain. These container plants were moved temporarily. Their new spot was exposed so I moved them close together to support each other in the gales. Watering was every couple of days. With all this rain, I did not think they needed much water, but given the exposure and the wind it is possible they may have dried out. It is also probable that they were too close together, hastening the spread of the mildew.

Does Mildew Affect The Fruit?

In my chosen books, authors mostly say that the fruit is fine; it is the leaves that are affected. Here are some author views:

Alan Titchmarsh (2009): the plants affected continue to produce crops but at a lesser rate (How to Garden, BBC Books, 2009).

Aaron Bertelsen (2020): it only affects the leaves (Grow Fruit and Vegetables in Pots, Phaidon, 2020).

Sally Smith (2010): if it is late August or in September that this happens, throw the plant away, simply because it will be unlikely to regrow another yield (BBC Dig In blog, bbc.co.uk, 2010).

With my own affected plants, I have removed the affected leaves. They all look like they’ve had an extreme haircut. Removing the leaves has revealed a few fruits emerging that had previously gone unseen. I’m going to throw out some of the fruitless plants, then retain the ones that look like they will produce courgettes and see what happens.

Can Powdery Mildew Be Treated?

There are chemical options mentioned to treat powdery mildew, but a common approach is care and future prevention.

Christine Walkden (2011): she suggests you grow a resistant variety or use a fungicide that is okay on edibles (No-Nonsense Vegetable Gardening, Simon Schuster, 2011).

Alan Titchmarsh (2009): feed the plant, water it well, and remove affected leaves by hand (How to Garden, BBC Books, 2009).

Guy Barter (2018): give the plant some care. Remove infected leaves as early as possible, water it and mulch it. He notes the home use of milk or garlic, but claims sodium or potassium bicarbonates are more effective. He gives a recipe for bicarbonate spray (image below). (RHS Can Anything Stop Slugs?, Mitchell Beazley, 2018).

Sally Smith (2010): She also mentions potassium bicarbonate spray, used weekly, as a remedy, as well as plenty of water (BBC Dig In Blog, bbc.co.uk, 2010).

Bicarbonate spray in Guy Barter’s 2018 RHS book

What to Do With Discarded Mildew Courgette Leaves?

Differing opinions. Discard, burn, or compost?

Most say discard the leaves, with no mention of where to put them, excepting these two authors I read who said:

Guy Barter (2018): powdery mildew is spread by spores. Burn the leaves and do not add them to your compost heap (RHS Can Anything Stop Slugs?, Mitchell Beazley, 2018).

Sally Smith (2010): she claimed you can compost them because the fungus will not survive the compost heap (BBC Dig In blog, bbc.co.uk, 2010).

I’ve not put them in my compost. I’d love to know if there have been any experiments to test mildew survival on compost heaps. Anyone know any studies?

Future Prevention

Tips offered in gardening literature to resist future mildew on your courgettes include:

Christine Walkden (2011): choose a disease-resistant variety (No-Nonsense Vegetable Gardening, Simon Schuster, 2011).

Guy Barter (2018): consistent watering, mulching, and disease-resistance varieties (RHS Can Anything Stop Slugs?, Mitchell Beazley, 2018).

James Wong (2015): try a diluted milk and aspirin spray (Grow For Flavour, Mitchell Beazley, 2015).

Mildew-resistant varieties of courgettes include Primula F1 and Dunga F1. Anyone else got a variety they can recommend?