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Downy Mildew infecting cut flowers & bedding plants

Posted by Lauren Barker - Horticulture Specialist on 25 July 2019 in Trees, Plants & Ornamentals
Downy Mildew infecting cut flowers & bedding plants

The recent humid weather has sparked an influx of downy mildew cases, and a surge of calls to our free technical helpline.

But what is it?

Downy mildew is an Oomycete, similar to a fungus. It needs high humidity to sporulate. Optimum temperature is between 15oC and 21oC, which is why there has been a surge in the past week or so.

Infection is very quick (sometimes in a matter of hours) and it can take five to six days before it begins to show up. You will only be able to see the pathogen when sporulation occurs. The process of sporulation will begin at 4.5oC and will stop at 27oC.

What does it do to my plants?

The first thing you will probably notice is that light green areas will start to appear on the top of the leaves. If you see this on some plants, you will need to look underneath the leaves to see if a white sporulation (white furry fungal growth - looks a bit like furry mould!) is also present. Both symptoms will spread across the whole leaf and eventually the leaf will turn yellow and fall off.

It's not good news for the flower either, they will often fail to open and can fall off. In cut flower production this is a very destructive disease.


We recommend planting young plants out as soon as possible to avoid them becoming wet. If they're under cover you will need to allow good air flow throughout the plants, and in a greenhouse or polytunnel you'll also need to keep an even temperature.

Something else I recommend is to avoid watering plants from midday onwards and particularly in the evening as this will allow the plants to have moisture sat on the leaves for longer periods of time. Ensure only plants that are dry need watering. You'll need to thoroughly clean tools, trays and pots in between handling plants as this can spread the disease. Remove weeds as you see them and clear any old leaves and other debris.

It is best to use fungicides from 3 different groups with different modes of action EG Paraat or Rigel, Percos, Revus, Dithane, Promess.

Any queries or need a problem identifying please get in touch and I can provide a treatment plan free of charge