Downy mildew

Spotted a white dust on the bottom of some of the leaves of my Laurel hedge. Curios - I thought - let's have a look on google to find out what it is.

The first site I came across said that it is downy mildew - the mildew will kill the plant - all is lost - there are no treatments.

After half an hour of running around waving my arms in the air in despair - I decided to ask google for another opinion.

It turns out that downy milder is akin to an algae - it needs moisture to survive.

The next site I came across said - if you have hot weather on the way - do nothing - the heat (lack of moisture) will kill the mildew. Much more palatable - do nothing - just my cup of tea.

So - don't believe everything you read on the internet - or at least check that the source is reputable before going off on one.

And....relax.....

 

Downy mildew, is that the same as powdery mildew?

I alway associate powdery mildew with dry roots.

 

Hi i thought they were the same but last year i had a lot of powdery milldew so was looking how toget rid of it naturally when l saw this
Powdery mildew is not the same as downy mildew. Both produce light-coloured spores on foliage, but downy mildew develops spores only on the undersides of leaves yet – powdery mildew appears everywhere even on shoots, buds and sometimes flowers. It thrives in warm weather when foliage is dry – wind spreads the spores, which can’t germinate or grow when foliage is wet

 

Having had a quick read up in my favourite pest and disease book, there are lots of both of these. Downy mildews are favoured by damp conditions and the general recommendation is to remove and destroy (burn) affected plants and improve ventilation. Two notorious examples being Aquilegia downy mildew and Impatiens downy mildew.
Powdery mildews are favoured by dry, hot conditions, so more of a problem when plants are stressed. Advice is to clear up affected plants and debris at the end of the season and either burn or compost. Try and keep plants growing and evenly watered.
As for treatment: for Downy mildews there is little available to gardeners; for Powdery mildew sulphur can help as can systemic fungicides, I've also seen recommendations for spraying with diluted milk.

 

There was some sort of mildew absolutely covering part of my Prunus laurocerasus a few months ago because of some scaffolding which was up for a few weeks. Thought I was going to lose that part of it, but after about 6 weeks of the scaffolding being removed you would never know there had been a problem.

Nick

 

Some interesting info. - thanks everyone .

Well - I decided to give the hedge a hair cut this morning - to use my new battery hedge trimmer for the first time and to have a go at the downy mildew.

There was white powder everywhere when I was trimming back - but apparently the leaves must be wet for the spores to set up home in them - so no more spraying water over the top of the plants.

When Monty says water around the base of the plant not over the leaves - this is why.

There is a systemic treatment available for downy mildew - but it's £60 GBP per litre. So that's not going to happen.

 

I always water at the base, but what I can't understand is that Mother Nature always waters over the leaves and it never seems to cause plants a problem. So what's the difference between me doing it and the rain doing it?

Nick

 

@Nikolaos no real idea, but a few thoughts: generally when it rains it's cloudy and by the time the sun comes out the droplets have dispersed, also when it rains the humidity is uniformly high and drops slowly afterwards.
Also dew generally forms on the upper surface of leaves first and a good dew will help the downy mildew spores to spread. Not sure how Monty and co prevent this.

 

I think some plants are just suckers for certain diseases.
If conditions are right it will happen, lots of supposition out there as to what causes what, I sometimes think we believe we can change things, often we can't it's nature.

If it was that simple the commercial growers wouldn't need the battery of chemicals that they use.

 

Thanks for your thoughts on that, @NigelJ!

@pete I can never get to the bottom of a lot of these little gardening theories and find it really frustrating at times! Like the idea that you shouldn't water plants in strong sun, I still err on the side of caution and avoid it, but I've never come across any scientific information to substantiate such a theory!

"...there is a widely-held belief among gardeners that watering plants in the midday sunshine can cause sunburn, because droplets of water supposedly act as tiny lenses to focus the sunlight onto the leaf surface. However, this myth was debunked in 2011 by researchers at Eötvös University in Budapest, Hungary. They used computer modelling and direct experiment to show that the refractive index of water isn’t strong enough to focus sunlight from a water droplet onto the surface of a leaf."

Why don’t plants get sunburn? | BBC Science Focus Magazine

Nick

 

Regarding pot plants, if it's dry it gets watered, regardless of time of day or weather.
I don't water much in the ground anyway.
The water droplet theory, I'm not sure about, I'd be slightly wary under glass.

I thought the main reason for watering late in the day was because evaporation is less but if it's dry, water it.

 

Hi nigel diuted milk worked for me
Last year i had a some of Powdery mildew on a couple of plants and leaf spot and a freind dad surggested i use half milk and half water for a week or 2 which i did and then i repotted one and now they healthy and grown twice as big so it worked for me

 

I like this very much

 

It's a privilege to be in a company of people who actually have "Favourite Pest and Disease"- books.