Here's a link and part of a theory, which backs up what I read.

http://cahe.nmsu.edu/ces/yard/1999/062899.html

But wait I think there's more...

Jonno wrote:
The water on the leaves story has been around for some time, and been
investigated. The problem is really caused by fungal infections due to
water on the leaves. The sun does not magnify due to different focal
length. But plants can suffer from straight out sunburn....Tomatoes for
instance get sun scald.

I'll try and find the story again if I can...

I wonder if some plants are susceptible to the pyrethrum?


Blackadder IIVX wrote:
"Jonno" wrote in message
u...
Maybe you burnt them due to the gas being a sort of refrigerant. In
other words youre spraying a bit too close to the plants.
Do it when there is no winf ie early in the morning when theres
very little wind. Then you can spray them with some distance in between.

Oh no. I use the hand pump manual add ons. I don't use a spray can.

Its just the heat of the sun. Even if you just spray cold water on
the plants- the sun magnifies the droplets and burns the plant.


John Savage wrote:
"Blackadder" writes:
Do you use the spray during the day?

Probably about mid-morning on a sunny day.

I used a similar spray and it ended up burning my roses - but it was on
account of the sun's heat. If I had done it in the evening or very early
morning it doesn't happen.

I'll bear that in mind. But I would expect there to have been on the
pack a prominant warning of possible plant damage. I'm surprised that
the matter hasn't been brought up on this group before.