Don't know about your dioxide idea.

I'm in Zone 9, extremely dry summers. Absolutely no rain. We still get
blackspot. A local rose company says the following about blackspot:

"Black spot consists of black/brown spots on the foliage that can defoliate the
plant. Black spot spores over-winter until humid, wet spring/summer conditions
are favorable. In the winter after pruning, when roses are bare, gather up and
dispose of all the foliage from the ground. Spray copper or Neem in the spring
and lime sulfur in the winter. Baking soda works -- somewhat."

I used Need oil until I got Orthenex as a gift several weeks ago. But with Neem
oil, I had control of the disease and stopped it from spreading. You can buy
the concentrate and add several tablespoons per gallon of water for spraying.
That's if you want a non-chemical approach to using a chemical like Orthenex.
Apparently Neem oil is some extract from a tree located in India.

If you're still looking for a simple substitute from your kitchen, this is what
they say about powdery mildew (maybe it works for blackspot too):

"Powdery Mildew looks like a white dusting on rose leaves. It is controlled by
Orthenex. The organic method is 1T baking soda, 1T vinegar, 1 T vegetable oil to
a gallon of water. Other good sprays are neem oil, horticultural oils or ultra
fine oil (spray on cool days). "

elfa



In article , "Mark. says...

I saw some product sold as a sort of horticultural disinfectant,
with a mention that it could be sprayed on roses with black spot
as it would kill any live spores on contact. It turned out that it
was "hydrogen dioxide" at a concentration of 27% or so.

Now, as far as I know, that's a synonym for hydrogen peroxide,
and the drugstore stuff can be as cheap as $1 a quart for the usual
3% solution -- about a tenth as strong. So suppose that one put a
pint or quart of the cheap stuff for each gallon capacity of a sprayer,
and diluted it. Spraying it on any live black spot spores (and one
would hope, various other fungal spores, and perhaps some fungal
infestations) would kill them, presumably. Now, it's not a sure cure,
it's not a long-term preventive, but it's certainly cheap and it's certainly
not toxic. Anyone tried it? Any thoughts?

(After a couple rainy weeks last month, almost every rose I have has
shown SOME black spot, and some have nearly been defoliated. I've
been alternating Daconil and Funginex sprays on a weekly basis. Also,
some of the rugosae have something that looks like rust.)

Mark., north Florida, zone 8b