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Jun 7, 2020 4:56 AM CST
Thread OP

I had to throw away all my basil plants after they developed this silvery leaf disease.
At first I tried removing the affected leaves, but it spread.
I also had to get rid of fungus gnats (started out with peppermint oil, soap, small amount of alcohol and garlic) and the spray damaged the plants further.

What looks like the same silvery leaf disease is also affecting a lettuce (growing in water), and possibly my Senecio (on the upper part of the stem).

These are all indoor plants.

The fungus gnats are now under control and hopefully gone after using the above spray, mosquito bits, sticky traps, and ultimately a peroxide wash once I'm sure they're gone.

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Jun 7, 2020 11:57 PM CST

That is downy mildew: the sporangia underneath the leaves are the telltale sign.
It's very commong for plants grown indoors because it's "triggered" by high moisture levels in the air and insufficient air circulation. Improve air circulation and use a copper based fungicide to put it under control.

About fungus gnat: there's a product called Gnatrol. It's BTI-based like Mosquito Bits but it uses a far more aggressive and tenacious strain. I've used it and it works really well, both on gnats and mosquitos. You also need far lower dosages than Mosquito Bits meaning you save a lot of money.
I am just another white boy who thinks he can play the Blues.
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Jun 8, 2020 2:36 PM CST
Thread OP

Very helpful advice, thankyou!

I'm not sure I will be able to improve air circulation, except for a couple of months in (UK) summer, by leaving windows open.
I will try the copper based fungicide ands also check out gnatrol.
So far the combined approach seems to have stopped them.

Do you think the Senecio would be treatable, or is it best to remove infected plants completely?
Jun 9, 2020 4:55 AM CST

Is your Senecio proper Senecio (read: old-man-in-the-spring) or Jacobaea (read: silver ragwort)?
I wouldn't bother with the former: it's an annual and it grows literally everywhere here. Silver ragwort is well known for not liking high moisture because... well you are seeing it. High moisture makes it extremely susceptible to fungine attacks, like you are seeing. You can try treating it, but the only real long term solution is to lower moisture in the air. Have you considered putting a fan in the greenhouse?
I am just another white boy who thinks he can play the Blues.
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Jun 10, 2020 4:19 AM CST
Thread OP

It's Senecio Himalaya.
There's no greenhouse, I'm in a first floor flat Smiling
Jun 12, 2020 3:30 PM CST

"Himalaya" is a cultivar of S. barbertonicus.
It's a very tough plant so it's worth giving it a second chance: just remember being a native of the South African veld it doesn't like high moisture or too damp of a soil.
I am just another white boy who thinks he can play the Blues.
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Jun 13, 2020 8:57 AM CST
Thread OP

Thanks I will make sure to let it dry out thoroughly between waterings
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