What is wrong with my Rebutia?

[Echinops32]

Discovered this today, I did not notice it before because the 'diseased?' side was facing the window. Is this a fungal/mould infection and if so should I bin it? I do have a couple more of these Rebutias one looks fine the other one looked suspect in the middle so I panicked and cut the center part of the plant out but after doing this I found the middle to look green and healthy It may just of been staining from watering with diluted liquid plant food (hopefully this one will callous over and survive ). But the one in the photo does not look happy at all, any ideas?

[Aiko]

My first thought: burned. But not very intensely, and at the same time very even. Strange for a burn, though. All affected sides were aimed at the window, and have you had any high sun intensity? I guess the latter not so much, beginning of November and being in northern Europe...

I don't think stains from watering has anything to do with it.

[Echinops32]

The strange thing is one of the other Rebutias that looks perfectly healthy is on a windowsill that faces the same direction (but in another room) so it would have got just as much sun as the 'diseased?' one. The one that looks diseased is right by a window which gets opened regularly (could draught have caused this, there have only been a couple of light frosts recently and I think the window was closed on those occasions) Also the 'diseased?' plant is in a small very small bedroom with higher humidity at night than the other healthy plant which is in a larger bedroom. The comment about the palnt food staining does not refer to the 'diseased?' plant but on another Rebutia that looks healthy (hope i'm making sense)

[jfabiao]

It could be a very severe red spider mite infestation. If you have a magnifying glass, check for minute reddish creatures. Do you spray your plant regularly, maybe from the side that stayed green?

[Echinops32]

I have had a lot trouble with spider mite on chilli pepper plants grown on my windowsills a couple of years ago but after closer inspection I cannot see any mites or webs. We just had the cavities filled in the house and my partner said there was a lot of dust when they did this and there is a gap by the windowsill where dust could have escaped but it does not wash off so I doubt that it is dust. I just cut one in half and it looks perfectly green and healthy under the white crust. I'm mystified.

[Aiko]

Keep an eye on it and see if / how it spreads. Could be bugs indeed.

[Echinops32]

The only other thing I can think that might have caused this is when I water my cacti I put them in a bowl of water with some liquid tomato/seaweed feed in it and sometimes if the soil is very dry the pots float and the plant gets a good soaking as well as the roots. I have read that plant food should not go on leaves but also some recommend foliar feeding

[Peter]

If the plant floats when it's put into a bowl of water I reckon that it's in a peat compost. Not the ideal, however the plant has developed well to date.

I would not recommend that the plant be drenched in seaweed based fertiliser. It's organic and the chemical basis of the product could possibly vary. Having stated that though, my impression is that it seems to be spider mite attack.

[BrianMc]

In my opinion it looks like Red Spider Mite

[Echinops32]

Although I cannot see any spider mites, I would not be too surprised if they were there because my chilli peppers grown on the same window sills were rife with spider mites a couple of years back (so much so that I gave up growing them). As I don't like using chemicals I will probably bin this one and keep an eye on the other two healthy ones, if they also get infected I will probably give up on Rebutias and grow only Cacti that are resistant to spider mites. Thanks for all your help and suggestions.