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I have owned a Mammillaria Nejapensis for the last few months which began showing unhealthy yellow spots a few weeks back and seems to be getting progressively worse. A photo of the cactus is attached.

Unfortunately, my room barely gets any natural light, which is why I chose to illuminate the cactus with an artificial plant-growth LED array from Amazon. I use a timed outlet to expose the cactus to 8 hours of LED light daily.

Would anyone happen to know if it is likely the the yellow spots are a result of insufficient lighting or some other cause I am unaware of? If it is the former, is there a lamp that might do a better job than the LED array I currently have?

cactus with yellow spots

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    Looks more like sunburn, maybe the light source is too close to the plant? – benn Mar 25 at 8:17
  • The array is not supposed to emit any UV light as far as I understand: only optical blue and red. Is it possible to sunburn a plant with optical light only? The source is, perhaps, 3-5 cm away from the plant. – Roman Mar 25 at 8:59
  • Sunburn in humans is caused by UV, but in plants it is also caused by high intensity light. I think 3-5 cm is pretty close for a 20 W lamp... Let your plants slowly adapt to high intensity light first, before you give them the full dose. – benn Mar 25 at 9:13
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That is not sunburn. That is a fungal infection similar to rust. You need to get an anti-fungal/anti-bacterial spray and use as directed. These generally contain sulphur or other fairly benign compounds.

I've had this on plants over the years. It (usually) won't kill it, but it does make marks. The marks don't go away, but the orange color will.

As the plant grows the marks will be less noticeable

This has nothing to do with artificial light. 3-5 CM looks about right. I don't see any UV stress or any etiolation from too little light.

The plant is a Mammillaria karwinskiana.

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