Spots on ponytail palm

Nolina recurvata? Closest picture I can find.
10 year old.
Full screaming hot morning and afternoon sun indoor houseplant (atrium at a College) Yellow spots on old growth, no pots on new growth. Yellow spots are block shaped like the parallel veination. Spots range in size from 1/8 inch to 1/4 inch.
No thrips, mites, mealy or scale. Minimal fert. in spring. Dishsoap, alcohol and lemonjuice to wash leaves occasionally.
Whats its problem? Any ideas?
Thanks for your advice
Susiecatt
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Beaucarnea recurvata - family Agavaceae - what does that tell you? Minimum water, full sun. Your yellow spots could be a fungus if they follow the veins. Could also be Magnesium deficiency. Reduce the water, feed with a little Magnesium sulfate.
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