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I found some G. elwesii plants with yellowish striped leaves, after strong frosts this symtomes are clearer to see. I think, this is a virus and selected this plants!
Yes it certainly looks well virused Harald-Alex, it obviously had virus to start with and the stress of the weather has increased the signs
Malcolm: I think those spots are nothing to worry about. Water them with mycorrhiza (Trichoderma harzianum for example) and your plant will be fine next year.