Can someone please help with the cause of yellowing of bottom leaves of bonsai umbrella tree. The new leaves look fine. I'm in Australia in autumn but still 28-15 deg so not sure if I should repot at this time. Thanking you
Various nutrient deficiencies often show as yellowing of specific areas on plant leaves, sometimes on older leaves first, sometimes new ones, depending on the particular deficiency. Unfortunately, that doesn't seem to be what's happening here - there is a mosaic or ringspot pattern appearing even on the leaves which are currently still green, so the most likely cause of the problem may well be viral infection. If it is viral, there's no treatment I'm afraid. Its possible that the leaves may remain on the plant, just completely yellow over time, or they may turn to yellow, then brown and fall, it's hard to predict what course any viral infection may take. Some information here on causes of chlorosis https://www.rhs.org.uk/advice/profile?PID=772
I hope I'm wrong and someone else will come up with another answer that might mean you can take remedial action. It's worth checking carefully beneath the leaves and all the stems on the off chance it's a pest invasion, in hopes it's something easier to treat like that.