Leaves always turn yellow before falling?

[karrolhk]

Do orchid leaves ALWAYS turn yellow before they fall off in the following situations?

- overwatering (root rot),
- water standing on leaves (crown/stem rot)
- problem in fertilizing,
- sunburn,
- extreme cold/hot temperatures,
- dehydration,
- fungal and bacterial infections

If not, in which of the above situations do they fall off prematurely and are still green?

[estación seca]

No. They can go directly to brown or black if frozen or sunburned. They can be mushy green with bacterial rots. Fertilizer burn turns tips brown; it rarely would cause a leaf to fall off unless the grower makes a huge goof in mixing.

[aliceinwl]

In Phalaenopsis and Vanda that I’ve had dropping green leaves has accompanied stem or crown rot. The rot itself was likely symptomatic of a fungal or bacterial infection that took hold due to cool temperatures and/or water that stayed in the crown or leaf axils.

[karrolhk]

Thank you. Would trimming healthy aerial roots also lead to leaves turning yellow and then falling?

[estación seca]

Trimming off healthy roots means the plant can't take up as much water as before. If it can't take up water it will drop leaves. I advise people not to trim roots.