I need some help on understanding why my dendrobium has had two lower leaves turn yellow in a relatively short time (last 2-3 weeks).
And on my cattleya, one leaf has turned yellow during the same time frame.
Please see attached pictures.
Dendrobium, top side of yellowed leaf
Dendrobium, bottom side of yellowed leaf
Cattleya yellowed leaf
If my memory serves me correctly, the dendrobium had some dark spots on them (mottled looking) when they first arrived about 4 weeks ago, but they were green. It seems that when I applied lighting with CFLs about 2 weeks ago, they yellowed much faster. This could very well be coincidental, because I think they had problems to begin with.
1) Any idea what is causing this? What disease?
2) Should I simply cut the leaves off immediately before it spreads?
3) Is there any preventive steps to protect the orchids going forward?
Any information and advice is much appreciated. Thanks in advance.