Question
I have in te past 10 months become a hobby orchid grower, and  
my family  has been very kind and given me an orchid from time
to time.  I now have 24!  Most are phalenopses and
dendrobiums.  Since I foolishly took the labels off each and kept
them together, rather than leave them with each orchid or found
some other way to mark  each pot, I cannot identify most of
them except by their species name.  Sorry.  My question
concerns the dendrobium leaves which are yellowing off from
time to time.  I do not believe it is too much water or sunlight.  I
water all of my orchids (except the small ones which I water
every week and are doing nicely).  And they are all in a shady
part of my back porch.  It does seem that when I bring some of
them inside (air conditioning)) they appear to be happier.  Could
you please tell me about the yellowing off of my dendrobiums?  I
must be doing something wrong!  Many thans, Betty Rogers

Answer
Hello Betty,

Congratulations on your new orchid hobby and being given so many orchids.  It sounds like your family really loves giving them to you because they know how much you're enjoying your new hobby.  

Too bad you took all the labels off the pots, but at least you know that you have phalaenopsis and dendrobium orchids, that helps.  

It is not unusual for dendrobiums to lose a leaf from time to time.  It is helpful to understand that there are two basic types of dendrobiums: evergreen and deciduous.  You can tell which yours are by where the flower stem grew from.  Evergreen types tend to produce flower stems from near the top of the plant while deciduous types produce flowers directly from the plump growth or pseudobulb.  Also, as the type name implies, evergreens keep their leaves for several years while deciduous ones drop their leaves after about a year but do not think the growth is dead, it is just dormant and getting ready to produce flowers.  .  

One other thing to look at are the leaves that have fallen off.  Is there a blackened area at the base of the leaf?  If there is, that would indicate a fungus infection.  How does fungus get there?  Fungus spores are airborne and all around us and when water is present overnight, the fungus rapidly grows and if left unnoticed, will eventually kill the leaf.  For this reason, it is always best to water orchids in the morning to allow enough time for any water on the leafs to dry before nightfall.  

Hope this helps.  If you have other questions, write back and I'll try to help with them.  It would be helpful to know where you live so I can tailor a response more suited to your area.   In the meantime, good luck and good growing.

Jim Kawasaki
San Jose, Ca.