The yellowing of orchid leaves are not always the sign of unhealthiness, since this may happened naturally to old leaves as a part of the ongoing aging process.
Orchid leaves are important indicators which can tell us at an early stage that something is going wrong before your orchid condition might get worse.
Healthy leaves suppose to have a light green color and eventually become darker when they are getting older.
Yellow coloring of newer or younger leaves seem to be the most common indication that you should worry.
This is a sign that your orchid is in trouble because of several possible reasons.
The most frequent cause is due to excessive or over lighting.
This often occurs during summertime when sunlight intensity is at the maximum while unintentionally placing your orchid at the wrong spot.
To simplest way to solve this problem is by moving your orchid to another spot with lower light or use a garden shade net if you manage an orchid garden.
Yellow leaves can also indicate over watering which is usually accompanied with a rotten root.
So, please check the condition of the roots.
If the yellow leaves also show some black areas this may indicate a fungal infection at the rotten roots.
Fungi like to stay in very humid environments.
You can solve this problem by stop watering your orchid for about one to two weeks, then start watering again once in a week.
The fungal infection can be treated by sprinkling the rotten root with ground cinnamon.
Aside of over lighting and over watering, yellowing leaves also indicates possible bacterial or fungal infection.
The leaves show yellow spots with black edges which differentiate it from the evenly spread yellow color due to over lighting or watering.
The suggested treatment for bacterial infection is Physan, while for the fungal infection a fungicide will do well.
A traditional remedy for both bacterial and fungal infection, as mentioned earlier, is to sprinkle the infected root with ground cinnamon.
Other deviation that can be encountered is the brown to black tips of the orchid leaf.
This condition may suggest overuse of a fertilizer that contains minerals.
This is also common when using water with high mineral content such as well water.
In those cases try to stop using the fertilizer and check whether you use the right concentration as recommended in the label.
If you are suspicious that your well water is the cause, it is best to check the mineral content of your well water or you can replace it with purer water such as rain water.
Brown tips that become larger may be a sign of fungal infection where you need to amputate the brown portion of the tips and treat the remaining part of the leaf with fungicide.
Remember to use a sterile cutter or razor blade and to cut the leaf at the healthy part.
Also check the humidity around your orchid that is probably caused by over watering or bad water drainage of your pot.
As mentioned earlier fungi favor humid environment and will eventually cause infection.
Mottled or striped brown to black leaves may imply viral infection.
To establish a sound diagnosis it is recommended to send your orchid to an agriculture research station, where they can treat your orchid and make a closer study and perform some experiments on your infected orchid.
Actually there is very little we - as an orchid hobbyist - can do when it comes to viral infection.
Shriveling of your orchid leaves is an indication of poor watering that causes low humidity in the potting substrate.
This problem can be solved by implementing the correct watering schedule to your orchid.
Aside of the signs that are relayed by a leaf, another distress can be expressed by the orchid pseudo-bulb.
If the pseudo-bulb becomes shriveled, then this a serious sign of dying roots, where it could be too late to save your orchid.