Thank you for your question. Where are you and your plants located? How much do you water them? Are they kept indoors or outdoors? Have you checked the leaves for spider mites? Thank you!

Dear Sir,
Thank you for your response.

1. These plants are located in UAE warehouse provided by ventilation.
2. Our staff is watering once in a week  depending on soil moisture.
3. Plants are arranged in indoor condition 

Thank you for the additional information.  The presence of dark spots on the leaves is indicative of a fungal infection.  Fungi grow on leaves when the plant is watered overhead, rather than at root level, since fungi need a moist environment to grow.  Many of the leaves have already been killed by the fungus, judging from the first set of pictures.

I'm giving you links to two sources of information for the maintenance and prevention issues you requested.  The first link is a scientific article out of the University of Florida.  The second link is a less scientific source, but is shorter and contains essentially the same information.  You will probably have to remove all of the leaves that are totally dead, as well as ones showing any spots, to remove the fungus.  And application of a fungicide, along with a change in the manner you water and increasing space between the plants, should save the remainder of your stock.

Hope this is helpful.  Good luck!