1). Leaf edges brown and dried:

Temperature is too high... Move plant to cooler area
Lack of sufficient humidity... Set plant on a shallow, pebble filled tray of water

2). Rapid defoliation:


Rapid changes in temperature or light... Was plant moved to a new location?
Overwatering possible... Check for root damage or rot
Underwatering... Provide enough water to support full foliage
Exposure to hot or cold drafts

3). Gradual defoliation (lower leaves become yellow and fall):

Overwatering possible... Check for root damage or rot
Underwatering... Provide enough water to support full foliage
Insufficient light... Move plants plant to a brighter location
Lack of, or incorrect components of fertilizer

4). Leaves drop continuously or new leaves on tip are small and curled:

Pollutants in the air... (fumes from gases, chemicals etc.)
Spray or vapor damage from cleaning fluids
Possibly aphid or mite damage

5). Spotted foliage:

Overwatering possible... Check for root damage or rot
Sunburn... Too much direct sun
Cold water on foliage... Use room temperature water for watering or misting
Fungal infection... Especially possible if plants are in very humid, wet conditions
Pollutants in the air... Fumes from gases, chemicals etc.