Diseases, disorders and deficiencies are difficult to diagnose. People can usually tell if a plant isn't completely healthy but to pinpoint the actual problem is not always easy. Once you notice your plant is struggling, the best way to diagnose the problem is to cross reference the recommended treatment with how its actually being kept. It could mean a great number of things if the leaves on your plant are turning yellow. If you check the guide and it recommends to keep the soil moderately moist at all times yet the pot your plant is sitting in isn't properly drained and therefore soaking in water, then we can pinpoint the problem.

This method of diagnosis works well if the problem is caused by improper treatment, but what about moulds and viruses? These are usually diagnosed by oddities in your plants appearance. Black spots, moulds, fungus, these are all signs of disease and may require some form of chemical treatment to combat the problem. Purchase the appropriate chemical from a nursery and be sure to follow the instructions carefully.

Keep in mind that resorting to chemicals may not be the best decision. I wouldn't recommend using chemicals if you're worried about small children or pets in the house regardless of how safe the product claims to be. Also, the cost of the chemicals may be more than the cost of a new plant. Replacing the plant may be the best alternative

Surface Mould: Overwatering

Sun scorch / Neglect

Lack of Nutrients

Neglect