Powdery Mildew is a fungal leaf disease which is common in commercial greenhouses and is becoming more common in indoor gardening all around the world. Left untreated, this disease will kill every leaf on a plant and destroy your crop. It is first seen as white powdery looking spots on leaves.
These spots grow and join together and soon the whole leaf becomes white. If you shake the leaf, you will see a white dust or powder flying around. This powder is the fungus' spores by which it spreads. The parts of the leaf that are white are the main body of the fungal infection.
The fungus grows on the leaf and sends root-like appendages into the leaf cells and suck out the cell contents. This kills the cell. As this process occurs, the fungus spreads, killing more and more leaf area until, eventually, nothing is left. If you kill the fungus on an infected leaf you can see for yourself the extent of the attack.
However, the damage is actually more extensive than you can see with the naked eye. Even though you have killed the Powdery Mildew with PMT you may still find that infected leaves may still die due to devastating damage caused by Powdery Mildew. Remember, Powdery Mildew damage is always more extensive and serious than it first looks.