Ours is not an ideal world, and not everything that comes into contact with our planes is beneficial. Even the best cared-for plant can fall victim to random attack by pests or diseases, or suffer due to a sudden physiological condition.
The key to bringing the plant through the problem with the minimum of damage is to identify and deal with it quickly. Keeping plants healthy a id well-fed also helps, because a plant under stress for whatever reason, is vulnerable to attack and lacking in resources to fight it off.
Cleaning is also important, as a build-up of dust and pollution deposits can clog the pores through which the plants breathe, so that they weaken and become more susceptible to infestation.
Not every condition will mean reaching for a chemical, although it may be the only answer to a really persistent problem. Often, inverting a plant into a bowl or bucket of soapy water may be enough to cure a minor attack of greenfly, for example.
If chemicals are required, they should be used strictly in accordance with the manufacturer’s instructions, and sensible precautions must be taken if recommended, such as wearing gloves or a face mask. Children and pets should be kept out of the way while the chemical is being applied.
Whenever a plant begins to look unwell, or just not as healthy as usual, it is worth running through a quick mental checklist to establish a cause. When was it last watered? When was it last fed? Has the humidity level changed recently? Is the pot size right? Is the plant getting enough light? Could it have been chilled or overheated? Is there a visible pest? Is there a sign of disease (such as the fur of a mold, or the slime of a rot)?
Correct identification of the problem is crucial to the treatment, but it can be hampered by the fact that identical symptoms may have differing causes. Having identified the problem, the next step is to deal with it effectively. This is easier for some conditions than others, and it may be preferable to put the plant into isolation in a separate room for two or three weeks while it is being treated, before it can become a hazard to its neighbors.
For instance, a fungus such as gray mold will attack any plant that has damaged tissue through which the spores can find a point of entry. If the plant is being treated with a chemical, either as a spray or a drench, it is safer to apply it outdoors on a calm, windless day.