The first question to ask yourself when you’re faced with a pest or disease problem on a plant is… is this a major problem or minor one? If it’s a major problem, if you risk losing the whole plant, if it might spread to other plants, of course, it’s perfectly acceptable to react, and to do so without delay at that (using the most environmentally-friendly method possible, of course!)
If the problem does not affect the plant’s long-term survival or value, however, there is no need to intervene. For example, when powdery mildew infests leaves late in the season just when the plant is on the verge of going dormant anyway, when there is only one caterpillar on the plant, when there are just a few spots on the lower leaves, etc. Remember the 15 pace rule: if you can’t see the “problem” when you back up 15 paces, it isn’t probably something you need to bother treating.
That is, I admit, a hard pill to swallow for many gardeners, who associate “garden” with “perfection” and instinctively want to grab the most toxic substance that exists to blast that problem to smithereens, but… a few holes in a flower or a leaf that is a little bit chewed on cause no significant harm the plant’s health. It’s important to understand that nature is very complex and when treating one problem, you tend to provoke others.
Quite often the wisest thing to do is to do nothing at all! ‘Nuff said!