Even in the best run garden things can go wrong - pests attack, diseases get hold, the weather strikes and, heaven forfend, gardeners may even make mistakes.
The secret of beating this avalanche of ailments is recognising them, or more often the damage they cause - and sorting the Bad from the Good and the Ugly . . . because most things that look as though they may be the devil's own work are actually harmless or even beneficial! (Or even things you can't do a lot about, like the Weather!)
"Kill as few things as possible - in life as well as in the garden." Has always been my motto.