The most important thing to learn as a pesticide-free gardener, is that not everything that looks like a problem, is a problem. Many insects are beneficial to plants and serve as predators to would-be pests. A problem that appears to be the result of disease or fungus, could actually simply be a case of too much sun, or too little water. Before you react to what you think is a pest problem, do a little research! If you reach for a pesticide before correctly identifying that you have a pest, you could actually make the problem worse or create new problems - by killing or deterring beneficial species from the area.
The websites below will help you identify common pest problems in the garden, and also suggest alternative ways of dealing with them!
Environment Canada: Roses and Other Flowers
Environment Canada: Healthy Trees and Shrubs
North Shore Recycling Program: Biological Pest Control
NCAP: Pesticide Alternatives Factsheets (from Aphids to Weeds)