Plant Disease Diagnostics
Do you need help diagnosing plant problems? Visit the University of Minnesota Extension's Plant Disease Diagnostics Web page for help.
The page is organized so that you can search by plant type, then by the part of the plant affected. Once you drill down to this level, you'll find photos of common problems affecting this plant. Click on a photo and you get general information about the problem, including the causal agent (which may be a fungus, insect, or environmental condition), plus control measures. In many cases organic or cultural controls are given.
Unfortunately, there isn't a corresponding Plant Insect Diagnostics - at least not yet. However, you can find links to information and photos about common insects at their Insect Journal page.
Identifying and Managing Garden Pests
Three of my favorite books for identifying and managing garden pests are all by Barbara Pleasant: The Gardener's Bug Book: Earth-Safe Insect Control (Storey Books, 1994, $14.95 ), The Gardener's Weed Book: Earth-Safe Controls (Storey Books, 1996, $14.95), and The Gardener's Guide to Plant Diseases (Storey Books, 1995, $14.95).
The books include an introduction to the subject, including how the various "pests" fit into the overall ecology of the garden. Then there is detailed information about individual pests, with details on "earth-safe" control measures. Emphasis is placed on maximizing plants' ability to fend off disease and insect attack and on gardening methods that keep pests in check. Detailed indexes and appendices make it easy to look up problems and solutions.