Given enough sunshine and rain, most plants will thrive with little care. That's why it's so disappointing to find sick plants in the garden. Gardening advice abounds in books, on the Internet and from well-intentioned neighbors. The best advice, though, is to observe your garden for a week or two and check the basics before investing in expensive chemicals or fertilizers to perk up ailing flora.
Describe discolorations of leaves; spots are generally signs of bugs and pale leaves between veins or visible veins are often indicative of fungal or bacterial infections. Start with the simplest problems; water and light; then consider more complex possibilities in your diagnosis. Make a list of environmental changes like heavy rains, heat or drought that have taken place as your plants began to sicken.
Most garden plants do best in friable, slightly acidic soil. Friable soil drains well but not too fast; water should drain within half an hour. Roots that sit in water rot, causing discolored leaves and die-back. Even after the surface is dry, friable soil holds some moisture at a depth of 2 to 3 inches so that roots can get both air and water. Most garden plants fade if soil pH does not fall between 6.5 and 6.8 but some prefer "sweeter", more alkaline soil. Lighten heavy soils with compost, humus and sand; add limestone to over-acidic soil and sulfur to alkaline soil according recommendations made by your state university agricultural extension agent after a soil test.
Aphids, mealybugs, thrips and whiteflies are common garden pests. Some pests, like aphids, can cover new growth and eat it up within days. True bugs suck the juices out of leaves and may leave lacy patterns or distorted tissue. Some pests, like whiteflies (which rise in clouds from disturbed foliage) are merely nuisances. If plants are wilting and stunted, fungus gnat larvae may be eating underground portions of stems.
Prune foliage that is under attack from pests; insecticidal soaps or sprays may help but will not completely eliminate most common pests. Match pesticide to the pest and use chemical controls only according to label directions.
Spotted, yellowed or curled leaves may be a sign of fungus or bacteria infections. Powdery mildew (looks like it sounds) takes hold when plants are crowded together or don't get enough sunlight. Fusarium wilt attacks plants located where infected plants of that species have lived in past years.
Botrytis blight causes splotchy leaves and twig die-back; hold back water during periods of high humidity and prune back affected growth. Reduce ground cover and clean up decaying matter where fungi and bacteria grow.
Avoid crowding or over-watering or over-fertilizing plants to allow them enough sunlight and air to help fight pests and disease. Shade faded or dehydrated plants at mid-day to allow them time to recuperate. Make sure garden plants that wilt in the afternoon get their weekly inch of water--more during drought--in the early morning, not evening when fungi and bacteria will have the cool night hours to grab hold of plant tissue. If all else fails, take your list and snip a part of an ailing plant and take them to your local state university extension office for diagnosis.