A sickly-looking plant may not be suffering from a disease or pest attack, but instead from, quite simply, stress.
Symptoms of stress include wilting, discolored foliage and leaves with ragged or dry edges. Stress in plants is most often caused by environmental factors and/or relates to the plant's position in the landscape.
Wilting can be caused by too little water or too much water; check the soil about 6 inches down to determine if it is too dry or waterlogged. Treat the plant by either watering or allowing the soil to dry out before watering. To prevent this type of stress, make sure the site has appropriate drainage for what you are planting, before you plant it. Also choose plants whose moisture needs will be met by the typical rainfall in your area.
Discolored foliage may be bleached or blackened. Bleached leaves indicate the plant is getting too much sun, or has been moved from a dimmer location (such as indoors) to bright outdoor light too quickly. Treat sunburn by moving the plant to a shadier location, either permanently or temporarily while it adjusts. Plants can usually outgrow sun damage. Prevent sunburn by choosing a site that matches the plant's light needs, and by moving houseplants and seedlings outside gradually, giving them just a few hours in the sun each day to start and working up to full-day sun.
Leaves that are black have been damaged by frost. Prevent this by putting plants outside after danger of frost has passed in the spring, and bringing them inside before the first frost of the fall. If frost blackens your plants in the spring, leave the black parts until the plant begins to grow again, then cut them away.
Leaves that have ripped, ragged edges or dried edges may be suffering from wind damage. Either transplant them to a more sheltered location or add some sort of windbreak near them, be it a trellis or more plants.
The bottom line with plant stress is to prevent it from happening. Understanding each plant's needs and siting it accordingly goes a long way toward this goal.