As with soil-based gardens, hydroponic plants require good pest and disease control habits.
The alternative is the same as with "ordinary" gardens: spindly or dead plants. Since the majority of hydroponic plants are fruits and vegetables, that means: plants not worth eating.
But the situation with hydroponics is even trickier, since disease and pests have it much easier in this setting. Plants are constantly kept wet, either immersed in water ("true" hydroponics) or continually sprayed (aeroponics) or reside in an always wet medium such as perlite or sand. Fortunately, as with soil-based gardens, there is an array of techniques available to manage the problem.
Using beneficial organisms is one popular way to control unwanted pests, including certain types of bacteria and fungi. These help control spider mites and other invaders by crowding them out, eating them or releasing compounds toxic to the pest. They're called beneficial because they do all that without damaging the plants themselves.
Pesticides of different types are available, too.
Pesticidal soaps have been in use since the 18th century and still provide effective and non-toxic ways to keep the pests down. One sub-category called botanicals are compounds released by plants themselves that have been packaged into an easy-to-use pest control method. Botanicals break down naturally from exposure to air and water and leave no harmful chemicals behind.
Neem oil can control over 400 different types of bug that commonly invade gardens, including hydroponic ones. A simple spray to the leaves can often eliminate common pests. The bugs absorb the oil, which interferes with their ability to reproduce, leading to a lower population.
For more serious infestations, many commercial pesticides continue to work well.
White flies, aphids, mites and other pests can be a problem in hydroponic settings, just as in soil-based gardens. Powdery mildew is a common problem. In fact, because of the continual moisture, they all have a "friendly" environment. Making it "unfriendly" is easy though, using fungicides and organicides. Sulfur-based compounds help control white flies, mealy bugs, thrips and more.
Pyrethrum continues to be a safe and effective means of control. Though it has a scary chemical formula and a "non-natural" sounding name, it's derived from flowers. This class of natural compounds released by plants are extracted and used in many commercial insecticides. Dosage is low, so the compound is safe when used correctly. Azatrol is a broad spectrum insecticide that provides easy control over most common pests.
Hydroponic gardeners have to exercise additional care when using any disease or pest control method, though.
Since no soil is present to secure the roots, it's easier to damage a plant when manipulating the leaves and stems. That means, for example, that if you pick off mites by hand - an effective method for low-number infestations - it's important to exercise extra care.
Since moisture is present, mildew and other fungi are more common. Keeping leaves dry and just the roots wet will help. Any insecticide sprayed on should be allowed to dry under the lights. For aeroponically grown plants, for example, that may require a temporary relocation.