The orchid family (Orchidaceae spp.) is a diverse group of flowering plants beloved by gardeners who don't mind a challenge. Many diseases and disorders may cause brown spots on these sensitive plants, though if the spots are also sticky, they are probably the result of scale. Scale are sucking insects that attack leaves, pseudobulbs and rhizomes, sometimes causing leaf drop. Female scale appear as brown spots, while the sticky substance is plant sap, which adults and juvenile insects feed on.
For a mild infestation, dip a cotton swab in rubbing alcohol and remove insects by hand by rubbing the swab against infested areas. For a more serious infestation, you will need more thorough coverage. The St. Augustine Orchid Society recommends putting alcohol in a spray bottle and spraying the entire plant once a week for three weeks. Attempt even coverage, spraying inside crevices. You may also apply insecticidal soap with a toothbrush or spray the plant with a narrow-range horticultural oil such as neem oil. Keep any insecticide products safely stored away from children and pets.
To prevent scale infestations, only purchase healthy, pest-free orchids. Examine all new plants before bringing them into the garden. After you've removed scale insects from one plant, check nearby orchids carefully to make sure the scale insects have not spread. Remove dead leaf and flower sheaths as they appear, as these provide potential hiding places. Keep ants away from your orchids, as they can transmit scale. You can create an ant barrier by placing bay leaves around the base of the pot.
Soft, watery brown spots that grow in size may be the result of bacterial or fungal infection. Remove the infected orchid from healthy orchids, and use a sharp, sterile tool such as a razor blade to cut off rotting tissue. Sprinkle with cinnamon to help seal the wound. After about a week, if the problem is solved you can put the orchid back with the others. Keep standing water off the foliage, and water your orchids in the morning to allow them to dry out before nightfall.
Healthy, strong orchids are less likely to develop pest and disease problems. Orchids have varying care requirements depending on genus and species, though generally orchids require moist, warm conditions and regular fertilization. Orchids grown in bark should be fertilized with a nitrogen-rich fertilizer such as a 30-10-10 or a 15-5-5, applied as per label recommendations. Orchids not grown in bark prefer a balanced fertilizer such as a 20-20-20. Only plant where hardy. Nun orchid (Phaius tankervilliae) is suitable for U.S. Department of Agriculture plant hardiness zones 9 to 11, where it prefers light, filtered shade.