A beautiful orchid may look perfect in the store but have a few slight brown spots on the leaves when you get it home. Orchids can bruise in transport or with handling, just like ripe fruit. But if the brown or black spots grow in size or become soft and watery, your orchid has a problem.
Phalaenopsis (Phalaenopsis spp.) and dendrobium (Dendrobium moniliforme) orchids can develop black or brown spot problems, but according to the American Orchid Society, the beautiful, showy cattleyas (Cattleya spp.) are especially susceptible to black spot problems caused by fungus. Phalaenopsis and cattleyas are hardy to U.S. Department of Agriculture zones 10 through 12, while dendrobiums thrive in USDA zones 9 through 12, but they are all commonly grown as houseplants in all zones.
Fungi in the water source or in water left standing in the drip dish may cause black spots on orchid leaves. Pythium ultimum and Phytophthora cactorum are likely culprits, and they are present only in water. If the fungi spores happen to land on an orchid leaf, they quickly penetrate and begin the disease process that produces brown spots.
You may notice tiny brown spots on the soil line of new seedlings or on new leaves of more mature plants. If untreated, the brown spots become black and grow in size. The affected leaves may turn yellow and spotted areas may exude water when pressure is applied. The disease progresses from outer leaves downward to the stem and crown of the orchid. When disease progression has reached the crown area of an orchid, it may be better to replace the plant entirely. The rot spreads quickly and at this point of progression, it may kill the entire plant.
Small yellow or brown spots which develop on either the upper or lower leaf surfaces may be caused by other types of fungal pathogens. Fungus problems can get started by inadequate air circulation combined with continuous over-watering.
Separate the diseased orchid from other orchid plants to begin treatment. Cut out all brown or black spots on the leaf and apply fungicide to the healthy tissue.
The fungicide may be as close as your spice cabinet, because cinnamon is a popular choice with experienced orchid growers. According to the American Orchid Society, you can apply the spice straight from the jar. Sprinkle it directly on the exposed areas where you cut off infected portions of the orchid.
Commercial fungicide products containing a mixture of copper sulfate and lime -- called a “Bordeaux” mix -- are often used to treat black spot problems on orchids. Apply the fungicide after the spots have been removed. Allow the cut leaf to dry and use wooden toothpick to apply the fungicide. Cover the entire wound edge, making sure no areas are left exposed. Press down the edges firmly so water cannot seep into the wounded area.
How to Prevent the Problem
“An ounce of prevention is worth a pound of cure” is certainly true in the case of black spots on plants. Orchids have a reputation for being fussy and hard to grow but they are simple once you know a few basic rules.
Orchids do not like water standing in their roots. Water about once a week, though temperature will affect your orchid's water needs.
Always use potting medium made especially for orchids. Typical orchid potting medium materials include fir bark, lava rock, sphagnum moss and perlite.
Ensure an open window or fan set on “low” provide good air circulation for your orchids.
Place your orchid in an area of the house that has several hours of either morning or afternoon sun. They prefer the filtered light provided by a curtain or bamboo shade.
Many growers recommend fertilizing orchids “weakly, weekly” with a balanced fertilizer made especially for orchids, containing little or no urea. Mix 1 tablespoon into a gallon of water and do not splash on the leaves as you irrigate the plant.