Diagnosing the Problem
Phalaenopsis and other orchids respond quickly to anything amiss in their environment. Black spots are one of the trouble signs. The first step in treating the dark spots on your orchid's leaves is diagnosing the problem. Some phalaenopsis have naturally mottled leaves, so it may be natural coloration. However, leaf spotting can also signify a bacterial or fungal disease. Bacterial leaf spot is fairly common among orchids, and it can be aggressive and dangerous on phalaenopsis.
Similarly, fungal disorders and cause leaf spots, especially if the plant is left exposed to moisture on chilly nights.
Unless the plant is valuable, the best approach is to discard it, as the disease is highly contagious and will spread from plant to plant from splashing water.
A danger to orchids any time the weather is rainy or damp for extended periods, black rot can quickly destroy an entire plant if left unchecked. Caused by one or both of the fungi Pythium ultimum and Phytophthora cactorum, black rot affects a wide variety of orchids
A Fungal Infection
The fungi that cause black rot in orchids consist of spores capable of independent movement, called zoospores, which swim through the water. If that water happens to be sitting on an orchid leaf, the zoospores can penetrate the plant tissue and begin the next stage in their life cycle. Once this occurs, the visible signs of infection - small, watery, translucent spots - expand quickly and change to brown and then black.
Left untreated, the affected plant or plants may infect others and will probably die themselves.
As it name implies, black rot shows up as dark blackish spots or lesions on the affected plant part. The black spot or spots enlarge rapidly and can spread throughout the plant. If allowed to reach the crown of a monopodial (single-stemmed) orchid, the rot will kill the plant.
Affected leaves may turn yellow around the infected area, and the lesions themselves will be soft and exude water when pressure is applied.
Unsterilized pots, media or water sources, as well as water splashed from affected plants nearby, are all excellent means of spreading the fungus. If your orchids are outside, keep them 3 to 4 feet above the ground to avoid splash contamination. Whether outside or inside, do not allow your orchids' leaves to remain wet for any extended periods. Good air circulation in the growing area is key. At first signs of infection, segregate all affected plants to avoid contaminating healthy plants. Finally, some growers recommend the use of a high-calcium fertilizer in the spring to help avoid black rot in new growth.
Cut Out the Rot
To stop the spread of black rot in your orchid, begin with a sterile knife and cut out the diseased portion of the plant. Remove the infected portion of the leaf, or the entire leaf if necessary, to halt the spread of the fungus before it reaches the crown. Place the plant in an area that receives good air circulation to allow the cut to dry.
Use a Fungicide
Once the infected plant parts have been removed, a fungicide should be applied to protect the remaining healthy tissue.
Cinnamon is an excellent fungicide, and ground cinnamon may be applied straight from the spice jar directly to the exposed area where infected portions of the plant have been excised. First Rays Orchids also recommends mixing cinnamon with either casein-based glue (such as Elmer's) or cooking oil to form a thick paste. This paste is fairly waterproof and can be used to cover the wound.
Drenching the affected plant with a fungicide is also an option. Drenches of a protectant fungicide such as Truban or Terrazole are recommended if the disease is diagnosed early. For more advanced cases, a systemic fungicide such as Aliette or Subdue is more effective. Captan, Dithane M-45, BanRot, Subdue and Physan 20 have also been recommended by some growers for control of black rot.