Orchids are generally disease-free houseplants, but are susceptible to some pathogens that can cause brown spots. Orchids can contract rot diseases, anthracnose, leaf spots and petal blight, all of which can present as brown spots. Providing proper light, water and a coarse bark-based potting media will help to prevent most diseases of orchids.
Black rot, brown rot, bacterial soft rot and bacterial brown spot are all rot diseases of orchids that can present as brown spots. All diseases first appear as small brown lesions or spots on either side of leaves. Unless the disease is severe, the first step of control is to remove the infected leaves. Apply a fungicide containing copper compounds as a treatment. Always observe label instructions, especially if applying chemicals indoors. In extreme cases, discard the entire orchid.
Anthracnose is a fungal disease which can present on orchid leaves as brown masses. The disease begins at the leaf tips and then works itself down toward the base if untreated. Implement a regimen of both systemic and protectant fungicides. Fungicides containing thiophanate methyl are effective systemic fungicides for orchids. Fungicides containing mancozeb are effective protectant fungicides. Anthracnose is most active in warm, humid and dark weather. Adjusting temperature and light levels as needed can help to prevent anthracnose.
Leaf spots on orchids can be caused by a range of fungal pathogens including Cercospora, Guignardia, Phyllosticta and Septoria species. Symptoms present and form as small yellow to brown lesions on either the upper or lower leaf surfaces. Spots may merge to form large brown blotches which, if untreated, lead to leaf drop. Treatment for leaf spot is the same as treatment for anthracnose. Alternating applications of both systemic and protectant fungicides containing thiophanate methyl and mancozeb will help to control these diseases.
Petal blight, a disease of orchids caused by the pathogen Botrytis, appears on the flowers, not the leaves. The flower petals develop a speckled appearance consisting of small brown spots. Untreated petals may develop a grey fuzzy mould. Petal blight decreases the lifespan of the orchid blooms and leads to premature petal drop. Remove infected flowers. Use a protectant fungicide containing the active ingredient daconil to treat orchids with petal blight.