Brown Sticky Spots on an Orchid

Aphids

• Aphids are green, yellow or pink insects less than 1/8 inch long, with soft bodies in the shape of a pear. Two tube-like projections called tail pipes poke out from their abdomens. Aphids cluster in groups, feeding on young orchids, flower buds and flowers. They are attracted to the fluid excreted by Cymbidiums and some other species of orchids. As they feed, they excrete a sticky substance commonly called honeydew on which fungi sometimes grow, forming sticky brown or black spots. Use warm water and detergent to wash aphids from your orchids or spray with insecticide soap containing the active ingredients diazinon, carbaryl, malathion or Orthene.

Scale Insects

• Scales appear as elliptical to round bumps from 1/16 to 1/8 inch long. They are brown, grey or white and usually feed on the bottom of leaves, under leaf sheaths and along the midrib. Both the common brown soft scale (Coccus hesperidum) and the Boisduval scale (Diaspis boisduvalii) leave honeydew on which sooty mold and other fungi can grow, causing sticky brown spots.

  Inspect orchids carefully for scale before you add them to your collection. When you find one that is infected, quarantine it until you can get rid of the scale. Use a swab dipped in alcohol to remove scales as you find them. If your infestation is severe, spray your orchids with an insecticidal soap containing carbaryl, diazinon, malathion or Orthene. Spraying is best against scales called crawlers that are too young to have shells. To kill crawlers as they hatch, spray your orchids weekly for several weeks.

Bacterial Spots

• Bacterial brown spot appears as a watery, brown blister on orchid leaves and spreads quickly. It develops when orchid leaves remain in cold, wet conditions. Phalaenopsis orchids are most vulnerable. Brown rot begins with a small brown spot on an orchid leaf that grows quickly and can kill an orchid if it reaches the crown. Phaphiopedilum orchids are most at risk. When these diseases appear you should remove all infected areas immediately. Spray your orchids with a fungicide containing a copper compound.

Septoria Leaf Spot

• Septoria leaf spot caused by the fungus Septoria selenophomoides develops on wet orchid leaves. The spots are at first sunken and yellow, growing to dark brown to black lesions. Make sure your orchid leaves are allowed to dry. If necessary spray monthly with a protective fungicide containing the active ingredient mancozeb, or a systemic fungicide such as thiophanate methyl.