Sticky Fungus on Orchids

Orchids are susceptible to scale insects, which cause sooty mold fungus.

Orchids are tropical plants often grown outdoors or used indoors as houseplants. More than 30,000 species of orchids exist in many shapes, sizes and colors. While orchids are generally healthy, they are susceptible to infestations of insects that cause sooty mold to develop. Sooty mold is a sticky black fungus that adheres to orchids, creating a gooey mess on the plant's leaves, flowers and stems.

Identification

Orchids are susceptible to infestations of several types of scale insect, according to the Missouri Botanical Garden's website. These insects feed on plant sap by sucking it from the leaves of infested orchids. Scale insects attach themselves to the leaves of infested plants, where they remain for life. These insects are usually brown and hard-shelled or have a white coating of powdery wax on their bodies. Infestations of scale insects can cause a fungal disease known as black sooty mold to develop on orchids.

Cause

Scale insects exude a clear sugary liquid known as honeydew after feeding on large quantities of plant sap. This sticky substance drops from the scale insect after it is excreted and lands on the surface of flowers, leaves and stems causing black sooty mold fungi to stick there. Sooty mold fungus feeds on the sweet honeydew and fungal spores may be carried on the wind to other plants nearby. Although sooty mold fungus does not damage the foliage or flowers of infested plants, it can cause reduced vigor because of low sunlight. Black sooty mold causes orchids to become unsightly and unattractive when covered with the sticky fungus.

Cultural Control

It is necessary to eliminate scale insects and other honeydew-producing pests to control sooty mold fungi. Spraying your orchids with a strong, steady stream of water two or three times each week can knock scale insects from the plant and reduce infestations. You can also remove scales with alcohol-soaked cotton balls. Place the cotton ball on the scale insect and wipe from the plant. This process works best with light infestations of scale insects.

Chemical Control

Insecticidal soaps can reduce heavy infestations of scale insects. These products must make contact with the insect to be effective. Horticultural oils are another product also used in control of scale insects on orchids, which work by coating the insect and causing it to suffocate. Horticultural oils can also loosen and help remove sooty mold fungi from orchids, according to NC State University. Some plants are susceptible to the effects of soaps and oils, making it necessary to test the product on a small portion of the plant before applying it all over. If no damage is visible after seven days, it is safe to apply to your entire plant.

References

• Iowa State University: Orchids As Houseplants
• MOBOT: Orchids: Problems
• NCSU: Sooty Molds-What And How; Steven D. Frank; Stephen B. Bambara