Houseplants Under Attack

By Katie Duttweiler
Plant Pathologist
Iowa State University

Houseplants are susceptible to attack by various pests and diseases. While rarely lethal, pest and disease outbreaks can compromise the aesthetic quality of indoor vegetation. Even tightly sealed houses and conscientious tending cannot always prevent attacks on our houseplants.  

Some common pests that attack houseplants include mites, scale, mealy bugs, aphids and whiteflies. Mite infestations can leave leaves yellowed, speckled, bronzed and covered in webs.  Plants infested with scale are covered with either hard or sticky brown bumps. The stickiness is due to the production of honeydew, the sugary excretion of the scales. Plants covered with sticky, white powdery clumps indicate a mealy bug infestation.  

If your plant becomes engulfed in a cloud of small flies whenever you nudge it, you may have an aphid or whitefly problem. Whiteflies can cause leaves to yellow and drop.  Aphids can cause stunted and deformed plant growth. Both produce honeydew, which can be a nutrient source for various gray to black fungi known as sooty mold.  

Other unwelcome organisms, such as fungi and bacteria, lead to a variety of plant diseases. One type of fungal disease, to which I’ve now lost four peperomias, is root rot.  Root rots are caused by various fungi that are adapted to life in the soil or on plant debris. These fungi invade and grow in susceptible roots, which then become dark and deformed. Infection can spread through the root system and cause browning and girdling of the stem. The fungal attack interferes with water movement, which explains why a common symptom of root rots is wilting and browning of leaves. Any infected tissue may also become soft, as I noted on the mushy stem of my dead peperomia, which was easily detached from the root system at the soil line.   

Other diseases blemish the appearance of houseplant foliage. Leaf spot diseases are caused by various fungi and bacteria. Leaf spot colors can vary from dark brown to light tan and can appear either dry or watersoaked. Some spots have dark margins, while others have yellow halos. The spots, which can vary from round to angular in shape, often begin as pinpoints and can grow to cover the entire leaf. Leaf spot infections inhibit normal functions of leaves, such as photosynthesis and respiration. Infected leaves may drop prematurely.  

White to gray, powdery leaf spots are due to another fungal disease called powdery mildew.  Unlike leaf spots, powdery mildew can be rubbed off the plant surface. This disease is favored by humid, cool environments such as damp basements or drafty areas.

Pests and pathogens can generally be controlled by following good plant care techniques. A good start is to be sure that houseplants brought indoors are pest- and pathogen-free. When purchasing new plants, inspect for bugs and diseased areas, and buy only those that are vigorous looking.  Thoroughly inspect any porch or patio plants that you bring in for the winter.

Pay particular attention to watering, since overwatering is one of the most common causes of plant problems. Root rots can be triggered by overwatering. Waterlogged soil does not contain enough air for roots to breathe, so they are too weak to fight off fungi. Rotted roots cannot uptake water, which causes the rest of the plant to wilt. While it may seem counterintuitive, the last thing a root-rotted, wilting plant needs is water. If watering does not perk up wilted plants, investigate the roots for symptoms of rotting. Also, be careful when watering to avoid drenching the foliage, since fungi and bacteria can be spread by water splashes.

If there is a pest or disease problem, isolate the afflicted plants. In the case of severe problems, discard the entire plant along with the soil, sterilize the pot with bleach or alcohol before use again with disease-free potting soil. In less severe cases of insect infestations, simply remove the infested plant tissue or smother the pests with a mild detergent solution.  In mild cases of disease, discard any infected plant tissue before removing the plant from isolation.  

Almost all houseplants are susceptible to any of the pest and pathogen problems discussed above. Fortunately, these particular houseplant problems are a nuisance at worst and are easily controlled as long as your houseplants get the attention they deserve.

Contacts :
Katie Duttweiler, Plant Pathology, (515) 294-0589,
Jean McGuire, Continuing Education and Communication Services, (515) 294-7033,