Diagnosing Houseplant Problems from Diseases

Growing indoors plants is a rewarding hobby that yields beautiful results.  But when problems arise with your indoor plants, it can be difficult to determine what to do to fix the problem.  

When you encounter a houseplant problem, first ask yourself, Are the environmental conditions appropriate for this species?

If problems arise, start by understanding the needs of that particular species for light, soil, water, humidity, temperature, and fertilizer.  If these environmental conditions are not ideal, they must be changed or the houseplant must be moved to a location where they are ideal.

Many problems are not caused by a single factor – they are the result of several factors coming together to cause an unhealthy or unattractive plant.  Always look for more than one factor that could be contributing to the issue and correct all of them to solve the problem.  It is also important to identify the primary issue.  Some problems are caused by other factors and while you can address the problem, if you don't solve the primary issue the problem will always return.

Diseases of Houseplants

Diseases can also be a serious problem for houseplants but are often secondary symptoms caused by poor environmental conditions and/or care.  Diseases are typically caused by fungi, bacteria, or viruses and while there are options to control these disease issues with things like fungicides, it is often more effective to address the primary issue caused by poor environment or care. 

Providing a proper plant-growing environment can prevent diseases.

• Gray Mold – Tan, papery patches usually covered with gray, fuzzy masses on leaves and flowers caused by the fungus Botrytis.  Very common on flowers or dying flowers.  Caused by overcrowding, low light, poor air circulation, and/or high humidity. See this article about Gray mold
• Leaf spots – Various fungi, bacteria, and viruses can cause these symptoms. Brown to black spots on leaves that enlarge with age. Concentric circles form with dead, dark centers often with fruiting structures.  Eventually, they cover the entire leaf. Caused by wet leaves (usually from misting or watering), poor air circulation, and/or overcrowding. 
• Virus– color break, off-color patterns (mosaic, mottled), ringspot, and blight.  
• Crown and/or Root Rot – Typically caused by water mold and fungi such as Phytophthora, Pythium, Rhizoctonia, or Thielaviopsis.  Dark, mushy, limp, easily broken roots.  Leaves change to yellow and blight (rapid death). Plants rarely recover.  Caused by pathogens that thrive in wet or waterlogged soils, no drainage holes or saucers full of water, frequent watering, and/or poor soil quality.
• Powdery Mildew – starts as small powdery colonies, and advances to across leaves.  Caused by overcrowding, low light, poor air circulation, and/or high humidity.

Control of Disease Issues

Management of these common houseplant diseases includes:

• Prevention. Scout often and deal with issues as soon as they are noticed.  Always work with clean soil, containers, and tools.  Sanitize them regularly.
• Throw the plant away.  It is better to get rid of the plant and prevent potential spread to nearby plants than deal with the disease.  Additionally, most disease issues cannot be cured, only prevented so if the problem is big, it's not likely to recover.
• Adjust Environmental Conditions.  Look at all factors and be sure they don’t favor disease development.
  • Increase air circulation
  • Space plants to allow better circulation
  • Provide adequate drainage
  • Water using a wet-dry cycle
  • Raise light levels
  • Lower humidity
• Fungicides. Keep in mind that fungicides prevent the further spread of disease issues but they do not cure current infestations.  Check the label to be sure the plant species and the disease are both listed.  Always use as directed on the label. See this article on Proper Fungicide Use.

More Information About Diagnosing Houseplant Problems