November 2020: Houseplant Horrors

Houseplant IconAs cold temperatures arrive in Wisconsin, many diehard plant enthusiasts are now concentrating on inside gardening activities involving their favorite houseplants.  In some cases, people have been growing their houseplants outdoors for the summer and recently (or perhaps not so recently) brought them back indoors to avoid plummeting fall temperatures.  In other cases, gardeners have visited their local greenhouses to pick out brand new plants to add to their indoor collections.  In either case, disease-causing organisms may be coming along for the ride.  Here are some of the common diseases that you may encounter as you garden indoors this winter.

Powdery mildews

I have talked about powdery mildews before on numerous occasions, but typically in the context of plants growing in outdoor settings.  One of the plants where I see powdery mildews indoors is rosemary.  Rosemary plants are often grown outdoors for the summer where they become infected and then begin to show severe symptoms once they are brought into the kitchen for the winter.  Higher humidity from cooking and dishwashing most likely contributes to development of the disease.  Relatively uniform, white, powdery growth on leaves is pretty typical for the disease on this host.  Management often involves cutting the plant back almost to the ground and allowing regrowth, in addition to placing the plant in a new location with lower humidity.  I also occasionally see powdery mildew on jade plants where typical symptoms are dark, sunken spots on leaves.  It’s only when you look carefully that you see a subtle network of white hyphae (i.e., fungal threads) of a powdery mildew fungus in the center of these dark areas.  Plucking off the infected leaves (typically there aren’t a lot) often takes care of the problem.

Root rots

Virtually any plant grown indoors may end up with a root rot problem.  Plants taken outside for the summer can become infected if soil from an outdoor garden accidentally gets splashed (e.g., via a hard rain) into a pot.  Garden soils typically contain at least some level of root rot pathogens.  Greenhouse-grown plants can harbor root rot pathogens as well, as these organims can survive in production facilities and retail greenhouses for years.  Plants may not show symptoms initially, but once brought into a home setting where owners tend to overwater, root rot pathogens can become very active.  They will cause root decay below ground and wilting above ground.  Root rots can eventually kill plants.  Often by the time indoor gardeners notice root rot symptoms, damage is so severe that discarding plants is the best option.  Plastic pots should be thrown away with the plants  Clay or ceramic pots are salvageable if you wash the pots well (to remove any remaining soil), then soak them for roughly 30 minutes in 10% bleach to kill off root rot pathogens.  Whenever you decontaminate pots, be sure to rinse thoroughly after treatment to remove bleach residues.  To minimze future root rot issues, cut back on watering, providing enough water to keep plants happy and growing vigorously, but not so much that root rot pathogens become active.

Bacterial leaf spots and blights

I see bacterial leaf spots and blights on a wide range of plants grown indoors including geraniums (brought in from outdoors to overwinter), poinsettias (purchased for the holiday season), and more traditional houseplants such as dieffenbachia, Chinese evergreen, elephant ear and philodendron (all in the Arum family).  These diseases are typically caused by bacteria in the genus Xanthomonas.  Plants initially may harbor sub-symptomatic levels of these bacteria, but eventually high enough populations develop to cause disease.  Typical symptoms can include angular, necrotic leaf spots (i.e., dead areas where veins border the dead tissue leading to very straight edges) or dead areas along leaf margins.  Dead tissue is typically surrounded by a distinct yellow halo.  Bacterial diseases are notoriously hard to manage.  You can remove symptomatic leaves, but healthy leaves typically still harbor the pathogen.  These healthy leaves often eventually development symptoms.  When bacterial diseases are a problem, you are typically left with the choice of living with the disease or throwing the plant out.  As I outlined above in my discussion of root rots, I recommend throwing out plastic pots and decontaminating clay and ceramic pots by bleaching them.

Need help or more information?

Whether I’m working with outdoor or indoor plants, there’s never a dull moment at the PDDC.  If you are having plant disease problems of any kind and need help diagnosing these problems, feel free to contact me.  For the PDDC’s current policy on sample submission, including submission of digital photos, check out the following link.  As always, be sure to check out the PDDC website for timely information on plant diseases.  Also, feel free to follow the clinic on Twitter or Facebook (@UWPDDC) to receive timely PDDC updates.  Or alternately, put in a request to subscribe to the clinic’s new listserv (UWPDDCLearn) by emailing

As always, hang in there, be safe, and stay healthy!