If you’re a houseplant owner, you’ve probably seen your plant’s leaves turn yellow and wondered what it meant. Maybe you checked the internet for answers and found, to your dismay, a laundry list of possible causes; plants can turn yellow from disease, or just from you overwatering or underwatering them.
Yellow leaves are to houseplants what coughing is to humans, said Carrie Harmon, Ph.D., a member of the University of Florida Emerging Pathogens Institute and an extenstion specialist in the University of Florida Institute of Food and Agricultural Sciences. In humans, a cough could signify various diseases or could simply be caused by mechanical issues like swallowing wrong or eating too much hot sauce. Neither coughs nor yellow leaves are definitive diagnostic symptoms, since they can be caused by a variety of issues.
What you should do, said Harmon, who directs the UF-IFAS Plant Diagnostic Center, is investigate. Stick your finger in the soil. Does it feel like it’s been soggy for a long time? Or is it dry and crumbling?
“If the soil is dry and crumbly, water the poor thing,” Harmon said. “If it’s wet and soggy, gently pull the plant out of the pot and look and see if the roots are white, or if they’re brown or black.”
Healthy roots should be white and firm. Overwatering can cause root rot, which makes the roots turn soft and become brown, or even black. The soil may also develop a sour, stinky smell as the oxygen-less soil conditions encourage anaerobic bacteria to grow.
But these bacteria aren’t necessarily pathogens — they simply prefer wet soil conditions and happen to produce an unpleasant smell. Similarly, root rot itself is not necessarily a disease, but a physical condition that may be caused by a pathogen or simply standing in water for too long.
In fact, bacterial diseases are actually quite rare in houseplants. If someone encounters a bacterial infection, Harmon said, it is usually because the plant was already infected when they brought it home from the nursery.
How to tell if your houseplant has a disease
If you’re concerned that your plant has a disease, there are several symptoms you can keep an eye out for.
Plants infected with bacteria will develop an area that looks dark, black and wet, with angular borders since bacteria struggle to punch through major veins in the plant. As bacteria move through the plant, they digest cells and leave the plant looking water soaked and mushy. In houseplants, bacterial disease is most likely to affect fleshy plants like orchids and succulents, causing soft rot.
Viral diseases, which primarily affect orchids, tend to create green spots. Harmon cautioned that these can neither be cured nor plucked out. Just because a part of the plant looks healthy doesn’t mean the virus isn’t there — only that it hasn’t yet replicated to levels high enough to cause symptoms.
Harmon’s advice for managing bacterial and viral houseplant diseases is simple. “Just throw it out,” she said. “Start over. There isn’t anything you can spray it with, and you can’t prune it out. Just get rid of it.”
For plants with a fungal disease, though, there is some hope. These can usually be treated with fungicides and, fortunately, most of the diseases that Harmon sees in the plant diagnostic clinic are fungal.
As the fungus pulls moisture and nutrition out of the plant, it creates leaf spots that can turn dry and papery. Another telltale sign is a dry, dark spot with concentric rings resembling a target. In preparation for treating the plant with fungicide, Harmon recommends pinching off any fungus-infected leaves to inhibit the pathogen’s spread to the rest of the plant.
But by and large, houseplant disease management is mostly about avoidance. “If a plant looks wrong in the nursery or greenhouse, don’t buy it,” Harmon said. “There really isn’t a way to get ahead of disease.”
To figure out if your plant is suffering from disease or mistreatment, Harmon suggested looking at how much of it is affected. If the problem is isolated to a single leaf, there’s a chance your plant has an infection. Pathogens take time to reproduce, spread and cause symptoms, so they will initially affect only one leaf or stem at a time. If wilting or mushiness is spread out across the entire plant, the issue is likely abiotic — something physical rather than biological.
In houseplants, abiotic issues are more common than diseases
Most people will only encounter plant disease in landscaping plants, Harmon said. So, if something is wrong with your houseplant, the explanation is most likely abiotic.
Common issues include overwatering, underwatering, not providing enough light or too much light. It’s also possible to physically damage a plant by leaving it in too small of a pot for a long time, causing the plant to become pot bound.
Harmon also sees a lot of people put the plant’s main pot — the one with drainage — inside a secondary vessel that’s more decorative and meant to keep water off the counter. That outer pot can hold onto water where the plant owner can’t see it. The roots, left in standing water and cut off from oxygen, begin to rot.
“Overwatering is actually the number one way we kill houseplants. The number two way is forgetting to water,” Harmon said. “So, water is a big deal.”
If you struggle with keeping track of which plants need more water or less water, you can always check soil moisture with your finger. When you poke about an inch into the edge of the soil, your finger should come out just slightly dusty, with some moisture. If the soil is completely dry and dusty, you need to water the plant – unless it’s a cactus, in which case you want the soil to be completely dry between waterings. And, finally, if the soil is hard, it’s time to repot your plant.
Avoid bringing diseases home when buying houseplants
When you’re deciding whether to bring a plant home, look underneath for leaves that are wet and squishy or dry and crispy. The first step to avoiding plant disease, Harmon said, is to leave a plant at the nursery if it doesn’t look right. At home, the next step is to provide plants with what they need.
“That’s called right plant right place,” Harmon explained. A maidenhair fern, for example, needs high moisture and low light, and would quickly dry up and die if it was kept by a sunny window in a screened porch. Garden centers and nurseries try to make this easy for potential plant owners by including tags that detail the water and lighting conditions necessary for success.
Harmon emphasizes that giving your plant what it likes is more important than worrying about disease management.
“Disease is the exception in plants,” she said. “Most of the time, if we give the plant what it needs, it’s healthy.”
For a plant to become diseased, it needs to be exposed to a pathogen under the right conditions. Pathogenic organisms need a plant that they can infect and a way in; if there are no wounds and if natural openings like stomata are closed, it won’t even matter if a bacterium is on the surface of a leaf. As long as the pathogen can’t get into the plant, it can’t cause disease.
Lucky for us, most bacteria, viruses and even fungi are really picky about the plants they can infect. So, if an orchid in your house has a disease, it’s very unlikely the infection will spread to your cacti or fiddle-leaf fig.
Ultimately, the healthier we can keep our houseplants, the less likely they are to develop a sickness. Since most diseases can’t be treated, focusing on maintaining a healthy houseplant should be the priority. And if your plant does develop an infection – and isn’t an expensive variegated monstera or decades-old jade plant — Harmon gives you permission to toss it out and go buy a new one.
Written by: Jiayu Liang