Foliar Fungal Infections: How to get rid of fungus on your plant’s leaves

What causes plant disease?

This concept is an old, but true one in plant pathology. The disease triangle should be known by heart by all horticulturalists who dare to call themselves professional. You, as a plant parent, should also be familiar with it.

Probably the most accurate disease triangle I fished out of the depths of the internet.


The environment plays a large role in whether or not a plant gets disease. Generally, if conditions are wet, this favors the formation of bacteria and fungi, and allows them to outpace the plant in growth, which means that infections become more severe. But this is not always the case. Dry environments favor spider mites and sap-sucking insects.

Additionally, sudden temperature flux can weaken a plant and invite fungal infections where the plant might not otherwise get sick. Cooler temperatures favor root diseases whereas warmer temperatures favor foliar diseases; but take this with a grain of salt – some fungi and bacteria will infect your plant no matter what temperature.

It is possible to have both the plant and the pathogen in the same space, but if the environment is not right, then no infection will occur. For example, soft rot (Erwinia infections) can only be transmitted by water splash. If an infected plant and a non-infected plant are placed next to one another (but not touching), and the humidity is low and there is no rain or water splashing, then the non-infected plant will not contract the infection.


The host, or your plant, is just as important as the environment and the pathogen. What not too many lay-people know is that each plant had virulence and avirulence factors for each pathogen. Pathogens and plants have evolved together over time, so it’s uncommon for a type of plant to be completely obliterated by a pathogen (that is, unless the pathogen is strong and the environment strongly favors the pathogen).

To simplify what we know about plant disease, we must understand the pathways by which disease occurs. Generally, pathogens have avirulence factors – proteins which help them penetrate, disable, hijack, and leech or destroy the infected cell. Plants, of course, have resistance genes, or R-genes, which create resistance proteins. Each interaction in this model is a one-for-one interaction. Each R-gene protects against one specific avirulence factor (remember, both encode proteins which are actually doing the interactions; genes don’t leave either organism!).

On a genetic level, the only way that R-genes work is if they recognize a specific avirulence protein. If there is a mismatch, or neither recognizes each other, then infection occurs. If the R-proteins recognize the pathogen avirulence proteins, then no infection occurs. Additionally, plants may produce secondary metabolites which aid in defenses by disabling fungal toxins or by other means.

Over time, different cultivars of disease-resistant plants have been bred. For houseplants and most ornamentals, disease-resistance is not a priority, so there are no known resistant strains, or cultivars of houseplants, but that does not mean that there aren’t a few plants floating around with some resistance to some pathogens. Before you get sad, do note that nearly all of the disease-resistance breeding goes into food crops rather than houseplants. Dunno about you, but I am very happy about that.


The strength of the pathogen depends on the avirulence factors described above, as well as the pathogen’s ecology and means of spread. Some pathogens are airborne, others are water-splash only, and others are carried via insect vectors. Some pathogens are host-specific, others are generalists. I didn’t have to tell you this, but obviously the pathogen has to be present for infection to occur.

One thing that gets missed in most pathogen talks is the influence of opportunistic endophytes on infections. When a plant is weakened or otherwise unhealthy, , the pathogen will switch from being “a team player” into a parasite (much like candida or yeast in humans). This is often the case in “clean” plants in a collection. Oncidium speckling in their leaves is a perfect example of this.

How can I treat for fungal infections?

First, you must know that many fungi are much larger and smaller than you think. That means that the organism itself spreads into larger areas than you think, and its parts are smaller than you think. Often times, when you see an infected lesion (usually bordered with yellow and the dead cells are brown), the active parts of the fungal hyphae are actually in the green just beyond the yellow, actively leeching off of the healthy tissue. The spores, which are usually only one cell big, are the key to spreading the fungus, and travel easily, as they are designed to do.

When dealing with infected plants, NEVER touch healthy tissue after touching infected tissues. I pretend that the disease is ebola, and I wash my hands and sterilize my tools each time I touch the infected bits. Obviously, if you can touch an infected part after touching another infected part. When trimming your collection, it’s a best practice to handle the infected plants last, to help reduce the spread of disease.

Like pests, fungal spores are everywhere. They exist in the air naturally, and throughout your life, you will inhale billions of spores. Your body kills most of these, and DO NOT fret – we are all swimming in the same fishbowl called earth, and it’s counter-productive to be grossed-out by nature. It’s happening whether you like it or not, so better breathe deep (obviously don’t go to a place where you can inhale known dangerous spores like black mold… or do something else stupid!).

The best ways to deal with fungal and bacterial infections are to:

  • Keep the leaves dry – STOP SPRITZING!!!
  • Spray the entire plant down – really drench it top and bottom with fungicide
  • For orchids, ferns, palms, and other thin-leaved sensitive plants, use Physan
  • For all other tropical plants, use this Copper-fungicide
  • For more details as to how drenched your plant must be, refer to my pest guide

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