Halloween may be over, but common fungal diseases on tropical plants ensure that spooky season never ends in our interior exhibits. According to the Ohio State University fact sheet on the topic, fungal diseases wreak more havoc on plants than any other group of plant pest or pathogen. Estimates hover around 85% of plant diseases are caused by fungal agents. The spores and hyphae of these mysterious, understudied organisms are lurking in your water, soil and air, waiting to strike like the masked psycho in a horror movie.

The very lack of what we understand about the Kingdom Fungi can make management difficult, however. They obtain nutrients by sending thread-like filaments throughout their food sources – you, me, a loaf of bread, your favorite plants, your jack o’lantern. They often hang around dead and decaying things (spooooky!) but not always. Sometimes they are straight-up evil parasites of living things. But, of course, evil is in the eye of the beholder – in this case, the beholder is the zoo horticulturist attempting to protect their tropical collections from the undead fungal hordes. Nevertheless, we should never forget the beneficial fungi that collaborate in vast mycorrhizal support systems for plant communities, as well as the yeasts that make sourdough bread and beer possible.

But we are here today to discuss the villains! Below are some of the common symptoms of fungal infections in interior plantscapes:

Leaf spots – Leaf spots are indicative of several different fungal species. Look for roughly circular tan/ reddish brown spots, concentric rings and small black fruiting bodies. If infection progresses, lesions may join together, killing off the growing tip or spread to branches of plants.

Powdery mildew – Powdery mildew is not just one but several fungal species. What starts as powdery spots on upper leaf surface can join together to cover the entire leaf. Powdery mildew is often found in temperate, humid climate conditions - temperatures above 86 do not support powdery mildew.

Stem canker – This infection is similar to an abscess- dark and mushy spots on stem. If an infection gets this far, the plant is usually not salvageable.

Black sooty mold – This fungus looks as grimy as a Victorian street urchin. Black sooty mold grows on the honeydew exudate of aphids, scale, and other sap-suckers. That high sugar content encourages the growth of black sooty mold. If the mold gets heavy enough, it can shade out the plant and stunt its growth. The horticulturist’s first response is to take care of the insect pest, which removes the source of the problem.

If fungal infections were like a horror movie, they would be like “The Thing”, starting off with small, seemingly innocuous symptoms but slowly infiltrating the plant completely and consuming it from the inside out. But unlike “The Thing”, the wise horticulturist does not need to blow up an entire Antarctic field station to contain the damage. Prevention and speedy diagnosis, as always, are going to be the most effective way to keep your plants healthy. Keep in mind that spores can live a long time and be carried by air currents, water, soil, tools and clothing, so our mantra for fungi should be “CONSTANT VIGILANCE” (shout-out to Mad-Eye Moody!).

• Include fungal symptoms in scouting regimen
• Check new plants for symptoms before installation
• Sanitize tools and containers regularly
• Eliminate cracks/ drafts if you suspect diseases are coming in from outer environment
• If fungal diseases are a significant and ongoing issue in your exhibit, consider resistant plant varieties
• May need lab analysis to pinpoint exact pathogen – most symptoms can be caused by wide array of fungal species

The next best way to manage fungal infections is to focus on plant health care and adjusting conditions to be better for plants, but worse for fungi. In the horror movie realm, this would be analogous to if you never build a summer camp, you are less likely to get a masked murderer hanging around.

• Reduce overall humidity
• Water early in the day to allow plant surfaces to dry
• Water to avoid wetness on surface of leaves, reduce overhead watering – dry leaves are less supportive of fungi. At Butterfly Pavilion, we often syringe our tropical exhibit for pest management, but we’ve gotten more focused over the years, so that we aren’t indiscriminately wetting leaf surfaces.
• Increase air circulation through strategic pruning and thinning.
• Remove debris from the ground regularly.
• Follow a balanced fertility program to avoid too much or too little of key plant nutrients
• Avoid wounding or stressing plant material as much as possible.

If cultural methods are not effective, you may want to consider the ultimate in physical methods – removing the infected plant entirely from the exhibit before it can be a source of new infections. Fortunately, you’ll have better luck with plants than in most horror movie franchises; those monsters just keep returning and returning!

Chemical controls can used to protect new tissue, but keep in mind that nothing can be done about the old infected tissue. Chemical fungicides, just like any chemical tool, require caution. Different fungicides are effective against different species of fungus, so checking the label is key. Fungicides can be irritating to skin, eyes and respiratory system, while chronic exposure may lead to negative health effects on the nervous system. Also, runoff and contamination, if it reaches the outside world, can negatively impact aquatic ecosystems and other organisms. For example, copper sulfate has been shown to have a negative effect on bees. Now that’s the real horror story!

Amy Yarger, Horticulture Director at Butterfly Pavilion