Black spots that start small but soon take over an entire leaf are common orchid problems called Black Rot. If not controlled soon, Black Rot will travel form the leaf down toward the crown, killing the pseudobulb, rhizome, and roots. This fungus, along with others in its family like Pythium ultimum which also cause Black Rot, is extremely fast-spreading. If not treated, within two weeks your orchid could be dead.
What Conditions Contribute to Black Rot?
Black Rot is commonly found in hot, humid climates, with temperatures ranging from 75º to86º F (24º to 30ºC) and relative humidity closer towards 80%—which also happens to be what orchids like.
This fungus along with Phytopthora (another fungus causing of Black Rot) are infamous for destroying corn plantations, apple orchards, and strawberry patches, since all have high water requirements.
I’ve added a link to an article that does have great pictures at the end.
I highly suggest you check it out.
How do I Know Its Black Rot and Not Something Else?
Black Rot will start of as a yellowing of the majority of a leaf or pseudobulb. If you have isolated brown or black spots, speckled over a small area of your leaf, then this article on Black Spots, which is a different organism entirely and also different treatment, will be more helpful.
Sometimes the fungus will be a purplish-black color, and not yellow. In both cases, the infection is well-defined and not blended in. It looks like a small child smudged black paint on the leaf.
In time, the leaf will wrinkle, crumble, and fall off. If this happens, Black Rot has already established itself in the crown of the plant and there isn’t much hope of recovering this orchid. If your orchid is wilting, and lifeless, check out this article. It talks about reasons why orchids wilt and lose their vitality.
What Orchids are Most Affected by Black Rot?
Cattleyas and Dendrobiums are the two orchids that have the most trouble with Black Rot, but if you overwater or water with wrong techniques, any orchid is victim to this fungus. After these two, which are more frequent, the Epidendrum, Vanda, Paphiopedilum, and Laeliocattleya.If you have a Cattleya and Dendrobium,
always observe closely for any yellowing sings that could lead to Black Rot.
How Does Black Rot Spread?
Although it looks similar to mold, Black Rot is a fungus that spreads through water. It starts with stagnant water droplets on top the leaves, result from sloppy watering. That’s why it’s so important to learn how to water your orchid correctly.
This stagnant water doesn’t evaporate (which is why you always need good air circulation) killing the leaf cells under it. Once the leaf barrier has been broken, Phytophthora cactorum—our fungus’ official name—settles in.
It also spreads through tools used in gardening. If you are repotting and pruning dead roots, don’t start on another orchid without sterilizing your equipment in between orchids.
Especially if you know that specific orchid is infected, don’t reuse tools without proper sterilization. This means table, bench, plant shelf, etc…
Viruses are the most common transitable pathogen when dealing with cut roots, leaves, and stems, but fungus like Black Rot can easily be spread this way.
The use of unsterilized pots is another facilitator. If you are reusing a pot from one orchid to the next, soak it in a bleach solution and scrub it out thoroughly before repotting.
Black Rot also spreads through proximity. If your plants are close together where the leaves are touching, the fungus can spread form one leaf to another easily, even without the air spores or water droplets.
How To Identify The Cause of Black Rot
To treat Black Rot, first, you have to identify what caused the appearance of this orchid-killer in the first place. Humidity and watering are the main culprits, and all the causes listed below are related to one or the other, sometimes both.
Humidity is an important factor in growing orchids, but if you’re overdoing your humidity requirements, black rot is an opportunist.
Is your humidifier blowing mist directly on your orchid?
Is the mist evaporating rapidly?
Is the humidity too high?
The most common cause of black rot is improper watering techniques. When you water under the sink, be careful not to splash water around in the crown of the orchid. Dry any remaining water droplets from leaves and lightly press a paper towel in between any crevices to soak up any extra water.
The second watering technique that could be wrong is misting. Misting will raise the humidity a slight bit, but only for an hour or two.
If there is no fan in your orchid room, or living room, or home office, then the water doesn’t evaporate. The mist just sits there on top the orchid leaves…in a hot room…with high humidity…asking to be taken advantage of.Another related problem with watering is how well the water is exiting the potting medium. If water takes too long to drain from the bottom holes, and has to travel in between highly compacted sphagnum moss and fine-grade bark, then you need to change your medium. Water should enter and leave your orchid quickly.
Potting medium that has been broken down over time will be more than an invitation for Black Rot to advance into the roots. This is called Root Rot, and is also caused by the same fungus as above, along with two more: Rhizoctonia and Fusarium.
Just like dominoes, once black rot installs, it takes over knocking down each orchid structure. Decomposed potting medium is a huge dominoes which topples over three or four more: (1) It changes the pH, which accelerates fungal growth; (2) it traps water, raising propagation of Black Rot; (3) it makes gas exchange almost impossible; and (4) doesn’t let in light for roots to photosynthesize.
Yes, roots on orchids also photosynthesize. That is why the roots toward the outer pot are greener and the ones hidden in the middle of the pot away form the sun are more yellow.
Yes, roots on orchids also photosynthesize.
That is why the roots toward the outer pot are greener and the ones hidden in the middle of the pot away form the sun are more yellow.
How do You Treat Black Rot?
1. Remove the Leaf.
The first thing you want to do is remove any part of your orchid with signs of Black Rot. With a sterilized tool, cut into the green tissue that surrounds the Black Rot. Don’t cut right into the rot, since it will just facilitate the spread of spores. It’s also gross… So just don’t.
2. Treat the wounds.
Make sure all parts of the Black Rot are removed to a surgical precision. Use cinnamon from your cooking cupboard to sprinkle on the open wounds. Cinnamon is a natural fungicide, and very powerful to contain the healthy tissue in good conditions.
3. In your potting medium, add charcoal.
Charcoal depletes certain minerals from the potting medium that are essential for bacterial and fungal growth. Charcoal also traps in the foul-smell from the fungus.
If you want to read more about charcoal, this article explains the types of charcoal, the benefits, how to use it, and how much to use.
To know if you have a bacterial infection or fungal infection in your plant, smell it. If it smells like rotting broccoli that has been sitting in the bottom of your trashcan for six days, then yep, you have a fungus. Fungus are the worst, and charcoal helps eliminate the harsh, necrotic smell.
Don’t Stop Learning!
If you want to be included in a more information and get a 14-page fertilization guide, please sign up for my newsletter. I don’t spam, but send emails out bi-monthly with some curious topics of interest. If you want more information, click here to go to a specific page on this website where I explain it more in detail.
Also, if you are looking for an orchid journal to keep your notes specifically about orchid care, check out my 2 solutions for that on this page. If note-keeping isn’t your thing, then there is a free excel spreadsheet that you can download. Click here for more information on how to do that.
If you subscribe to my newsletter, I will send you a 14-page guide on the main tips of orchid fertilizer. It is downloadable and you can print it out on your computer. I designed the guide to double up as a coloring book, just to make it fun.
4. Treat your Orchid with Fungicide.
You’ll need to check with your local nursery or greenhouse to verify what is an option for you, but the most commonly used fungicides are Captan and Alliete.
There are many, many others.
If your orchid is completely taken over with Black Rot, then these fungicides won’t be much help to restore the roots and leaves. This stage is used for prevention, to hinder more Black Rot from forming than necessarily treatment.I wish I had some pictures of Black Rot, but unfortunately, I don’t. Here’s an excellent article with pictures from St. Augustine’s Orchid Society. I highly recommend it.
I hope that this article clarified some ideas on how to prevent, treat, and maintain an orchid that has Black Rot.
One thing is important: once you see that Black Rot has started in one orchid, remove it from your collection.
This isn’t like mold: it’s a live fungus which spreads quickly. If the orchid is taken over by Black Rot, discard it. I know it’s hard, but losing one is better than losing four or five. Black Rot spreads faster than bad news. Eliminate it as soon as possible.