Black spots on orchid leaves could mean one of three things: (1) the orchid was bruised during transport or handling, (2) the leaves have had access to more sunlight then they require, or (3) bacteria has infected the leaf.
Since the first two reasons are fairly easy to resolve and there’s no need to panic, this article will focus on the bacterial infection. But as a guide, brown spots can occur during rough handling, and the leaves bruised. They will recover in a month’s time.
As for sunlight, move the orchid away from direct sun, since the only commonly sold orchid that like full bright sunlight are Vandas.
There are many others, but not as widely known and accessible to the public.
Black spots on orchid leaves are a direct result of a bacterial infection caused by Acidovorax (syn. Pseudomonas), known as Bacterial Brown Spot. The spots start off as brown smudges, minuscule in size and appear waterlogged. When pressed, the leaf will ooze sappy liquid.
This initial stage could be mistaken with overwatering, since the leaves can’t eliminate the excess water faster than the roots can absorb it. Water will cause blisters on the leaf, which eventually turn brownish-black.
Contrary to overwatering, with Bacterial Brown Spot, the infection will appear a nasty green color, slowly fading to yellow circle-like halo around the spot, then black.
Black is the final stage, when the blister sinks down into the leaf, leaving an “open pond”, or whole.
First, let’s eliminate what black spots are not.
- If the spot is bigger than a small circle, like a black smudge that takes over 1/3 of the leaf, then you are dealing with Black Rot, not Bacterial Brown Spot.
- If the spots are on the pseudobulbs or roots and not the leaves, then its also not Bacterial Brown Spot.
- If the tips of the leaves are turning brown, then black, it’s not Bacterial Brown Spot. This is more common in Anthracnose, Colletotrichum & Glomerella spp.
- If the leaf has elongated smudges that cover more than a pencil eraser size, it’s probably Guignardia Leaf Spot. These lesions will act like long, thin lines that elongate parallel to the veins of the leaf.
- Phyllosticta Leaf Spot is more similar to Bacterial Brown Spot, starting off as small yellow spots, which later evolve to black sunken spots. The main difference here is that we are dealing with a bacterium, not a fungus. In Leaf Spot, the fungus has evolved to the reproductive state, spores will appear in the middle of these spots.
What Orchids are More Likely to Have Black Spots on Their Leaves?
Phalaenopsis are the orchids that most commonly get Bacterial Brown Spot. In Phals, this disease spreads quickly, and if not treated can spread to the crown of the orchid, killing it. In two-three weeks’ time, your orchid can die. While the black spots are only on the leaf in your Phal, then you can still save it.
In second place, come Cattleyas, but Bacterial Brown Spot isn’t as fatal in Cattleyas—for some reason unknown to me. Bacterial Brown Spot in Cattleyas will start off as yellowish-green dots on older leaves where wounds once were. They may also appear where you have removed a leaf of flower spike. Rarely do they develop into something more.
How Did the Black Spots Get on my Orchid in the First Place?
Bacteria love high humidity. They also love high temperatures. If your orchid is outside, then it might be in a place where the temperatures are too high, combined with too much humidity in the air.
If your orchid is inside… well, needless to say, I haven’t had black spots yet since there is no way on earth I can get my humidity too high, even if I tried. If you have a greenhouse, then higher humidity can be achieved.
First, move your orchid away from the humidifier. Make sure you keep the fan on even during night, so proper air circulation will evaporate any droplets that you missed when watering.
Bacteria builds up through stagnant water droplets on the top of orchid leaves. With the sun, the water acts as a magnifying glass and burns the place where it lays.
This burnt layer no longer acts as a barrier, and bacteria enter the wound, spreading to other places along the leaf.
Avoid watering where water can splash from one orchid to the next, spotting leaves and stem. Be extra careful when watering an infected orchid—in fact, place this orchid away from your collection so it won’t contaminate the rest of your orchids.
Video: Black Spots on Orchid Leaves
How Does Bacterial Brown Spot Spread?
Acidrovax, the bacteria which cause these unsightly spots, spreads through water. It’s important that the water you are using is not “shareable”.
By this, I mean don’t reuse water that you are watering one orchid in to water another orchid. Keep watering separate and don’t soak multiple orchids in the same basin or bucket of water.
It also can be spread by the use of a contaminated misting bottle. If you touch the leaves on one orchid and mist another orchid, also touching it, then you are spreading bacteria and possible viruses, too.
Keep all your tools sterilized and clean.Off the tangent a bit, but still important: Viruses are mainly spread by contaminated material when removing a leaf, flower spike, or roots. So always sterilize your equipment before using them, and I between different orchids.
How do I Treat Black Spots on My Orchid Leaves?
By using 3% Hydrogen Peroxide and a cotton swab, you can touch spot the areas that have been infected. If you prefer, you can swipe the upper side of the leaf, but never the underside of the orchid.
This is because the majority of the stomata (pores that allow oxygen-carbon dioxide trade) are located on the bottom of the leaf.
You can also use cinnamon, which is a natural fungicide. Mix the cinnamon you use at home to bake with, and add a few drops of water, just enough to form a paste, like glue.
With the cotton swab or spoon, spread the cinnamon paste over the spots. Just make sure that you don’t use the same spoon on different orchids.
If you prefer the more chemically-related answers, you can purchase a bactericide at your local nursery. A well-known one is Physan 20.
If the black spots are too numerous and are reaching the crown of the plant, then remove the entire leaf. Use a sterilized utensil and slice back the leaf. Apply cinnamon and spray your orchid whit Hydrogen Peroxide to clean it of any possible bacteria that are left behind.
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By the way, if you’re interested, check out his article on yellow leaves, which is another common problem with orchid growers. I spent a lot of time and research to put it together, to provide a well-written and comprehensive guide to treating the yellowing of orchid leaves.