If you own oncidium or cattleya orchids, you’ll find that those plump rounded growths called “pseudobulbs” at the base of the leaves and above the roots are susceptible to rot.
The problem of bulb rot in sympodial orchids (e.g. oncidiums, cattleyas, dendrobiums) is similar to stem rot in monopodial orchids (e.g. Phalaenopsis).
Bulb rot usually occurs due to planting the orchid too deep in the growing medium, problems with the medium, and watering technique, causing the plant to become vulnerable to fungal and bacterial infections.
The symptoms of bulb rot are:
- Mushy to touch
- Yellow or brown color
- Foul odor
- Gray or bluish mold
- Yellowed leaves
- Stunted growth
Similar to root rot, treating bulb rot in orchids is certainly possible, but the time of treatment is essential. The longer you wait to treat the bulb rot on your oncidium or cattleya, the more likely it is that the disease will spread and irreparably damage the plant.
Read on to learn how to identify and treat oncidium and cattleya orchid bulb rot.
What does a rotten orchid bulb look like?
Rotten bulbs are identified by the following symptoms:
- Soft, shriveled bulb: The major sign of rotten orchid pseudobulbs is that they are not firm and plump like the healthy ones. Instead, they are mushy or soft to the touch. They can also be wrinkled.
- Yellowing: A rotten bulb that is soft and mushy to the touch is often accompanied by a change of color from green to yellow or even brown. Beware that the bulbs turning yellow alone, without being soft and mushy, may only be a sign of nutrient deficiency where nutrients stored in the old growths are mobilized to the new growths.
- Attached to yellow leaves or rotten roots: The rot from bulbs, if left untreated, often spread upwards to the leaves, resulting in leaves turning pale, yellow, or even dry and papery. Similarly, the rot can also spread to the bottom resulting in root rot but this would only be noticeable when you unpot the orchid.
- Stunted growth: A sick orchid is often accompanied by stunted growth as shown by a lack of new growths and flowering as the plant tries to reserve its energy to heal.
- Foul odor: In more serious cases, rotten pseudobulbs will smell like rotten vegetation.
- Mold: some, though not all, rotten pseudobulbs even have a gray or bluish mold on them.
Why does my orchid pseudobulb rot?
The causes of bulb rot in sympodial orchids (e.g. oncidiums, cattleyas) are similar to those of stem rot in monopodial orchids (e.g. Phalaenopsis), related to problems in the growing medium and bacterial or fungal infection.
1. Planted too deep
The major reason for rotten bulbs for sympodial orchids such as oncidium or cattleya is that the orchid is planted too deep into the growing medium.
For sympodial orchids, pseudobulbs grow from one horizontal stem called a rhizome. If the rhizome is planted too deep with the bulbs buried deep into the moist medium, it is easy for the bulbs to become rotten especially because oncidiums like to stay moist around the roots.
Planting your orchid too deep into the medium makes it more vulnerable to fungal or bacterial infection.
2. Growing medium problems
Bulb rot can also be caused by a soggy growing substrate that does not allow for enough air ventilation around the roots.
This problem is often mistaken as overwatering.
Oncidiums are in fact thirsty plants whose roots do not prefer to dry out between waterings during their growing season. Depending on how fast the growing medium dries out, oncidiums actually prefer to be watered every other day or even every day.
Although their roots like to stay moist, they should not be soggy and their roots need to be well-ventilated instead of being soaked in water.
Growing substrates become soggy, retain too much water, and do not have enough ventilation when they are too decomposed due to a lack of repotting.
Incorrect water techniques such as soaking the orchid for too long can also be a problem.
Being in constant contact with moisture makes the orchid become vulnerable to diseases and infections.
3. Disease
This problem goes hand in hand with all the problems mentioned above.
When the plant is planted too deep into the moist medium, resulting in bulbs being in constant contact with moisture, they become more vulnerable to various types of bacterial and fungal infections.
Black Rot
This notorious diagnosis is, according to the Missouri Botanical Garden, the disease that is most damaging to orchids. The most telling signs are, as the name suggests, black spots on the plant’s pseudobulbs, roots, and leaves. The disease will spread upwards to the crown of the plant, effectively killing the orchid.
The two fungus strains responsible for black mold are Phytophthora and Pythium, both of which are water fungi.
Brown and Soft Rot
True to its name, these bacterial infections can be identified by wet, brown patches on the orchid’s pseudobulbs and leaves. They progress rapidly and, if left untreated, can kill the plant in just a few days.
Brown rot occurs when too-low temperatures are combined with too much humidity. The bacterium responsible is Erwina sp., which thrives in cool, high-humidity environments.
In most cases, brown rot advances too quickly to effectively save the plant. If pseudobulbs are affected, remove the rotten pseudobulb from the plant – taking care not to damage the rhizome that connects the plant – and dispose.
Make sure to clean the open wound with a fungicide.
Rhizoctonia Root Rot
While “root” may be in the name of this ailment, make no mistake that the pseudobulbs are just as affected by this fungal disease as the roots. This disease is directly caused by overwatering. Signs and symptoms of rhizoctonia root rot are thin, shriveled, and yellow roots and pseudobulbs.
Rhizoctonia root rot is a slowly progressing disease that you may not notice right away but can have devastating effects to your orchid if overlooked for too long. In cattleyas, rhizoctonia spreads from older to younger pseudobulbs. A pseudobulb affected by rhizoctonia will turn brown, surrounded by husks.
To prevent rhizoctonia, change your orchid’s potting media frequently and check often for signs of overwatering and/or poor drainage.
Southern Blight
Southern blight is caused by the fungus sclerotium and is identified by sclerotia on lower leaves, roots, and pseudobulbs.
Unlike many diseases that affect orchids, a plant afflicted by Southern blight cannot be salvaged and should be disposed of quickly to avoid the infection spreading to other healthy plants.
How to remove a rotten pseudobulb
Since a rotten pseudobulb has no chance of recovering and becoming healthy again, the only course of action is to remove and dispose of the affected bulb so the rot doesn’t spread to the rest of the plant.
The steps to removing a pseudobulb are as follows:
- Remove the orchid from the pot and potting medium to check the rot has spread to the roots.
- Replace potting media if media is degrading – ignore this step if orchid has recently been repotted.
- Grip the affected pseudobulb by the base and twist, gently disconnecting it from the rest of the plant. Be careful so as not to damage the rhizome that connects the rest of the plant. If the affected pseudobulb is firmly attached to the healthy bulbs, make an incision to remove the pseudobulb from the rhizome.
- Spray the open wound with a fungicide such as Physan 20 to prevent contaminants from entering.
- Dry the area around the open wound with a clean cloth or paper towel.
- Dust a cotton swab with cinnamon and apply it directly to the wound. Only apply to the wound, not to other areas of the plant, as cinnamon is extremely dry and can rob moisture from healthy parts of the plant.
- Pot up the orchid in a new growing medium and water it thoroughly.
Keep in mind that cross-contamination is a major risk when it comes to disease spreading from sick plants to healthy ones, so make sure to use gloves when removing a rotten pseudobulb and to sanitize your cutting instruments between uses.
Can you pot a rotten orchid bulb?
After removing the rotten bulb, you should not attempt to propagate it. An infected and rotten will not propagate into a new orchid. It should be discarded to avoid infecting other plants.
What happens if you leave rotten pseudobulb on orchids?
Rotten pseudobulbs won’t recover.
Rotten pseudobulbs are similar to stem rot in monopodial orchids such as Phalaenopsis, in that the rot can spread down to the roots or upwards to the leaves if left untreated.
So, they must be removed in order to keep the rot from spreading to the rest of the plant. Leaving a rotten pseudobulb on an orchid will doom the rest of the plant to rot.
As with all ailments that affect your orchid, early detection is key for proper treatment and the long-term health of your plant.
Happy gardening!
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