Cacti have become some of the most popular houseplants over the years because of how low maintenance and resilient they are.
They also come in many different shapes and sizes that allow plant enthusiasts to choose which varieties suit them.
Cacti do not encounter as many problems as other houseplants, but that does not mean that they do not have their fair share of issues.
One of the most common symptoms observed on cacti is when they turn black. The most common reasons why your cactus is turning black are not enough water, too much water, disease and temperature changes.
In this article, we will discuss the different causes of a cactus turning black and what you can do to save it. So if you are having this problem and wish to learn more, just keep on reading.
Why is my cactus turning black?
Not enough water
A common misconception about cacti is that they do not need to be watered. This is certainly not true because it is a plant that needs a certain moisture level inside it to remain healthy.
Yes, cacti need watering much less than regular houseplants, but that does not mean that you can just neglect watering it all the time.
Cacti use water in the soil as a sort of vessel to absorb the nutrients and minerals from it. If there is no water in the soil around the roots, the cactus will not be able to get the substances that it needs to survive.
If you leave the plant underwatered for long enough, the plant will dry up and turn brown or black and then die completely.
You will know that your cactus is not getting enough water when the soil around the plant looks and feels bone-dry and if the plant itself looks limp and dried out.
If you think that your cactus is underwatered, water it immediately. Make sure that you soak all of the soil in the pot so that all of the roots of the cactus get their share of moisture.
In order to avoid underwatering again, you should know when to water the plant. You will know if the cactus needs to be watered if the top inch of soil in its pot is dry to the touch.
Too much water
Cacti are desert plants that have evolved over the years to develop characteristics that allow them to survive in these extreme conditions. One of these is the ability to absorb and store plenty of water in their stems. This water is what gets the plant through long dry spells in the desert.
Rainfall is scarce in the desert and the plant knows to capitalize on this to ensure its survival.
Because the plant can store a lot of water and use it effectively over a long period of time, it does not need to be watered as often as the regular houseplant.
In fact, overwatering is one of the most common mistakes cactus owners commit. This can be due to you giving the plant too much water every time you water it, giving the plant water more often than you need to, using a poorly-draining potting mix and pot or if you do not adjust your watering habits to changes in weather, season and climate.
An overwatered cactus will turn yellow and become mushy because it has been absorbing water from the soil continuously and the plant cannot use up the water fast enough.
Overwatering that is not corrected immediately can lead to a more serious condition called root rot.
Root rot happens when an overwatered cactus is left to sit in the waterlogged soil for long periods of time and the plant’s roots will drown and die. The dead roots will become susceptible to opportunistic pathogens and these pathogens are going to make the rot more aggressive until it spreads to the entire plant.
When the rot has consumed the entire cactus, it will turn black and cannot be saved.
If you think that your cactus is overwatered, stop watering it immediately and place it in a sunnier spot to allow the soil to dry out quicker.
If you suspect root rot, you will need to remove the plant from the pot. Once the plant has been removed, wash off as much of the old soil from the roots as you can. Do this gently because the roots are quite fragile at this point.
Inspect all of the roots and search for sections that have turned brown or black. These roots are rotten and will have to be removed. Use a sterile pair of scissors to remove the dead roots until only the white, healthy roots remain.
Place the plant on a dry surface to allow the roots to air dry for a few hours.
Prepare a new pot by filling it with fresh cactus potting mix. Place the plant in the middle of the pot and cover the roots with more soil if you need to. Make sure that the pot has drainage holes at the bottom.
Place the plant in a sunny spot with good air circulation. Wait at least seven days before watering the cactus. Water the plant only when the soil in the pot is dry to the touch. If the soil is still a bit damp when you touch it, wait one or two days before checking the soil again.
Crown rot is a fungus that lives in the soil and thrives in wet, heavy soil. It affects trees and shrubs. Cactuses, on the other hand, thrive in light, airy soil that dries quickly. However, this does not mean that your plant is immune to this fungal disease. When it comes to gardening, knowing the basics of crown rot is essential if you want to grow a healthy garden. Crown rot can affect almost any plant, shrub, tree, or vegetable patch, so knowing the basics of this disease is essential.
Initially, the soil will display symptoms that are similar to those of dry rot, which indicates that the cactus is suffering from crown rot. There may be other signs that go unnoticed until the affected areas become dark, tan, or brown in color. By this point, it is almost always too late to save your houseplant from certain death. This is why crown rot is so lethal because you may not even be aware that your plant has been infected with it until it is too late.
Once the fungus has established itself in the soil, it is virtually impossible to eradicate.
Once crown rot has progressed to the point where your cactus has become blackened, it is extremely difficult to reverse the situation. In conjunction with proper watering, fungicides can aid in the prevention of diseases; however, once a disease has taken hold, it is difficult to eradicate it permanently from the plant. As previously stated, crown rot begins in the soil, so you should consider repotting a cactus and replenishing the soil around it as a precaution.
Phyllosticta pad spotting
Infection with Phyllosticta pad spotting, also known as prickly pear leaf spot, is a fungal disease that affects the prickly pear cactus as well as other members of the Opuntia family. Cactus pads become infected with phyllosticta fungus after tiny spores from the fungus colonize them and cause lesions.
If your cactus plant has phyllosticta pad spotting, you should remove it from any other plants you own as soon as possible because the infection can spread very quickly between plants. In addition, you should carefully remove the infected pads from your prickly pear cactus in order to prevent the disease from spreading further.
Fortunately, the damage caused by this fungal disease is primarily cosmetic in nature and will not result in the death of your cactus plant. Prickly pears are most commonly affected by phyllosticta concava, which is also known as dry rot and is the most common cause of the disease.
When you have phyllosticta pad spotting, you will notice black circular lesions on your cactus’ skin that are 1 to 2 inches in diameter. This is one of the first symptoms you will notice. The dark colors are produced by reproductive structures known as pycnidia, which will release spores when the conditions change. When the conditions change, the spots will fall out of the cactus and leave a scar.
Bacterial necrosis is a type of fungal disease that affects the saguaro cactus plant. It is a fairly simple infection that rots the tissue of the plant and can be both smelly and life-threatening to the saguaro cactus if left untreated. If left untreated, the infection can spread throughout the plant and kill it.
It is possible to smell and feel the softness of the blackened spot on a Saguaro cactus when it is affected by bacterial necrosis in the first stage of this disease in the plant. It will eventually progress to the formation of dark lesions that exude an odorous fluid and the formation of necrotic pockets that will spread throughout the body. Necrotic pockets are pockets of dead plant tissue that, if left untreated, will kill your cactus plant if it is not removed. If you catch a saguaro cactus disease early enough, it will have an 80 percent chance of surviving.
You will also notice that the necrosis of the saguaro cactus develops a corky patch on the cactus. This is the plant’s attempt to heal itself, but any breach of the corked area will allow the bacteria to escape and infect your plant even more severely. The patch will be blackened to give it a more edgy appearance.
The only way to treat bacterial necrosis is to manually remove the infected area with a clean, sharp, sterilized knife, which is the only treatment available. However, because the bacteria can survive in the soil or ground, your cactus plant may not be completely saved.
Cacti are desert plants, which mean that they do not do particularly well in cold weather. This is also why it is no surprise to experienced cactus growers to see problems pop up when winter rolls around.
The very biology of cactus plants will not survive, let alone flourish, in winter conditions. The tissues in the surface of the cactus will react negatively to the sudden drop in temperatures such as black spots on the skin and can even result in the death of the plant.
If you are growing your plant outdoors, make sure that you take it inside your house before the frost comes and do not allow it to spend long periods of time out in the cold. Try to keep the cactus in an environment with a temperature between 75 to 80 degrees Fahrenheit.
After keeping the plant indoors for a week, note the changes in the black spots on the plants and see if they have lessened in number or size.
If the spots do not go away or there are no changes, the spots may be because of a completely different cause.
Cacti are a diverse group of plants that come in all shapes and sizes. They have become some of the most popular houseplants because of the ease at which they are grown and cared for.
One of the most common problems encountered by cactus growers is when the plant suddenly turns black. No matter the variety of cactus, if any of them turn black, that means that there is something wrong with it that you need to resolve.
The most common causes of a cactus turning black are not enough water, too much water, disease and temperature changes.
The most important step in fixing this problem for your cactus is to correctly identify the cause of the blackening in the first place. The faster the cause is determined, the faster the treatment is administered and the faster the plant is going to be able to recover and flourish again.
Image: istockphoto.com / Boyloso