Fungal issues plague almost every form of plant. The sheer number of fungal organisms is staggering and most survive by being dormant for long periods of time. Fungal lesions on cactus may be caused by any number of fungal types, but the important things to note are what causes them to colonize and how to prevent their damage. In this way, the gardener is armed with knowledge on how to treat fungus on cacti should any disease symptoms develop. Some fungal diseases simply cause cosmetic damage while others can develop into rots that completely eat the cactus from the inside out.
The vast amount of cacti species can only be outcompeted by the huge quantity of fungal varieties. Fungus spots on cactus pads are common, as in the case of Phyllosticta pad spot. It is often quite impossible to diagnose which fungal organism is causing the spots, but often that is unimportant since treatments are generally the same.
A few fungi types damage the roots and eventually the whole plant, so once their visual damage is seen, it is too late for the plant. Simple topical fungal spots are much easier to combat and are usually not life threatening to the cactus provided steps are taken to control the offending fungus.
Lesions on cacti may present in many different ways. They may be round, irregular, raised, flat, and any other shape. Many are discolored but, again, the tones can range from yellow to brown and all the way to black. Some are corky, while others are weepy. These may ooze brown, rusty, or black fluid, evidence of severe infection.
The cacti most frequently plagued by fungal lesions are Opuntia and Agave. Fungal lesions on cactus usually start as water spots or slight discolorations on the plant’s epidermis. Over time, as the fungi mature and spread, the symptoms can broaden and even eat into the cambium as the surface skin cracks and allows the pathogen to enter.
Outdoor cactus can come in contact with fungal spores in various ways. Spores may be blown in from wind, in soil, or contracted from splashing water. Plants with consistently wet pads or stems are the worst affected. Conditions where rain or high humidity combine with warm temperatures promote the formation of fungal lesions.
Fungus spots on cactus pads are more prevalent in the springtime. They are also enhanced by overhead watering and in areas where humidity is high. Greenhouse specimens may be particularly susceptible unless there is adequate ventilation. Condensation adds to the ambient humidity and promotes spore growth.
Soil is another contributing factor. Many soils harbor fungal spores, which can persist for years until the right set of conditions occur. Even purchased potting soil may be contaminated with fungal spores.
Once there is a fungus affecting your cactus, it can be difficult to stop. If damage isn’t severe, a fungicide spray can usually help. If the plant is rife with lesions, it may be best to find some uninfected healthy material and start a new plant with a cutting. Use a sterile knife to take the cutting and dust it with sulfur to kill any possible adhering spores.
Controlling cultural conditions with plenty of heat, under stem watering, sterile potting medium, and ventilation will halt many fungal outbreaks. Another way to save a plant is to cut out the infected tissue. This doesn’t work with all fungi, but it may be effective at times. Again, sterilize your cutting implement and remove more tissue than appears to be affected to ensure all the pathogen is removed. Keep the area dry as it calluses and watch carefully for signs of reinfection.
Epiphyllum is the name of a genus that is part of the Cactaceae (Cactus) plants. The Epiphyllum genus comprises 19 different species of epiphytic plants. The plants originate in Central America. They are also frequently known as leaf cacti and orchid cacti. The plants produce pleasant, showy and large flowers that range from pale white to vibrant red. As with all plants, there are several diseases that may affect epiphyllum.
A cactus with brown or black spots or even one that’s entirely discolored is the plant’s cry for help. While internally, the damage may have been going on for a while, you’re just now seeing it externally.
There exists a few fungal and bacterial diseases that are most likely responsible for a blackened cactus. While I touched on them in the intro, I’ll discuss each one in more depth now.
If you grow other houseplants besides the cactus variety in your indoor garden, then you’ll never have to worry about them having bacterial necrosis.
That’s because bacterial necrosis strikes cacti only, and only certain species of cactus at that. These include the organ pipe cactus, barrel cactus, prickly pear, the cholla, and the saguaro cactus.
The Erwinia bacteria, which earned its name after Erwin Frink Smith (a well-known plant pathologist), is what triggers bacterial necrosis in cacti. All it takes is any branch and trunk wounds in your cactus and it’s possible for bacterial necrosis to enter.
The cactus then develops necrotic pockets, or areas of plant tissue that have died. These pockets allow the disease to travel much more easily due to the weakened areas throughout the cactus.
Some species of cactus, including the saguaro cactus, may not have these exposed areas from bacterial necrosis forever. These spots soon become patchy and cork-like.
This is the cactus’ attempt at self-healing, but the corky areas still have bacteria within them that furthers the spread of this disease.
These cork areas progress even further, eventually turning black. By this point, any healthy tissue is now dead and completely rotting away. The tissue can even reach the point where it cracks and releases a liquid in a dark brown hue.
While bacterial necrosis is treatable, the sooner you catch this disease, the better. At its earliest stages, your plant’s survival rate is around 80 percent.
Do keep in mind that that number does steadily decrease the longer the disease goes on.
To prevent edema from causing unsightly blemishes on orchid cacti leaves, and to prevent excessive moisture from causing other diseases that can damage or potentially kill plants, always be mindful of the manner in which plants are watered. Try to keep water from splashing onto plant leaves. Always water in the morning so that the warmth of the sun or additional artificial light can dry the leaves faster. Also make sure there is sufficient air circulation in the space where plants are growing. High humidity and inadequate air flow may cause condensation to drip onto leaves.
A change in color becomes a cause for concern if many of your cactus’ leaves are noticeably purple.
Other signs of a problem include wilting, stunted growth, or soggy foliage. It’s important to determine the cause of the color change to determine if your plant is in any serious danger.
Cacti change colors when they experience stress. Cacti contain a purple pigment called betalain, which they produce more of as a stress response.
Let’s go through the potential causes and figure out how to treat them.
Even though cacti evolved to do well in bright sunlight, the one you have at home might be having trouble adjusting.
Store-bought cacti have usually been grown under shade in a greenhouse. This means that they aren’t used to so much direct sunlight.
Cacti need bright light, but it’s best for the light to be indirect and dispersed. Suddenly exposing a cactus to bright light can scorch its skin, causing it to turn a purplish-red color.
If your cactus is new, or if you have recently moved it to a sunnier spot, there’s a good chance that it’s sunburnt.
Luckily, the fix for sunburn is relatively easy. Move your cactus over to a spot that gets less direct sunlight.
Don’t go moving it to your basement just yet though — cacti still need a whole lot of sun!
Light that falls directly onto the plant, for example through a south-facing window, is direct sunlight.
The other windows in your house will offer indirect sunlight, which is more dispersed and easier on the plant.
Moving your plant over to a window facing in any other direction can help with sun scorching.
If you only have south-facing windows, try making a DIY sun filter. Simply prop a paper towel up over your cactus to give it some much-needed shade.
Purple leaves can be a sign of temperature-related stress. Cacti sometimes turn reddish-purple when their roots overheat.
Cacti can also turn purple when they get too cold. If the plant is suffering freeze damage, its cells burst and it is no longer able to hold liquid.
The ideal temperature is somewhere in the middle, so it’s important to avoid placing your plant in extreme temperature conditions.
To prevent your cactus from getting too cold, keep it away from drafty places like open doors and windows. Also avoid places that get too hot and dry, like fireplaces and heating vents.
Since the plant’s roots are the most likely to overheat, keep your cactus in a pot that stays cool. Avoid black plastic planters, and instead go with a material like clay.
Purple leaves could also be a sign of root rot, which results from overwatering and poor drainage.
If the soil stays wet for too long, your plant’s roots will die and will be unable to take in any more water and nutrients like magnesium. As a result, your cactus may turn purple.
Begin by pruning off the damaged roots and leaves with sterile scissors, and removing as much of the soggy soil as you can.
Transfer the plant into a sterile pot with fresh potting soil. Don’t water the cactus for a few days after moving it, and let the top inch of soil dry out between waterings.
Most of the time root rot is the consequence of overwatering. I have written an article on how you can save your overwatered cactus. Also, you will learn how to water them correctly.
One possible reason why your cactus is turning purple is because it doesn’t have the proper nutrients that it needs to survive. If your plant is wilting and turning purple, this may be a sign of a magnesium deficiency.
Christmas cacti are particularly likely to develop magnesium deficiencies. That said, all types of cacti are susceptible.
If your cactus is experiencing a magnesium deficiency, fertilizer is the solution. You can buy a magnesium-enriched fertilizer, but you can also go the DIY route with an Epsom salt treatment.
To make a magnesium treatment, mix the following ingredients into a spray bottle:
Use a spray bottle to spritz the leaves of the cactus, making sure to also get the undersides of its leaves. Keep using the spray mixture every two weeks until your cactus turns back to its normal color.
Crowded roots are another potential reason for your cactus’ color change. If a plant is grown in a container that is too small, its roots can become overly crowded, or “rootbound”.
Rootbound plants are unable to properly absorb water and nutrients from the soil. The nutrient deficiency can cause the leaves to turn purple as a stress response.
With time your cactus root system will also grow bigger and it may not fit within the pot it used to be.
If you notice that some parts of the roots are trying to get out through the drainage hole then it’s time to repot your cacti to a bigger container. Normally consider repotting your cactus every 3-4 years.
If your cactus’ roots have become overcrowded, you’ll need to move it into a bigger home. As a general rule, you should repot cacti when you can see their roots through the drainage holes at the bottom of the pot.
For faster-growing cactus varieties, this generally takes two to three years. Slower growing cacti should be repotted only about every three or four years.
Not all species of cactus have the same needs, so it’s important to research the ideal conditions for your cactus.
For example, some types of cactus such as Christmas cacti actually do well when they have crowded roots.
For this reason, a Christmas cactus should not be repotted unless it has been in the same pot for at least a few years.
Here are the steps to repot your cactus:
A purple cactus could also be a sign of an infection. There’s a chance that your plant is infected with a pathogen called Cactodera cacti, more commonly known as cactus cyst.
Cactus cyst happens when a cactus is planted in infected soil. Reddish-purple leaves, as well as stunted growth and wilting, are all possible signs of infection.
The most obvious sign of the cactus cyst, however, is tiny white spheres that appear on the plant’s roots. (Source: Plantwise Knowledge Bank)
It is very difficult to treat the cactus cyst once it has infected the plant, so it’s better to focus on prevention instead. To prevent your cactus from getting sick, plant it in clean, sterilized soil in a new pot.
If your cactus does get infected, your best bet is to discard the plant. As hard as it is to throw away your beloved cactus, it’s important to prevent the infection from spreading to your other plants.
Have you ever dealt with a cactus that is turning purple? What caused it, and what did you do to revive your plant?