Hoya plants are one of the plants native to tropical forests with beautiful flowers and healthy, green leaves. This plant is relatively easy to grow and does not require much care. Therefore, many people choose Hoya plants to grow in pots and decorate their home or office space.
When growing Hoya plants, you will not face many problems with diseases or unusual signs of health. However, black spots on hoya leaves are a phenomenon that you are sure to face several times in the process of caring for this plant. The appearance of black spots on leaves has various causes such as improper care or disease attack. To learn about the detailed causes and how to fix them, let’s dig into the details in the article below.
Top 5 Causes For Black Spots On Hoya Leaves
The cause of black spots appearing on the leaves of Hoya plants can be related to improper watering and causing waterlogging. In addition, the infestation of insects, diseases, and fungi is also the cause of black spots appearing on the leaves of Hoya plants. Here are the causes and detailed remedies for the appearance of dark spots.
Over-watering Leads To Waterlogging
Proper watering is one of the most important factors in maintaining the health of any plant species. Each plant species has different water needs in different environments. Therefore, you need to learn about the water needs of Hoya plants before planting them.
Hoya plants are native to tropical forests, so they prefer moist environments to grow quickly. However, if the soil is too wet and lacks oxygen exchange, this plant will also have many serious health problems.
Over-watering and poor drainage due to poor-quality potting mix will lead to waterlogging. Waterlogging will make the root system unable to transport nutrients, oxygen, and other essential substances to the plant. From there, Hoya plants will have some signs such as yellowing leaves, black spots appearing, wilting or drooping leaves around the pot, root rot, and gradual death.
Waterlogging that takes place for a long time will make the entire leaves and stem of the plant always in a wet state. Moist growth is an ideal environment for fungi to grow. The fungus then releases the fungal spores into the air and the spores attach to the leaves and cause black spots.
How to handle
- Stop watering completely until the soil is completely dry again.
- Move the pot to an area with indirect sunlight to promote evaporation from the soil and help the soil dry faster.
- Prune off yellow, brown, and black leaves or rotten branches to prevent the disease from spreading to other leaves.
- Inspect the entire root system and cut off any rotten or diseased root branches.
- Replant the plants in a new potting and soil mix to keep harmful bacteria from lurking in the soil.
- The soil mix should include ingredients such as perlite or gravel, and coir at the bottom of the pot to speed up drainage.
- The pot must have at least one drainage hole and you need to clean it regularly to avoid clogging.
- Do not water again until the soil has completely dried by 2-3 inches.
Fungal diseases are another common cause of black spots on the leaves of Hoya plants. Some common fungi that cause this condition are black spot fungus, sooty mold, leaf spot, and anthracnose. Each fungus has different pathogenic mechanisms and prevention methods. So, check out the details below.
Black Spot Fungus
Black spot (Diplocarpon rosae) is a fungus that most commonly causes the appearance of black spots on the leaves of Hoya plants. This fungus will appear as small and dark spots on the leaves if you look at it with the naked eye. These small spots will grow and join together to form large black patches on the leaves and cause the leaves to fall when they have completely lost their energy. Black spot fungus will often appear if the environment is wet or flooded for a long time. This fungus will grow quickly in an ideal environment.
How to handle
- Prevent humid environments by properly watering and maintaining humidity.
- Do not water directly on the leaves, especially at night, it will create a humid environment for fungi to grow.
- Once plants have been infected with this fungus, you need to prune diseased leaves to avoid spreading to healthy leaves.
- Make sure tools used like scissors or knives are disinfected before and after pruning to prevent the disease from spreading to other potted plants in the space.
- Prioritize the use of fungicides of natural origin such as neem oil.
You can easily spot the appearance of sooty mold because the fungus will create a black coating on both surfaces of the leaves. This disease usually appears after the attack of aphids or other parasitic scale insects on Hoya plants. These insects will secrete a sticky substance like honey. This is also an abundant source of food to nurture the development of sooty mold.
If this fungus grows rapidly, it will create black coatings on the entire leaf and cause the leaves to turn yellow or brown uniformly. In particular, the black film on the leaves of Hoya plants, also known as carbon black, will prevent the plants’ photosynthesis and oxygen exchange.
How to handle
- The simplest and most effective treatment is to remove scaly insects and aphids that are parasitic on plants.
- Use insecticides or catch them by hand.
- Prune severely damaged and unrecoverable leaves to prevent the fungus from spreading to other leaves.
- Use neem oil to clean the black film on the two surfaces of the leaves.
Fungal Leaf Spot
Leaf spot is also a common fungal disease of both perennials, mature plants, and annuals. The humid environment also causes this fungus to grow quickly and is difficult to control completely once the plants are infected.
The symptoms of this fungus will vary from plant to plant. However, the common sign is usually the appearance of black spots on the leaves. If plants are severely infected, the entire leaves will turn yellow, and brown, and fall off simultaneously.
How to handle
- Prune infected leaves with clean disinfected tools to prevent the fungus from spreading to other healthy leaves.
- Use fungicides such as neem oil or horticultural oil.
- Water only the root part instead of the leaves to avoid creating a humid environment for the fungus to grow.
Anthracnose is also a fungus that causes black spots to appear on the leaves of Hoya plants. This fungus can survive in the soil, leaves, or stems of plants. They hide in leaf crevices or crevices in leaves and stems. Therefore, you will have difficulty seeing this fungus with the naked eye. Anthracnose will enter leaves and stems through open wounds or stomata on the leaf surface.
Once this fungus has entered the plant, it will begin to grow rapidly and produce airborne fungal spores. These fungal spores will attack the entire leaves and stem of the plant when given the opportunity and cause black spots to appear on the leaves.
How to handle
- Prune infected leaves with sterilized tools to prevent the fungus from spreading and to create space for air circulation.
- Move the pot to a well-ventilated and well-lit location to stimulate the regeneration of new cells.
- If the condition is more serious, you need to use specialized fungicides.
- Using the correct and sufficient dosage as recommended by the manufacturer is an important factor in the treatment of this fungus.
Read this post: Do Hoyas Like Humidity? (How To Increase Humidity)
Another common cause of black spots on the leaves of Hoya plants is a bacterial infection. Common diseases caused by this bacterium are leaf spot and blight.
Bacterial Leaf Spot
The bacterium that causes the leaf spot of Hoya plants has the scientific name Xanthomonas campestris pv. Vitians. This type of bacteria will enter plants through open wounds on the leaves, stems, and roots of plants.
After plants are infected, leaves will appear as small black, brown, or dark spots. They will then spread and cover the entire leaves of the plant if you do not stop them in time. The entire leaves will turn yellow, wither and fall off. Bacteria can be spread through water, wind, soil, or garden tools for long periods. Therefore, you should clean pruning tools such as knives or scissors with specialized disinfectant solutions.
How to handle
- Create an open space between the leaves by removing diseased leaves.
- Place the pot in an area with good light and air circulation.
- Limit watering during this time and only water when the ground is completely dry.
- Use a copper-based fungicide or microbicide as recommended by horticulturalists.
Leaf blight is also a common disease caused by harmful bacteria that attack and cause black spots on both sides of leaves. This bacterium has the scientific name Pseudomonas syringae pv. Syringae. It also attacks plants through open wounds on leaves or stems. After plants have been infected, black spots quickly appear and cover the entire leaf. A yellow halo will cover the dark spots that you can see with the naked eye.
How to handle
- Clean the leaves weekly with a specialized solution to kill bacteria.
- Use neem oil or a mixture of baking soda and water to clean the leaves of Hoya plants.
- Use a copper-based fungicide to control the rate at which bacteria spread.
- You can also try using a copper-based fungicide to help control the bacteria.
- Control the dosage and duration of use of fungicides according to the manufacturer’s recommendations to avoid overdose.
Another common cause of black spots appearing on the leaves of Hoya plants is a viral infection. Many viruses attack plants such as tobacco mosaic virus, cucumber mosaic virus, and impatiens necrotic spot virus.
These viruses will attack plants through butterflies, scaly insects, or contaminated garden tools. Some signs when plants are infected with the virus are deformed, dry leaves, yellow spots, or black spots on the leaves.
How to handle
- There is no single method that can completely cure the virus.
- You just have to remove or isolate the potted plant from your garden or other potted plants to prevent the virus from spreading.
- Always clean garden tools before pruning to prevent the virus from spreading from plant to plant.
- Place the potted plant in a light and airy area to limit insect attack.
Pests are the ultimate cause of black spots on the leaves of Hoya plants. Common insects that cause this condition are aphids, scale insects, and mealybugs. They attack the leaves and stems directly by sucking the sap, causing the leaves to be stunted, and deformed, black spots appear and mass fall.
Aphids: This insect is small and pear-shaped. They are black, brown, green, or white. They often hide in clusters on the leaves or stems of plants. Aphids will secrete a sticky substance called sweet nectar. This fungus is favored by many insects, and soot fungi also rely on this honey to grow rapidly.
Scale: They are small, hard-bodied, black or white insects. This insect will suck the sap directly from the leaves and stems of Hoya plants. From there, the leaves will be deformed, turning yellow or brown. This insect also secretes sweet juices to attract other insects to attack the plant.
Mealybugs: These insects have a small oval shape and their entire body is covered with a white substance. Mealybugs also suck sap directly from leaves and stem, then they secrete a sweet juice to attract companions and other insects.
How to handle
- Use a soft and clean towel to clean the sides of the leaves and remove any insects that cling to the leaves.
- You can also use a strong stream of water to spray the leaves and remove them from the plants.
- If plants have been severely attacked by insects, you need to use natural or chemical insecticides.
- Use according to the dosage and duration recommended by the manufacturer to avoid overdose.
- Spray a mixture of dishwashing liquid with water to clean the surface of the leaves.
- Continue cleaning the leaves daily until the insects are gone.
The appearance of black spots on the leaves of Hoya plants can be related to over-watering leading to waterlogging, fungal diseases, insect attacks, viral infections, or leaf spot diseases. These diseases often have some signs such as black spots, yellow spots, and wilted, dry, or drooping leaves around the pot.
To prevent this, you need to water properly to limit waterlogging. This leads to a humid environment that allows fungi to grow quickly. Cleaning leaves and stems regularly is also a way to prevent fungal and insect diseases. Alternatively, you can also use natural or chemical fungicides to kill insects.