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How to Identify and Manage 10 Common Plant Diseases

Just like any other living organism, plants get sick, too. If you have a garden or a significant number of indoor plants, dealing with such ailments is practically inevitable. When that happens, it’s often difficult to identify the issue and know how to treat the plant without harming it with harsh chemicals and treatments.
Learn to diagnose and deal with 10 widespread plant diseases and how to prevent them from spreading through your outdoor or indoor garden.

1. Powdery Mildew

Common Plant Diseases Powdery Mildew
Powdery mildew is one of the most common and easily identifiable ailments that occur in plants. The disease shows up on many flowering plants, including lilacs, cucumbers, peas, daisies, grapes, and apples, and is caused by a fungus. Luckily, mildew is one of the least harmful plant diseases. 
What it looks like: Powdery mildew is easy to spot, as it leaves white dust on the plant’s leaves, stems, and flowers.
What you can do: Collect and get rid of all infected leaves and parts of the plant to prevent the spread of spores. The fungus that causes powdery mildew requires a damp and dark environment to multiply, so it’s useful to avoid watering plants in the second half of the day. There are dedicated fungicides available at garden centers, but you can also make your own solution to get rid of the fungus by combining 1 teaspoon of baking soda and 4 cups of water.

2. Rust

Rust is another fungal disease common in plants. It affects both woody and herbaceous plants, but it’s most common in roses, daylilies, and tomatoes. Luckily, rust rarely kills the plants it infects, but it may make them produce fewer flowers and less harvest. 
What it looks like: There are many types of rust, but it usually appears in the form of orange or yellow spots on the bottom of the leaves and the stems of the plant. With time, the spots turn black. The shape of leaves may become distorted and the plant may lose some of its foliage over time.
What you can do: The disease spreads through the spores present on the affected leaves of the plant, so it’s important to remove all the infected parts and destroy them. For the same reason, it’s recommended to clear any debris and weeds in-between plants to prevent the spread of the disease. Fungicides that address rust are available at gardening supply stores.

3. Root Rot

Common Plant Diseases Root Rot
Image Source: Reddit

Root rot is another widespread issue, especially in potted plants. Root rot occurs in two scenarios: some kind of fungus living in the soil or prolonged overwatering. In most cases, both overwatering and pathogens are involved. Slowly but steadily, the plant’s roots start to rot away. 

What it looks like: If you notice that the leaves of your plant are turning yellow and wilting for unknown reasons, you’ll need to take out the plant from the pot and check the roots. Gently remove the soil from the roots and examine them. Affected roots may turn black, feel mushy to the touch, or fall off altogether. Healthy roots, on the other hand, feel flexible and firm.

What you can do: First and foremost, it’s important to act fast because the rot can spread to healthy roots and kill them rather quickly. Gently wash the roots of the plant with water and trim away the affected roots, occasionally disinfecting the shears with rubbing alcohol.

If you had to remove a significant part of the root system, prune one-third of the plant’s leaves as well. Discard the old potting soil and wash the pot using bleach. Repot the plant, making sure that it has good drainage. Avoid overwatering the plant and only water when the top of the soil is dry.

4. Mosaic Virus

There is a large variety of mosaic viruses, but they all make the leaves of the plant have a mottled appearance, hence the name. Mosaic viruses infect a variety of flowering plants, such as petunias, roses, marigolds, as well as many edible crops, including cucumbers, tomatoes, peppers, beans, potatoes, corn, apples, spinach, pears, and cherries.
These viruses are more widespread in hot weather. Tobacco is a common carrier of the virus, so smokers should wash their hands before handling gardening supplies and plants.
What it looks like: Leaves affected by mosaic viruses have yellow, white, and green spots or streaks. Sometimes, the affected leaves are curled and misshapen. Importantly, these viruses may result in fewer flowers, a smaller yield, and malformed fruits.
What you can do: Getting rid of mosaic viruses can prove to be difficult. It’s important to destroy all infected plants entirely and avoid planting species susceptible to the virus in the area for 2 years. Monitor the surrounding plants closely, and repeat the same steps if they start exhibiting signs of the disease. Do not compost the plants because the virus can survive in your compost pile, so it could eventually spread to other parts of the garden through compost.

5. Cylindrocladium

Common Plant Diseases Cylindrocladium
Image Source: Scot Nelson/ Flickr

Cylindrocladium fungi can be the reason behind many common plant ailments, including root rot, lower stem blight, and leaf spots. The disease can affect legumes, trees, and ornamental plants.

What it looks like: Since the fungus can infect any part of the plant, the symptoms can vary from dark spots on the leaves, yellowed and wilted lower leaves, and root rot. Cylindrocladium spores are transmitted through water, so splashes of water can cause leaf spots. If several plants are placed on a flat surface, water leaking from the infected planter can get to the root systems of other plants and spread the disease.

What you can do: Treating Cylindrocladium is difficult because fungicides can only prevent the disease from spreading to other plants, but will not cure the affected plant. Hence, affected plants should be discarded. To prevent Cylindrocladium in plants, it’s important to make sure that your potted plants are well-drained. It's also important to keep the leaves of the plants dry, especially in the evening.

6. Verticillium Wilt and Fusarium Wilt

Common Plant Diseases Verticillium Wilt and Fusarium Wilt

Image Source: Scot Nelson/ Flickr

Verticillium wilt and fusarium wilt are also caused by soil-borne fungi, and they affect hundreds of plant species. Trees, shrubs, ornamental and edible plants are all susceptible to these pathogens. Famously, fusarium wilt, commonly known as Panama disease, lies behind the extinction of the Gros Michel banana species in the 1960s and is threatening banana plantations worldwide today.

What it looks like: These fungi make their way to the plant’s stem through the roots, blocking the vascular system that provides the leaves and stems with nutrients. As a result, the foliage of the plant suddenly turns yellow, wilts, and falls off. The disease can also cause root rot.

What you can do: These fungi can survive in the soil for years and no effective fungicides are available to home gardeners. If fusarium wilt or verticillium wilt appears in your garden, destroy all the infected plants immediately and avoid planting species susceptible to the fungi for 5 years. Sterilizing all the tools with a bleach solution after use is also recommended.

7. Downy Mildew

Common Plant Diseases Downy Mildew

Downy mildew is caused by parasitic oomycete microbes that live and feed on plants. The ailment occurs in many edible plants, such as lettuce, broccoli, and cauliflower, but it’s a particular nuisance for those who grow fruits and vegetables on vines, such as grapes.

What it looks like: Usually, dowry mildew appears in wet and humid weather. The top part of the leaves becomes patchy and discolored, whereas the bottom part develops gray mold.

What you can do: Once again, fungicides are not effective at treating this ailment. Therefore, it’s important to observe the plants and consistently remove and get rid of infected leaves or even entire plants if they’re badly affected. To prevent the ailment from spreading, avoid crowding plants and keep the garden clean.

8. Anthracnose

Common Plant Diseases Anthracnose
Image Source: Scot Nelson/ Flickr

This is another fungal disease that usually attacks plants in cool and humid weather. The ailment may become dormant during the summer heat and then reappear in the fall because rainy weather creates the perfect conditions for the fungus to spread. The disease can reappear every year, but it is rarely fatal for the plant. Anthracnose can infect edible plants, grass, trees, shrubs, and even potted plants, such as palms, ficus trees, and succulents. 

What it looks like: Anthracnose lesions appear on the leaves and twigs of plants. They look like small, dark, sunken spots. Notably, the bottom of leaves affected by the fungus will be dotted with brownish spots the size of a pinhead.

What you can do: To limit anthracnose, trim the affected leaves regularly and clear away any fallen leaves or twigs from the ground. The fungus can overwinter inside fallen twigs and leaves, so make sure to get rid of them regularly.

9. Mold

Common Plant Diseases Mold
Image Source: Scot Nelson/ Flickr
There are many mold varieties that can affect different plants. Sooty mold is one of the most common varieties. It appears on parts of plants bitten by insects and then starts spreading to the leaves and stem of the plant. Snow mold is another common variety of mold that attacks turfgrass during the winter. It starts growing beneath the snow and becomes visible in spring.
What it looks like: As the name suggests, sooty mold appears as a black powder that covers the plant’s leaves and possibly stem. The mold prevents the plant from photosynthesizing, stunts plant growth, and may lead to foliage loss. Snow mold, on the other hand, looks like light tan or gray matted grass.
What you can do: Since sooty mold is caused by insect bites, you need to get rid of the parasites to limit the growth of the mold. Learn more about common plant parasites and ways to get rid of them here: Houseplant Pest Guide - How to Identify & Fight Them.
As for snow mold, all you have to do is rake the affected grass to allow fresh grass growth. It’s also a good preventative practice to trim the grass shorter in the fall.

10. Black Spot

Common Plant Diseases Black Spot
Like many ailments on this list, black spot is rarely fatal, but the fungal disease can stunt the plant’s growth and makes them more susceptible to other problems. Black spot is commonly found on roses, but it also appears on fruits and other flowers. 
What it looks like: Since black spot is caused by a type of fungus, it thrives in cool humid weather. You will notice it as small black spots that start appearing on the foliage in spring. The spots then expand and turn the entire leaf yellow or brown. Eventually, leaves affected by black spot fall off.
What you can do: Limit the spread of black spot by trimming and getting rid of affected leaves and twigs, as well as any debris and old leaves around the plants. It’s also a good idea to plant species susceptible to black spot, such as roses, strategically in places where the morning sun will evaporate any moisture from the leaves. Fungicide can prevent the spread of black spot.
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