23 Common Garden Diseases and How to Treat Them

When it comes to garden diseases, the most effective method of fighting them is preventing them in the first place. A few best practices are key to prevention: providing good airflow between plants and limiting overhead watering are the top two ways to prevent diseases from spreading throughout your garden. But sometimes, a disease will appear anyway. 

Below, we’ll discuss how to treat each disease if treatment is possible. Unfortunately, some garden diseases aren’t curable. Removing infected plant material to prevent its spread to nearby healthy plants is sometimes the best approach. The right course of action also depends on the type of disease.

Let’s dig into the most common diseases you may encounter in your garden, and what you should do about them.

Powdery Mildew

This is by far the most common garden disease, and at a certain point in the year, usually in the late summer or fall, powdery mildew loves to make an appearance. Good garden practices like maintaining adequate airflow between plants and avoiding overhead watering can help prevent or delay powdery mildew, but plants in the cucurbit family, like squash and cucumbers, are particularly susceptible. 

Powdery mildew is easy to identify by the powdery white spots it produces on the undersides of leaves. This fungus thrives in moist and humid conditions. If you catch it early, you can remove infected leaves and prevent its spread. However, if it spreads, it will cause the leaves to die, which will eventually result in plant death.

If it is the end of the season and for example, you have the last of the season’s pumpkins ripening on the vine, then you might choose not to treat it at all since the season is coming to a close. If it’s earlier in the season and you want your plants to hold on, you can treat it with a copper fungicide spray that can help slow the spread to a degree. 

Downy Mildew

This type of mildew is also a fungal infection. Downy mildew is very similar to powdery mildew in appearance and attributes. You’ll be able to differentiate downy mildew as it appears as yellowish spots on the leaves with a purplish-gray powder surrounding each spot.

Humidity, poor airflow, and warm days with cool nights are the ideal conditions for this fungus. This is why both of these mildews generally appear in the late summer and fall as the nights cool off. Remove diseased leaves as soon as they appear and dispose of them to prevent the spread to nearby plants. Treat downy mildew with a copper fungicide spray if necessary. 

Root Rot

Root rot is another garden disease that thrives in overly wet conditions. Increasing the drainage around your plants and avoiding low-lying areas where water can pool (like close to a downspout) will help you avoid root rot. Overwatering houseplants is the most common cause of this type of rot, but it can affect outdoor plants as well. 

Even though you are watering the plant consistently, you may notice wilting. This is a symptom of root rot. If you dig up the plant, you will see mushy, brown, and rotten roots. It’s tough to recover from root rot, but it can be done if there are still fresh white roots on the plant. If the plant is growing in a container then dig it up and move it into a container with dry soil.

If the plant is growing in the ground, then let the soil dry completely before you begin watering it again. Moving forward, be sure to err on the side of less water rather than too much. Sticking your finger into the top few inches of soil will tell you if it’s time to water. If it’s dry, then give your plants a drink. 

Early/Late Blight

These types of blight commonly affect tomatoes and potatoes. Early blight appears as dark brown or black leaf spots. The black spots may even spread to the tomatoes themselves. Early blight infections generally occur in May or June following a very wet spring. Late blight appears as greenish-black spots on leaves, then stems, and finally will be found on the fruits. Late blight generally appears in August and can spread very quickly. 

As with the above-mentioned fungal infections, you can treat advanced cases with a copper fungicide spray. Preventative methods include good airflow and avoiding overhead watering as much as possible. Of course, you likely have little control over how much it rains! This is why these blights usually appear in particularly wet seasons. 

Leaf Spot

There are many different types of leaf spot. They generally appear as yellow or brown lesions, or a combination of the two, that look like burn marks on the leaves. Septoria leaf spot is the most common of these types of fungal infections. Overly wet conditions and poor airflow are the top contributors to leaf spot infections.

Treat this fungal infection with copper fungicide sprays. It’s important to note that these sprays will not cure infected leaves, but they will protect healthy leaves from becoming infected. It’s always best to remove infected leaves and dispose of them before you begin treating the disease. 

Blossom End Rot

Also referred to as a calcium deficiency, or BER, blossom end rot has likely been experienced by every tomato grower, both beginner and experienced, since the beginning of time. Although it most commonly appears in tomatoes, it can affect peppers as well. The symptoms will appear as dead, black tissue localized to the blossom end of the fruit. 

Its link to a calcium deficiency has given way to odd remedies like burying tums or egg shells in the ground underneath tomato transplants. And while you can’t really hurt your plants by doing this, it probably isn’t necessary, and eggshells take a long time to break down and become available to your plants. A lack of water or inconsistent shallow watering is most often the cause of this type of calcium deficiency. 

Inconsistent water reduces the plant’s ability to uptake calcium from the soil. Too much fertilizer can also cause blossom end rot since an excess of nitrogen in the soil also reduces the plant’s ability to uptake calcium from the soil. Suggested treatments include more consistent, deep waterings for your plants.

The plant still needs to be able to uptake calcium from the soil which is accomplished through the roots taking up water. For this reason, adding calcium to the soil might not do much. Adding organic matter or compost to the soil is a better call since it can help add nutrients and also help with moisture retention. Using drip irrigation on a timer can help give regular and even water to your plants. 

Gall

Leaf gall is a swelling that occurs on plants. It can occur on the leaves, stems, and roots. Galls can be caused by a wide range of organisms, including insects, mites, nematodes, fungi, bacteria, and viruses. The first step to treating gall is by identifying the cause.

A quick internet search with the affected plant name and “leaf gall” can give clues as to the cause. Insects, namely wasps, are the cause of most galls. Galls can be harmful when they are numerous on young plants, but generally speaking, they don’t cause any lasting damage and are mostly cosmetic. 

Root-knot nematodes are a common pest of tomato plants, and they can cause damage to the plant and eventual plant death by feeding on the plant roots. The name root-knot nematode comes from the fact that when these nematodes feed on the roots, they cause galls to form, which appear as knots in the root system.

The strong scent of plants like marigolds deter root-knot nematodes. They make an excellent companion plant for tomatoes but they don’t actually emit the chemicals that deter nematodes until the second season in the area they are planted. Regular crop rotation in your garden can also help you avoid these pests.  

Corn Smut

The fungus Ustilago maydis is responsible for the corn smut disease. This fungus forms galls on corn kernels. It distorts the corn kernel growth and causes it to appear twisted, blistered, and explode in growth away from the cob.

Corn infected with this fungus will not produce perfectly straight rows of kernels. There are currently no fungicides available to effectively treat corn smut. It’s best to prevent it by practicing crop rotation or utilizing resistant varieties. 

However, corn smut is actually a desirable trait in some cases since the infected corn is still edible. In Mexico, corn smut is a delicacy known as huitlacoche where it is used as a filling in tacos, in quesadillas, and in soups.

The presence of corn smut also changes the composition of the nutrients in the corn cobs, making them much higher in protein. Farmers in Mexico spread the spores around their fields intentionally, whereas most farmers in the United States consider corn smut to be a blight on their crops. 

Rust

Rust is another fungal disease, and it gets its name from the fact that it appears on the foliage of plants like little spots of reddish-brown rust. Hollyhocks and garlic are two of the most common plants affected by rust for the home gardener.

Remove infected leaves as soon as they appear to help limit the spread. Treat severe infections of rust with a copper fungicidal spray. Again, this does not remove the infection but prevents its spread to healthy leaves. Rust is best managed by removing affected leaves. It generally does not kill the plant, but in advanced cases will cause extremely stunted growth and production. 

Black Knot

This fungal disease affects plants in the Prunus genus, like plum, apricot, cherry, and chokecherry. It causes wart-like growths to appear on stems and branches that will turn into black knots in the fall. Prune these knots off while the tree is dormant in late winter. It is important to do this since spores from old knots can infect new growth.

There are several species in the Prunus genus that are resistant to black knot. Planting these varieties is a good preventative measure. Only employ the use of fungicides for severe infections or for young trees, otherwise, pruning has shown to be most effective for controlling and reducing the spread. 

Botrytis

This fungal disease is also known as gray mold. It appears as a gray fuzzy mold on fruits, causing mushy spots and decay. It initially forms on dead or dying plant material, but with wet conditions, it can spread and infect healthy plant material as well. Peonies, hostas, roses, strawberries, and raspberries are particularly susceptible, although there are resistant cultivars.

Placing your plants in a sunny area with good air circulation is a good preventative measure. Remove infected leaves and fruits as soon as they appear. Fungicides are only minimally effective here. 

Anthracnose

This fungal disease causes dark sunken lesions on the stems, leaves, and fruits. This disease thrives in warm and humid conditions. Trees such as sycamore, ash, oak, and maple are particularly susceptible.

For trees, you can prune out the deadwood to help promote airflow and remove infected leaves. Since this fungus thrives in wet conditions, it will often clear up as the wet spring weather clears and dry and hot summer weather moves in. 

Fire Blight

The scorched appearance of the infected foliage is what gives this bacterial disease its name. Fire blight can be spread from tree to tree through contaminated pruning tools and can cause major damage to apple and pear trees.

When pruning your trees, always be sure to clean your tools with an alcohol-based cleaner, like rubbing alcohol, when moving from one tree to the next. Streptomycin sulfate sprays can slow the spread, but severe fire blight can cause trees to die, and there is no cure. Remove severely infected trees completely.  

Fusarium Wilt

This type of wilt is caused by a soil-borne fungus that affects both ornamental and edible plants. In the annual vegetable garden, it most often affects tomatoes, potatoes, peppers, eggplants, squash, cucumbers, and melons. It causes leaves to turn yellow, then brown and dry and die off. Eventually, the loss of foliage will cause plant death.

Fusarium wilt can survive in a wide range of soil conditions and can be found worldwide. Crop rotation is not an effective method of control since it can survive in the soil for long periods of time. It spreads quickly in soils that are consistently moist with bad drainage, so amending your soil and providing good drainage for plants will help limit its spread. There are resistant cultivars of tomatoes, cucumbers, and melons that make for a good preventative measure as well. 

Apple Scab

This type of scab affects apple trees. Infected leaves will have green spots with feathery margins that turn brown or black. The infected leaves will turn yellow, then brown, and eventually drop from the tree. The fruit can also become infected with circular brown spots.

This fungal disease can survive over winter in infected foliage that has fallen to the ground. Planting resistant apple cultivars is a good preventative measure. Prune trees to increase airflow and limit the conditions that this fungus prefers to thrive. Be sure to rake up fallen leaves to avoid providing a place for the fungus to survive over winter. Treat extreme infections that have lasted a few years in a row with a fungicide spray. 

Peach Leaf Curl

This type of leaf curl infects peaches, plums, and nectarines. This fungal disease commonly occurs in cool and wet environments and generally appears in late winter and early spring. Infected buds will produce leaves that are severely curled.

It is hard to control in areas that receive a lot of rain due to the constant overhead water falling on the leaves. This can be curtailed by planting your peach tree under the eaves of a house or overhang of another structure. Spray trees with a copper fungicide to limit the spread. 

Verticillium Wilt

This fungal disease affects a wide range of deciduous trees, herbaceous perennials, and annual vegetables. This soil-borne disease enters through the roots of the plant and affects its ability to take up nutrients and water, which causes the plant to wilt. The first symptoms will be yellow leaves, overall wilting, and eventual death of the plant.

There is no cure for this type of wilt. Remove and destroy infected plants. Verticillium wilt can live in the soil for years before suddenly infecting a tree. It is thought that trees and plants experiencing drought stress can become more susceptible. If you discover that verticillium wilt is in your soil, then you should plant a resistant species in that spot. 

Aster Yellows

Aster yellow is caused by a phytoplasma or small bacterium. It can survive in the gut of the leafhopper, who then spreads it to a healthy plant when feeding on the plant’s tissue. Symptoms will appear as stunted growth, yellow leaves, and malformed flowers.

The phytoplasma does not survive in dead plant debris. There is no cure for this disease, and infected plants should be removed and destroyed to limit the spread to healthy plants. Controlling leafhoppers is another way to prevent the disease’s spread.

Mosaic Virus

There are many different types of mosaic virus. Generally speaking, mosaic virus refers to any virus that causes the foliage of the plant to become mottled in appearance, like a mosaic. The most common mosaic virus is cucumber mosaic virus (CMV).

Despite its name, it affects many, many more plants aside from cucumbers, including ornamentals, perennials, and a wide range of vegetables. This virus can cause significant crop loss. It survives overwinter in susceptible perennial and biennial plants, but it can’t survive in the soil. 

Most commonly CMV is spread by aphids feeding on infected plant tissue and then transmitting it to healthy plants. There is no cure for CMV. Remove and destroy infected plants. A good preventative measure is to keep aphid populations in check by either manually removing them from plants with a strong blast of water from the hose, or encouraging healthy populations of predatory insects like ladybugs. 

Black Spot

This fungal disease commonly affects roses. The symptoms will appear as circular black spots with fringed edges. Severely infected leaves will turn yellow and this will ultimately affect the amount of blooms that are produced.

Remove infected leaves and dispose of them to prevent the spread of this fungus. Avoid overhead watering your rose bushes as a preventative measure. Use copper fungicide sprays to prevent this disease, before it appears. There are also resistant varieties that will help you avoid this disease altogether.

Cedar Apple Rust

Cedar apple rust is similar to the above-mentioned fungal rust. However, this fungus specifically attacks apple trees and trees in the cedar family. It will appear as bright orange-yellow raised spots on the leaves. The spots appear in mid-spring and cause both leaves and fruits to drop from the tree.

This fungus primarily survives in eastern red cedars and junipers. For this reason, it is a best practice to plant apple trees at least 200 feet from any cedar trees on your property to prevent the spread. Many apple cultivars are resistant to cedar apple rust. 

Leaf Curl

Leaf curl is generally the symptom of a viral disease. There is such a wide range of viruses that can cause this symptom that it might be hard to pinpoint that exact cause, but if you’ve ruled out the possibility of other diseases and the curling is not due to extremely hot weather or lack of water, then your plant likely has a virus.

This is especially true if the leaf curl is affecting newly formed, young leaves. There are no reliable treatments for plants infected with a virus, and they should be removed and destroyed. Control aphids as a prevention method, as they tend to spread the disease.

Bacterial Canker

This bacterial disease afflicts stone fruit trees like cherries, peaches, plums, and apricots. It spreads during the cool and wet weather of the spring. Small black spots appear on the leaves and the fruits as well. Cankers will form on twigs and small branches. There are no known resistant cultivars. Prune out cankers and dead twigs and branches during dry weather to limit the spread.