12 Hydrangea Diseases: How to Identify, Prevent, and Treat Them
Did you recently plant hydrangeas in your garden, but aren't quite sure how to identify the most common diseases that may attack them? There are many different hydrangea diseases that may attack your plant, but knowing how to identify, prevent, and treat them is critically important. In this article, gardening expert and hydrangea enthusiast Jill Drago walks through the different hydrangea diseases that are most likely to strike.
Are you wondering what those brown spots are on your hydrangea leaves? While hydrangeas for the most part are resistant to many pests there are still a few diseases that can affect them. These diseases fall under the categories of bacterial diseases, fungal diseases, or viruses. All three are problematic, and can prevent your hydrangeas from blooming to their fullest potential.
While many symptoms of these diseases look similar, they have very different causes, and some have different treatment options. So, where do you begin? The very first step in treating any hydrangea disease, is to properly identify the problem. Once you’ve identified the issue, it’s a matter of treatment, if treatment is an option.
It’s important to note, that not all hydrangea diseases are readily treatable. With some diseases, you may end up just needing to replace the plant altogether. But many common diseases can be treated, and successfully eradicated. Of course, the best treatment option is disease prevention. So, continue reading to help diagnose the disease impacting your hydrangea and find ways to treat and prevent them!
Bacterial diseases in hydrangeas are quite common in hydrangeas, and unfortunately most bacterial diseases are not able to be treated. This means prevention with bacterial diseases is critical, so you don’t end up needing to completely pull your entire plant.
Bacterial diseases can spread from plant to plant, so it’s important to prevent them before they occur. Let’s look at the two most common bacterial diseases you may come across.
Bacterial wilt can affect most species of hydrangea. Bacterial wilt disease in hydrangea occurs mainly after heavy rains and hot weather. Typically, bacterial wilt affects the flowers and leaves, but in extreme cases the plant itself will wilt. If this happens, it will become susceptible to root rot and the plant will eventually die.
Bacterial wilt is easily identifiable because it is one of the few diseases that directly affects the flowers. Look for wilted flowers, and brown, wilted leaves on the hydrangea plant itself.
Remove infected plant tissues to prevent any further infections. Keeping the base of the hydrangea plant clear of leaves, grass clippings, and other plant materials.
There are currently no options to treat bacterial wilt in hydrangeas.
Bacterial Leaf Spot
Bacterial leaf spot is similar to fungal leaf spot which we will talk about soon. In this case, the disease is spread by bacteria that enters through any natural wounds or openings the plant may have, like stomata on the underside of the leaf.
Natural wounds may be from an insect or from pruning. The initial symptoms of bacterial wilt appear as water soaked spots. These spots will slowly darken and spread. Occasionally these spots will enlarge and cause the death of mature leaves.
Bacterial leaf spot is easy to identify. You’ll notice them as water soaked spots, that look like brown spots on the plant’s leaves themselves.
To prevent bacterial leaf spot, keep your plants and the area around them free of leaf debris. Remove any potentially infected plant tissue as soon as you notice it, and be sure to clean your clippers after. If it is necessary to remove the entire plant due to the disease, do not plant another plant in that spot. This will help to ensure the bacteria doesn’t spread to your new plant.
Just like bacterial wilt, there is no known chemical option to treat bacterial leaf spot.
Fungal diseases arise when a fungus takes hold of your plant. They can be difficult to distinguish from both bacterial diseases and viral diseases. Fungal diseases can spread from plant to plant, and across different types of plants.
Similar to leaf spot (and most fungal diseases for that matter) anthracnose likes warm, wet weather. Anthracnose will spread from water droplets splashing onto the plant. This could be from rain or irrigating, and will worsen with the humid weather.
Anthracnose begins as small brown spots and will grow quickly, affecting both leaves and flowers. The centers of the spots will turn light brown and begin to look like a bulls-eye. As the disease progresses, the spots will grow into more angular patches. The leaves will turn a darker shade of brown and eventually drop.
Anthracnose looks a bit like leaf spot, except the color of the infection shows up as dark brown spotting on the hydrangea’s leaves. Eventually the leaves turn dark brown, and will fall off the plant.
Anthracnose can be prevented by keeping your hydrangeas free of debris. Do not water overhead, doing your best to keep flowers and leaves dry. If you have diagnosed your hydrangea as having anthracnose, remove all infected debris, bag it up and throw it away. This will keep it from spreading to nearby plants.
If extra treatment is needed, a copper based fungicide such as Bonide copper fungicide will help. Spray the leaves and flowers every two weeks. If you have a diseased hydrangea one year, be sure to spray with the same fungicide in the spring to prevent a second infection.
Botrytis blight is a common disease that attacks hydrangea. You will notice small wet spots on the leaves that will quickly expand into brown irregularly shaped blotches. This disease can also affect the flower buds.
Botrytis blight thrives in humid and wet conditions. This fungus will survive on fallen plant leaves, meaning moisture from the air or irrigation can splash spores up onto your healthy plant and spread.
Also known as gray mold, this shows up as a mold on the hydrangea’s flower heads that can spread through to the leaves. You’ll notice this as a whitish-gray mold on the flower heads as the infection progresses.
The best way to prevent this fungus is by clearing plant debris from around the plant. Also, pruning away hydrangea branches that have grown too closely together will increase air flow throughout the plant. Be sure to keep your pruning shears clean after pruning, as this will help to prevent the spread of diseases if they are present.
If you think your hydrangea has botrytis blight, remove any infected flower blooms or leaves. If further treatment is needed, a horticultural oil such as neem can help. Fungicides, such as Bonide Fung-onil or propiconazole, are available at garden centers and can be applied as directed on the bottle.
Bud blight is a disease that can be found in areas of your garden that may receive a lot of overhead watering. Symptoms begin as water-soaked spots on petals, which turn into irregular reddish-brown blotches that may eventually cover the whole flower.
Petals turn brown and fall. Leaf spots may form after making contact with faded infected petals, and flower buds may die before even having a chance to open.
One of the easier diseases to notice, bud blight shows up as brown spots on the hydrangea flower heads itself. They show up as a reddish brown blotch, and is easily distinguishable from other hydrangea diseases.
Take care when pruning or working on your hydrangea. Damaging the plants or branches can invite this disease into the plant. Avoid overhead watering by watering only around the base of the plant. Remove debris from around the plant, as well as potentially infected plant tissue, on a regular basis to allow for good air flow.
Using a multi-purpose fungicide can help keep this disease under control. Be sure to pair it with good maintenance and the mechanical control methods mentioned above.
Fungal Leaf Spot
Hydrangeas are susceptible to leaf spot which can be caused by a number of different fungi. Leaf spot could present itself as light brown spots with a darker brown border. As the disease progresses the spots will grow, the leaves will turn yellow and eventually fall off.
Fungal leaf spot looks similar to bacterial leaf spot. The difference is, this type of leaf spot can be caused by many different types of fungi. Fungal leaf spot will cause your hydrangea leaves to spot, and turn yellow.
Leaf spot can be prevented by minimizing moisture on the leaves. Avoid overhead watering as much as possible. Also, water earlier in the day so the water that is left on the surface of the leaves has time to evaporate before night falls.
Leaf spot can be treated with copper based fungicides. Treat shrubs in the late spring with copper fungicides to prevent leaf spot. If you wait until leaf spot presents itself you will need to spray every two weeks. Be sure to spray both sides of the leaves!
Powdery mildew can occur on all hydrangeas but is most common on bigleaf hydrangea. You will see a white fungus which resembles powder on the surface of the leaf. In some instances you could see yellow or purple leaf blotches.
Powdery mildew typically does not hurt the plant, but can be aesthetically unpleasant. However, if you do leave the powdery mildew untreated it may infect buds that are just beginning to develop, causing them to produce smaller and misshapen flowers and leaves.
Another common disease, powdery mildew is easily identifiable as it shows up as a whitish-gray mildew on the plant’s leaves.
Prevent powdery mildew by not planting hydrangeas too closely together. Avoid overhead watering. And, don’t forget to clean up fallen leaves and dead plant tissue to help prevent new infections.
Treatment with a fungicide may be necessary. Fungicides for powdery mildew can be found at your local garden center. You may also use a horticultural oil such as neem oil.
If you are paying close attention to your garden, it will be hard to miss the signs of root rot. You will notice root rot by one or more shoots drooping and not responding to a heavy watering. These plants will eventually die.
Plants that are drought-stressed are most susceptible to catching root rot. The fungus will produce white mats under the bark or near the soil line. After heavy rain you may also notice mushrooms growing above the damaged roots.
Root rot is a little harder to identify at initial glance, even though the symptoms are fairly straightforward with this disease. You’ll notice your plant’s leaves yellowing, and the plant showing other signs of struggle. You’ll notice root rot by digging up some of your hydrangea’s roots, and seeing white patches of rotting roots in the soil.
Good growing conditions are the best way to keep your hydrangea protected from root rot. Grow your hydrangea in well-draining soil. Continue your good plant maintenance practices to help keep your hydrangea strong. This will aid in their ability to fight off diseases.
There are no chemical treatments for root rot.
Rust is a leaf disease that affects Hydrangea arborescens, or smooth hydrangea. Rust is an interesting disease as it needs two hosts to carry out its life cycle. The type of rust that affects hydrangeas needs two distinctive hosts. In this case a hemlock is the alternate host.
The first thing you might notice on a plant that is infected with rust disease are brownish orange bumps on the bottom of leaves. These bumps will cause yellow spots to appear on the upper side of the leaf.
Rust is easy to identify, with rusty spots on your hydrangea’s leaves. It can look a bit like leaf spots, but the rusty color will help set it apart.
Keep your pruning shears cleaned while you are pruning a rust infected plant. Avoid overhead watering. Keep the hydrangeas clear from leaves and other plant debris around the base of the plant. If you are concerned about rust, you can plant Hydrangea arborescens ‘Frosty’; This variety is resistant to rust disease.
Rust can be difficult to control. You can use diluted aspirin tablets or hydrogen peroxide in a spray bottle, and spray the affected areas of the plant.
Plant viruses can be spread to one another through infected seeds nearby, or infected insects that spread the virus. Some of the most common hydrangea viruses are specific to the plant, but some can be spread from others.
Hydrangea Mosaic Virus
Hydrangeas infected with this virus will have a pattern of yellow mosaics on their leaves. You may also notice the veins of the leaves are yellow as well. Leaves can appear wrinkly and may grow smaller than anticipated.
This virus can be spread through many different ways. It can be spread through garden tools, insects, or transmission from other infected plants, including weeds.
Easily identified by patterns of yellowy rust across the leaves, mosaic virus has a similar color to rust, but shows up in patterns rather than spots.
The best way to prevent mosaic disease on your hydrangea is to keep your gardens neat and tidy, sanitize your tools, and to treat for insects using a broad spectrum insecticide.
There is no chemical treatment for hydrangea mosaic virus. If your plant has become infected with this virus you should remove it from your garden and dispose of the plant. Do not compost the plant! This will cause the virus to spread through your compost and will eventually infect whichever plants come in contact with the compost.
These are two different viruses that are very similar in appearance, as well as in how they are transmitted. Hydrangea ringspot virus can be first identified by the yellowing of leaves. You may notice that the plant slowly begins to decline.
This disease is spread by the pollen of infected plants, as well as by nematodes. Nematodes are microscopic insects that dwell in the soil. They spread diseases by eating plant tissue, and inject the diseases into the wounds they have made on the plant. The wounds from the nematodes make great entrance points for other bacteria and fungi as well.
Spread through pollen, ringspot shows up as small yellow rings on the leaf of your hydrangea plant.
Using sterile soil free of nematodes, is one way to prevent this disease. Removal of infected plants will also help the rest of your garden be safe. Be sure not to place your infected plants in your compost bin, as this will further spread the disease.
There is no chemical treatment for the virus.
Impatiens Necrotic Spot
This is a nasty virus that affects many garden plants. Thrips are responsible for the spread of this disease. They spread diseases from one diseased plant to the next through their mouthparts.
If you do not have an infected plant in your garden it is unlikely that the thrips will spread the disease. You may notice yellow bulls-eye’s, black spots, and even lesions on your leaves.
Necrotic spot is easily identifiable due to the yellow bullseyes that show up on the leaf of the plant.
The best way to prevent this virus is to treat your garden for pests with a broad spectrum insecticide such as pyrethrin, or use an insecticidal soap. If you can keep the thrips at bay, you will be setting yourself up for a healthy garden.
There is no chemical control.
General Prevention Tips
Many of the above diseases have similar prevention methods. Here is a further breakdown of how to keep your gardens free of diseases.
It is always a good idea to clean up your gardens in the fall and again in the spring. Sometimes this can feel like a big undertaking, and I get it, sometimes the leaves just never stop falling. However, doing a fall and spring clean up, as well as keeping your plants free of debris during the growing seasons, will help keep your garden healthy.
In the fall or the spring when your hydrangeas just look like sticks, take the time to pull the dried leaves out from the crown of the plant. This will give your plant room to breathe. It will also remove any lingering diseases out of the immediate area.
Throughout the growing season pluck leaves from the base of the plant, and try to keep the area around your plant as free from leaf litter and weeds as you can.
When watering your hydrangea by hand, aim for the base of the plant and the soil surrounding it. If you water from above, the water that is left sitting on the leaves will make diseases more likely to appear on your plant.
If you have a large plant, and are watering from above, there is a good chance that a lot of that water won’t actually make it to the ground. This will cause the plant to suffer from drought stress. Plants that are dried up are an easy target for diseases.
If you use an irrigation system, drip irrigation is the best option for your hydrangeas as it will keep the flowers and leaves dry, but well watered.
Proper tool maintenance will keep your garden tools working the way they are supposed to for as long as possible! Even if there are no signs of disease on your plants it is a good idea to clean the tools, in this case pruning shears, by removing dirt and any other debris.
To sanitize your clippers you can purchase a specific cleaner for garden tools. Or, you can use some household cleaners such as bleach or isopropyl alcohol.
To use bleach, dilute to a 1 to 9 ratio of bleach and water. Allow the clippers to soak for about 30 minutes. When the 30 minutes is up, rinse the clippers and leave to dry.
To use isopropyl alcohol there is no need to mix with water. Simply wipe the clippers with a cloth, or spray with a spray bottle, and allow to dry.
Whichever method you choose to clean your tools be sure to do so outside or in a well ventilated area as these chemicals can produce harmful fumes.
While caring for your hydrangeas, do your best to keep their growing conditions optimum. This will be your key to prevention, and to keeping their bright blooms going all season. To recap on how to keep diseases away from your hydrangeas: