Hydrangea diseases can mean the difference between a garden full of beautiful summer blossoms and a brown, soggy mess of shrubs. And, unfortunately, hydrangeas are susceptible to many diseases.
In this article, we’ll examine nine of the most common diseases that afflict hydrangea shrubs. Once you’ve identified the disease, you’ll know how to treat it, if possible, and what steps to take to prevent it from reoccurring.
If you need some tips or reminders, learn all you need to know about growing hydrangeas in our article before we jump in. Then, come back here, and we’ll discuss the following:
Common Diseases
1. Botrytis Blight (Gray Mold)
Are there reddish-brown blotches on your hydrangea’s petals? If so, you’re likely dealing with botrytis blight. This is one of the more common hydrangea diseases and also the most easily avoidable one.
It’s caused by the Bortrytis cinerea fungus, which thrives in cool, moist conditions.
Essentially, this blight occurs when moisture accumulates in between flower petals, causing them to rot, and spreading between blossoms. Fungal spores flourish in those damp areas, and spread out to affect the rest of the flower heads.
If you spot this issue early, you can treat it with a fungicide and snip off affected areas so the blight can’t spread.
To prevent Botrytis blight, water your plants at ground level during the day rather than from above or in the evening. Prune away excess foliage so there’s plenty of airflow and light exposure, and don’t allow your plants to get overcrowded.
If the blight starts to advance, prune drastically to remove any affected areas, which will eliminate the fungus entirely.
2. Bacterial Leaf Spot
If your plants have been injured at all—either by sap-sucking insects or by accidental nicks from pruning shears—then they’re at risk of developing bacterial leaf spot. The pathogen can also enter through natural openings like stoma.
Openings in the plant’s stem or leaves, whether natural or from injury, allow Xanthomonas campestris bacteria to slip in and cause significant damage to your shrubbery.
You’ll recognize this issue by the brown, yellow-ringed spots that appear on your hydrangea’s leaves. These will expand and deepen in hue until they’re blackish-brown blotches.
The middle portions of these spots may actually rot off, leaving jagged holes in the leaves. The spots eventually spread and combine, killing the leaf.
If you’re dealing with bacterial leaf spot, copper-based fungicides may help a little bit, but nothing really cures this issue.
Your best bet is to cut the plant back and burn affected parts. In the future, be sure to tend your plants carefully to avoid injury—especially to oakleaf hydrangeas, which are particularly susceptible—and never water them from above.
3. Fungal Leaf Spot
Are your plants’ leaves turning yellow and covered in little brown spots? Furthermore, do you live in a warm climate, or have you been experiencing a stretch of hot, humid weather? If so, the leaf spot illness your plant is suffering from is likely fungal rather than bacterial.
Some hydrangea diseases can cause similar symptoms but thrive in different environments. As such, you can determine which issue you’re dealing with by noting the weather conditions that may have caused it.
Fungal leaf spots may be caused by Colletotrichum spp., Cercospora spp., or Phyllosticta hydrangea pathogens. These pathogens are either air—or soil-borne and thrive in warmth and humidity.
You can treat them with sulfur—or copper-based fungicides, but if the fungal infection is too advanced, it’s often better to destroy the plants instead.
4. Root Rot
Do your hydrangea plants seem wilted and droopy? Are there mats of white fungus on the soil around their stems? Then you’re likely dealing with root rot.
This is one of those hydrangea diseases that can be quite difficult to prevent or manage. It’s caused by the plants having “wet feet,” which usually occurs after periods of heavy rain that follow drought-like conditions.
Unless their soil is extremely well-draining, the water will accumulate in and around your hydrangea’s roots, at which point fungal pathogens such as Fusarium oxysporum, Phytophthora nicotiana, Globisporangium splendens, G. spinosum, and Armarilla spp. to flourish.
These pathogens get inside the plant’s roots and stem, damage the cells within, and disrupt all cellular functions (including nutrient uptake) until the plant finally dies. Depending on which pathogen is present, there might not be any treatments for this condition.
For instance, armillaria root rot initially infects just half the shrub, so only part of it wilts. Adding water won’t help. If your plant is infected with this disease, there is no cure. Learn more in our guide.
If you have yellowing leaves and wilting, visit our guide to Fusarium.
You can send a sample of the roots to your local extension office to confirm which disease is present, or try treating the soil with copper fungicide to see if it helps. You’ll need to repeat every few weeks for at least a month or two.
If there is no change after several months, pull up the diseased plants, burn them, solarize the soil to kill off remaining spores, and refrain from planting more hydrangeas there for three to five years.
5. Powdery Mildew
If your hydrangea leaves look like they’ve been dusted with talc, powdery mildew has made a terrible appearance.
It’s caused by the airborne spores of Erysiphe poeltii, Golovinomyces orontii (formerly Erysiphe polygoni), Oidium hotensiae, or Microsphaera friesii fungi, and generally appears during periods of hot, humid weather.
Unlike many other hydrangea diseases on this list, powdery mildew doesn’t carry a death sentence. Snip off any affected leaves when you find them, and treat the remaining plants with 50:50 milk and water, horticultural oil, a product that contains potassium bicarbonate, or neem oil.
To avoid further fungal issues, prune excess foliage for better air and sun exposure and keep the soil clear of fallen plant matter.
6. Anthracnose
Are your hydrangeas’ leaves covered in brownish-black spots and lesions? Are many leaves turning brown and falling off? Then anthracnose may be the culprit. You can tell the spots aren’t caused by botrytis because they’re round or maybe slightly irregular at first.
Cercospora spots, on the other hand, start out purple and then develop raised, tan “frogs eyes” at the center.
This is another fungal pathogen likely to appear at the height of summer. It’s caused by the Colletotrichum gloeosporioides fungus, which usually lurks in the soil until conditions are favorable for it to leap into action.
It needs heat and moisture, like high humidity or summer rain, to thrive. If you water on the plant rather than the soil using sprinklers or your hose, this also creates the ideal conditions.
Avoid it by watering at the soil level and taking care not to overfertilize.
Prune off affected areas and treat the remaining plants with a copper-based fungicide or Bonide’s Fung-onil. Repeat treatment every two weeks. Alternating the product you use helps prevent the fungi from developing a resistance.
Remove any and all plant debris from the soil around them, and once the growing season is over, turn over the soil to solarize it. This should kill off many spores within the soil, but you can also treat the area with fungicide to eliminate as many of them as possible.
7. Bacterial Wilt
Although many hydrangea diseases are fungal in nature, this is another one that’s caused by bacteria.
In this case, the culprit is Ralstonia solanacearum, which is most often found affecting nightshade (Solanaceae) plants such as tomatoes, peppers, eggplant, potatoes, and tobacco.
The first symptoms of this issue are wilted, drooping leaves starting at the bottom of the plant. You may also find wedge-shaped areas of necrotic tissue on the leaves. If you cut the stem, a white, runny substance may ooze out of it.
The wilt will continue to spread upwards until the entire plant keels over and dies.
There is no cure for this, so all you can do is remove the plant burn it, and dig up as much of the surrounding soil to dispose of as well. Eliminate any dead plant debris at the soil level, and if you choose to grow hydrangeas again in the future, try to choose wilt-resistant cultivars.
8. Ringspot Virus
Are there yellow rings showing up on your hydrangea leaves? Are the leaves distorted and the plant stunted?
If so, you’re probably dealing with tomato ringspot virus. This is spread via pollen or nematodes, and causes the aforementioned yellowing and rings on leaves, followed by leaf rolling and drop, and eventual plant death.
Like some of the other hydrangea diseases on this list, ringspot virus has no known cure. Your only option is to destroy the plant, and don’t cultivate hydrangeas there for three to five years.
When you do plant any, try to choose resistant varieties so you don’t have a repeat performance.
9. Hydrangea Mosaic Virus
Although this virus also causes leaf yellowing, it’s in a mosaic-like pattern rather than rings. Unlike tomato ringspot virus, this disease can also turn the leaves’ stems and veins yellow as it progresses. As it does, the yellow patterns turn to a deep rusty hue before the leaves die and fall off.
Unlike some other mosaic viruses, this one isn’t transmitted via insect vectors, but through cross-contamination from gardening tools. Sterilize your tools between uses, and clear away plant detritus as it accumulates.
When and if this virus appears, destroy any affected plants and don’t cultivate hydrangeas there again for several years.
A Note on Hydrangea Diseases
There are many other hydrangea diseases out there, including rust (Pucciniastrum hydrangea), hydrangea ringspot virus, bacterial wilt (Pseudomonas solanacearum), and virescence.
While many of the hydrangea diseases we’ve talked about have no known cure, most of them can be prevented with good gardening hygiene and planning.
Try to buy disease-resistant cultivars whenever possible, keep your tools and gardening clothes clean, and maintain the soil surface so no dead or decaying plant matter accumulates.
Finally, check your plants daily so you can take quick action of any of the symptoms mentioned above rear their heads.
Hydrangeas are beautiful, showy plants that can offer a lot of joy in the garden, so be diligent, always water at soil level, and enjoy every beautiful blossom when it arrives!
