What causes brown spots in hydrangeas? - Plant Care Guide

Brown spots on hydrangeas are a common concern for gardeners, and they can be caused by a variety of factors, including fungal diseases, environmental stress, and even pest infestations. Accurately identifying the cause of these leaf spots is the first step toward successful treatment and prevention, helping your hydrangeas stay healthy and vibrant.

What are the Most Common Fungal Diseases Causing Brown Spots on Hydrangeas?

Several fungal diseases are frequent culprits behind brown spots on hydrangea leaves. These diseases thrive in specific environmental conditions, often spreading rapidly if not managed. Understanding each one helps in proper identification and treatment of hydrangea leaf spots.

• Cercospora Leaf Spot (Cercospora hydrangea): This is one of the most common causes of brown spots on hydrangeas. It typically starts as small, circular, reddish-purple spots on the lower leaves, which eventually enlarge and turn tan or brown with a distinct purplish halo. In severe cases, leaves may yellow and drop prematurely. It thrives in warm, humid conditions.
• Anthracnose (Colletotrichum gloeosporioides): This fungus causes irregular brown spots that often appear near the leaf margins or tips. These spots may have a darker border and can sometimes develop a target-like appearance. As the disease progresses, large sections of the leaf can turn brown and crispy. It can also affect flowers, causing unsightly brown blotches.
• Powdery Mildew (Erysiphe polygoni): While primarily known for its white, powdery growth on leaf surfaces, severe or untreated powdery mildew can cause the affected leaf tissue to turn yellow and then brown as it dies. It prefers high humidity but dry leaf surfaces, often appearing in areas with poor air circulation.
• Leaf Scorch (Non-pathogenic, but can look like disease): While not a fungus, leaf scorch can appear as large, irregular brown patches, particularly on leaf edges, often mistaken for a disease. It's caused by the plant losing water faster than its roots can supply it, usually due to too much sun, wind, or insufficient watering. This leads to the drying and browning of leaf tissue.
• Phyllosticta Leaf Spot (Phyllosticta hydrangeae): This causes small, circular to irregular brown spots with a distinct light tan or gray center and a darker brown or purplish border. Tiny black dots, which are the fungal fruiting bodies, may be visible within the lighter center of the spots.

Identifying the specific fungal infection is crucial for effective hydrangea disease treatment. Using a hydrangea fungicide specifically designed for ornamentals can help manage these issues.

How Do Environmental Factors Cause Brown Spots on Hydrangeas?

Beyond diseases, various environmental stressors can also lead to brown spots on hydrangeas. These issues are not caused by pathogens but by suboptimal growing conditions or sudden changes in the plant's environment.

• Sun Scorch/Heat Stress: Hydrangeas, especially the bigleaf (Hydrangea macrophylla) and oakleaf (Hydrangea quercifolia) types, prefer some shade, particularly during the hottest part of the day. Too much direct sun, especially in hot climates or during heatwaves, can cause leaves to develop large, irregular brown patches, often starting at the edges, as if they've been burned. The affected areas can become crispy.
• Water Stress (Both Too Much and Too Little):
  • Underwatering/Drought Stress: Hydrangeas are thirsty plants. Prolonged periods without sufficient water will cause leaves to wilt dramatically, turn brown and crispy at the edges or entirely, and eventually shrivel. This is a common cause of brown spots on hydrangea leaves.
  • Overwatering/Poor Drainage: While less common for direct brown spots on leaves, constantly saturated soil can lead to root rot. Damaged roots cannot absorb water and nutrients, causing the leaves to yellow, wilt, and eventually turn brown and die due to lack of uptake. This often appears as a general decline of the plant. Using a soil moisture meter can help prevent both extremes.
• Nutrient Deficiencies/Imbalances:
  • While not typically causing distinct brown spots, severe deficiencies can lead to discolored foliage that eventually turns brown. For example, severe potassium deficiency can cause browning along leaf margins.
  • Fertilizer Burn: Applying too much granular fertilizer directly to the foliage or not watering it in sufficiently can lead to brown, scorched spots or edges on the leaves due to the high salt concentration.
• Cold Damage/Frost: Late spring frosts or early fall freezes can damage tender new growth and leaves, causing them to turn brown or black and become limp. Flowers and buds can also be affected, turning brown.
• Wind Burn: Strong, persistent winds, especially when combined with dry conditions, can strip moisture from leaves faster than the plant can replenish it, leading to browning and crisping of leaf edges, similar to sun scorch.

Providing the correct site conditions for your hydrangea variety, including appropriate sun exposure and consistent watering, is essential for preventing these non-disease related brown spots.

Can Pests Cause Brown Spots on Hydrangea Leaves?

While less common than diseases or environmental stress, certain pests can cause feeding damage that results in brown spots or discolored areas on hydrangea leaves. Their feeding activity often leads to compromised leaf tissue that then browns.

• Spider Mites: These tiny arachnids are nearly invisible to the naked eye. They feed by piercing plant cells and sucking out sap. Initial symptoms include tiny yellow or stippled dots. In severe infestations, especially during hot, dry weather, the affected areas of the leaves can turn a dull bronze or brownish hue as the cells die. You might also notice fine webbing on the undersides of leaves.
• Thrips: These small, slender insects feed on young leaves, buds, and flowers. Their feeding can cause leaves to become distorted, streaked, or mottled, and in some cases, develop brown patches where the tissue has been damaged. On flower petals, thrips cause brown discoloration or a "scorch" look.
• Leaf Beetles or Caterpillars: While these typically cause more obvious holes or chewed edges, severe feeding by large numbers of these pests can weaken leaves to the point where large sections become necrotic and turn brown. The browning would be associated with visible chewing damage.
• Scale Insects: These small, immobile pests attach to stems and sometimes leaves, feeding on plant sap. While they usually cause yellowing, wilting, or stunted growth, heavy infestations can lead to a general decline in plant health, and severely weakened leaves may eventually turn brown and drop. Look for small, raised bumps on stems or leaf undersides.

Regularly inspecting your hydrangeas for signs of pests is an important part of proactive hydrangea care. Catching infestations early can prevent widespread browning and more significant damage to your beautiful shrubs. Using an insecticidal soap for plants can often control soft-bodied pests.

How Can I Prevent Brown Spots on My Hydrangeas?

Preventing brown spots on hydrangeas involves a combination of good cultural practices and proactive measures. A healthy hydrangea is naturally more resilient to diseases and environmental stresses.

• Choose the Right Hydrangea for Your Site: Select hydrangea varieties that are well-suited to your climate and sun exposure. For example, Bigleaf hydrangeas (Hydrangea macrophylla) prefer morning sun and afternoon shade, while Panicle hydrangeas (Hydrangea paniculata) can tolerate more sun. This prevents sun scorch.
• Proper Site Selection and Planting:
  • Sunlight: Ensure your hydrangea receives the appropriate amount of sun for its type. Too much sun can lead to brown spots from scorch.
  • Air Circulation: Plant hydrangeas with adequate spacing between them and other plants. Good airflow helps foliage dry quickly, preventing fungal diseases.
• Consistent Watering: Hydrangeas are thirsty. Provide deep and consistent watering, especially during dry spells and hot weather. Water at the base of the plant to avoid wetting the leaves, which can encourage fungal growth. Early morning watering is ideal.
• Good Sanitation:
  • Remove Affected Leaves: Promptly remove and destroy any leaves showing brown spots or other disease symptoms. Do not compost diseased material.
  • Clean Up Debris: Rake up and dispose of any fallen leaves under the hydrangea bush, as these can harbor fungal spores.
  • Sterilize Tools: Clean pruning shears with rubbing alcohol between cuts and between plants to prevent spreading diseases.
• Balanced Fertilization: Fertilize your hydrangeas with a balanced fertilizer specifically formulated for acid-loving plants, if applicable. Over-fertilizing, especially with too much nitrogen, can lead to lush, tender growth that is more susceptible to disease. A hydrangea plant food can help provide the right nutrients.
• Mulch: Apply a 2-4 inch layer of organic mulch (e.g., shredded bark, compost) around the base of your hydrangeas. This helps retain soil moisture, keeps roots cool, and reduces fungal spore splash-up from the soil onto the leaves.
• Pruning: Prune away dead, damaged, or crossing branches to improve air circulation within the shrub's canopy.

By implementing these preventive measures, you can significantly reduce the incidence of brown spots and keep your hydrangeas healthy and beautiful throughout the growing season.

What is the Best Way to Treat Cercospora Leaf Spot on Hydrangeas?

Cercospora leaf spot is one of the most common causes of brown spots on hydrangeas, and effective treatment involves a combination of cultural practices and, if necessary, fungicides.

1. Sanitation is Key:
   • Remove Affected Leaves: As soon as you see the characteristic brown spots with purple halos, carefully pick off and destroy all affected leaves. Do not compost them, as the fungal spores can survive.
   • Clean Up Debris: Rake up and dispose of any fallen leaves, twigs, or other plant debris from under the hydrangea bush, especially in the fall. This is crucial because the fungus overwinters in plant debris.
2. Improve Air Circulation: Prune your hydrangea to thin out dense interior growth, allowing better air flow through the plant. This helps leaves dry faster after rain or dew, discouraging fungal growth.
3. Water Correctly: Always water hydrangeas at the base of the plant, directly on the soil, avoiding wetting the foliage. Water in the morning so any accidental moisture on leaves has time to dry before nightfall.
4. Fungicide Application (If Necessary):
   • For severe or persistent Cercospora leaf spot infections, a fungicide may be needed.
   • Look for a fungicide labeled for leaf spot diseases on ornamentals, specifically mentioning Cercospora. Active ingredients like chlorothalonil, myclobutanil, or azoxystrobin are often effective.
   • Apply according to label directions, typically every 7-14 days during periods of active growth and high disease pressure (e.g., warm, humid weather).
   • Start applications preventatively in spring if you've had issues in previous years, or at the first sign of spots. You can find general ornamental plant fungicides that may contain these ingredients.
5. Long-Term Strategy: Once the disease is under control, continue with good cultural practices (sanitation, proper watering, air circulation). If your hydrangea variety is consistently susceptible, consider replacing it with a more disease-resistant hydrangea cultivar.

Treating Cercospora leaf spot effectively minimizes leaf damage and premature leaf drop, helping your hydrangeas maintain their lush appearance.

When Should I Use a Fungicide for Hydrangea Spots?

Using a fungicide for hydrangea spots is a management tool, not always the first step. It's most effective when used preventatively or at the earliest signs of a widespread fungal disease, especially if you know your hydrangeas are susceptible or conditions are favorable for disease.

• Preventative Use: If you live in an area prone to fungal issues like Cercospora leaf spot or anthracnose, or if you have highly susceptible hydrangea varieties, you might choose to apply a fungicide before symptoms appear. This creates a protective barrier on the leaves. Start applications when new growth emerges in spring.
• Early Detection and Treatment: If you notice the very first few brown spots (or other discolored spots) characteristic of a fungal disease, apply a fungicide promptly. Catching it early can prevent widespread infection and defoliation.
• During Favorable Conditions: Apply fungicides when weather conditions are ideal for fungal growth – prolonged periods of rain, high humidity, or warm, damp weather. This is when spores are most likely to germinate and spread.
• After Cultural Practices Fail: If good cultural practices (sanitation, proper watering, air circulation) alone are not enough to control the disease, then integrate fungicides into your hydrangea care routine.
• Consider the Specific Disease: Different fungicides target different diseases. Ensure the product you choose is labeled for the specific hydrangea fungus you are trying to control.
• Follow Label Directions: Always read and strictly follow the product label for mixing ratios, application frequency, and safety precautions. Over-application can harm your plants or the environment.

Fungicides are part of a comprehensive disease management plan for hydrangeas. They work best when combined with proper cultural practices that create an environment less favorable to fungal growth.

What is Hydrangea Virus and Does it Cause Brown Spots?

Hydrangea virus refers to various viruses that can infect hydrangeas, with Hydrangea ringspot virus (HRSV) being one of the most common. While these viruses do not typically cause distinct brown spots as fungal diseases do, they can lead to leaf discoloration and other symptoms that might eventually involve browning of dying tissue.

• Primary Symptoms of Hydrangea Virus:
  • Ringspots: Often characterized by light green or yellowish rings, lines, or blotches on the leaves. This is how "ringspot" gets its name. These patterns might become necrotic (dead) and turn brown over time.
  • Leaf Distortion: Leaves may appear stunted, crinkled, puckered, or otherwise deformed.
  • Stunting: Overall plant growth can be significantly reduced.
  • Reduced Vigor: The plant may appear generally unhealthy and produce fewer or smaller flowers.
  • Necrosis/Browning: In advanced or severe infections, the discolored or distorted areas of the leaves can become necrotic (die), leading to irregular brown patches. This browning is a secondary symptom of tissue death rather than a direct viral lesion.
• How it Relates to Browning: The browning that might occur with a hydrangea virus infection is typically a result of damaged leaf tissue dying off, rather than a primary symptom like the distinct spots seen with fungal infections.
• Treatment: Unfortunately, there is no chemical cure for hydrangea viruses.
  • Removal: The best course of action for a confirmed viral infection is to remove and destroy the infected plant to prevent the virus from spreading to other healthy hydrangeas. Do not compost virused plants.
  • Prevention: Viruses are often spread through infected sap (e.g., via pruning tools) or by sap-sucking insects.
    • Sterilize Tools: Always clean and sterilize your pruning shears with rubbing alcohol or a bleach solution between plants.
    • Pest Control: Manage sap-sucking insects like aphids, which can sometimes transmit viruses, though insect transmission for HRSV is less common than mechanical transmission.
    • Buy Healthy Plants: Purchase new hydrangeas from reputable nurseries to ensure they are virus-free.

If you suspect hydrangea virus due to unusual patterns of discoloration and overall plant decline that eventually includes browning, consulting with a local plant diagnostic lab can confirm the diagnosis.

How Do I Identify Different Types of Hydrangea Leaf Spots?

Identifying the specific type of hydrangea leaf spot is crucial for choosing the correct treatment and ensuring effective hydrangea care. While many issues can cause browning on leaves, key characteristics can help distinguish them.

Always use a magnifying glass to examine the undersides of leaves for pests or fungal growth. Observe the pattern of the browning and the overall health of the plant. If unsure, consider consulting a local extension office or master gardener for an accurate diagnosis of your hydrangea problems.

Should I Remove Leaves with Brown Spots on Hydrangeas?

Yes, in most cases, you should remove leaves with brown spots on your hydrangeas, especially if you suspect a fungal disease like Cercospora leaf spot or anthracnose. This is an important step in hydrangea disease management and helps prevent the spread of infection.

• Why Remove Them?
  • Prevents Spread: Diseased leaves contain fungal spores. When these leaves fall to the ground or are splashed by water, the spores can spread to healthy leaves and other hydrangeas. Removing them reduces the "inoculum" (infectious material).
  • Improves Aesthetics: Removing unsightly brown-spotted leaves improves the appearance of your hydrangea.
  • Promotes Airflow: While indirect, removing some leaves can slightly improve air circulation within the plant, which is beneficial for overall health.
• How to Remove Them:
  • By Hand: Carefully pluck off the affected leaves. Be gentle to avoid damaging the stem.
  • Pruning Shears: For larger or hard-to-reach areas, use clean, sharp pruning shears.
• Disposal:
  • Do NOT Compost: Never put diseased hydrangea leaves (or any other diseased plant material) into your compost pile. Fungal spores can survive composting and reinfect your garden later.
  • Bag and Dispose: Place the removed leaves in a sealed bag and dispose of them in the trash.
• Ongoing Maintenance: Continue to monitor your hydrangeas regularly and remove any new brown spots or diseased leaves as they appear throughout the growing season. Also, rake up any fallen leaves from around the base of the plant.

While removing brown-spotted leaves is a vital step, remember it's part of a larger strategy that includes proper watering, good air circulation, and potentially preventative fungicide applications to keep your hydrangeas healthy and prevent widespread browning.

Can Nutrient Deficiencies Cause Brown Spots on Hydrangea Leaves?

Yes, while not always the primary cause of distinct brown spots, severe nutrient deficiencies can lead to discoloration that eventually results in browning on hydrangea leaves. When hydrangeas don't get enough of essential nutrients, their physiological processes are disrupted, leading to visible symptoms and, in extreme cases, tissue death.

• Potassium (K) Deficiency:
  • Often appears as browning or scorching along the leaf margins (edges), especially on older leaves. The inner part of the leaf may remain green initially.
  • Can also cause a general yellowing or bronzing of leaves.
  • Potassium is crucial for water regulation, disease resistance, and overall plant vigor.
• Magnesium (Mg) Deficiency:
  • Typically causes yellowing between the veins of older leaves (interveinal chlorosis), while the veins remain green.
  • In severe cases, the yellowed areas can turn brown and necrotic (dead tissue).
  • Magnesium is a key component of chlorophyll.
• Phosphorus (P) Deficiency:
  • Less common to cause direct browning. It usually results in purplish discoloration of leaves, particularly on the underside.
  • However, severely deficient tissue can eventually become necrotic and brown.
• Boron (B) Toxicity (Excess, not deficiency):
  • While deficiencies are problems, excess boron can also cause issues. Too much boron in the soil can lead to brown or burnt-looking leaf tips and margins.
  • It's important to test your soil or ensure your fertilizer is balanced.

To determine if a nutrient deficiency is contributing to your hydrangea spots, a soil test is the most accurate method. This will tell you what nutrients are lacking or in excess in your soil. Once identified, you can amend your soil or use a balanced hydrangea fertilizer or a specific nutrient supplement to address the issue. You can find acid-loving plant food that is formulated for hydrangeas.

What is the Impact of Poor Air Circulation on Hydrangea Health and Brown Spots?

Poor air circulation is a significant factor contributing to the development and spread of brown spots on hydrangeas, especially those caused by fungal diseases. Hydrangeas need good airflow around and through their foliage to stay healthy and dry.

• Fungal Disease Promotion: Many common hydrangea fungal diseases, such as Cercospora leaf spot and powdery mildew, thrive in humid, still environments. When air doesn't circulate well around leaves, moisture remains on the leaf surface for extended periods after rain, dew, or watering.
• Prolonged Leaf Wetness: Fungal spores require moisture to germinate and infect plant tissue. Prolonged leaf wetness provides the ideal conditions for these spores to take hold and spread rapidly across the plant.
• How Poor Air Circulation Occurs:
  • Overcrowding: Planting hydrangeas too close together or too near other dense shrubs and structures restricts air movement.
  • Dense Interior Growth: Unpruned, thick growth within a single hydrangea shrub creates a humid microclimate where diseases can flourish on the inner leaves.
  • Obstructions: Planting hydrangeas too close to solid walls, fences, or building foundations can block natural airflow.
• Consequences for Brown Spots: When leaves stay wet, it creates a perfect breeding ground for fungi, leading to more frequent and severe outbreaks of brown spots, yellowing, and premature leaf drop. This weakens the hydrangea plant, making it more susceptible to other stressors and diseases.
• Solutions:
  • Proper Spacing: Ensure adequate spacing between hydrangeas and other plants when designing your garden.
  • Strategic Pruning: Regularly prune your hydrangeas to improve air circulation within the shrub's canopy. Remove any crossing branches, dead wood, and dense interior growth. This allows light and air to penetrate, helping leaves dry quickly.
  • Site Selection: Choose a location that naturally has good airflow and avoids "dead air" pockets.

Prioritizing good air circulation is one of the most effective preventative measures against brown spots and a cornerstone of healthy hydrangea care.