Why Does My Arrowwood Viburnum Shrub Have Brown Spots?

If you’ve noticed unsightly brown spots marring the leaves of your beloved arrowwood viburnum shrub, you’re likely dealing with a fungal disease Brown spots on viburnum are typically caused by fungal infections like anthracnose or bacterial leaf spot But don’t despair! With prompt treatment and proper cultural care, you can get your arrowwood viburnum looking lush and healthy again.

What Causes Brown Spots on Arrowwood Viburnum?

The two most common diseases leading to brown spots on arrowwood viburnum are

Anthracnose

Caused by the fungus Elsinoë viburni anthracnose first appears in spring as small, rounded brown spots with purple margins on emerging leaves. As the disease progresses, the spots enlarge and spread eventually turning the entire leaf brown. Heavy infections can defoliate the shrub.

Moist, humid weather promotes anthracnose development. Spores spread easily via splashing water. Crowded, poorly ventilated viburnum shrubs are most vulnerable.

Bacterial Leaf Spot

Pseudomonas syringae bacteria infect viburnum leaves, causing small, angular brown spots between the veins. These spots may drop out, leaving shot-hole lesions. In severe cases, entire leaves turn brown and shrivel.

Wet, warm weather encourages bacterial spread. Overhead irrigation and spring storms disperse the bacteria.

What Does Anthracnose Damage Look Like?

Anthracnose damage starts as small, round brown spots on emerging leaves in spring. The spots enlarge into irregular dead patches with purple or reddish-brown borders. Severely infected leaves turn entirely brown and curl up. Defoliation can occur.

Look for the spots mainly concentrated along the veins and leaf edges. In humid weather, you may see salmon-pink spore masses in the center of older spots.

Anthracnose progressively worsens, so early detection and treatment is key. It helps stop widespread defoliation that can seriously weaken or even kill viburnum shrubs.

How to Treat Anthracnose on Arrowwood Viburnum

Several treatment options can help cure and prevent further anthracnose damage:

• Prune infected branches – Cut off affected twigs and branches well below visible symptoms to reduce fungal spread.

• Rake and remove debris – Rake up and dispose of all fallen leaves and debris to lower spore levels.

• Apply fungicides – Use broad spectrum fungicides containing chlorothalonil, myclobutanil, propiconazole or thiophanate methyl. Treat at bud break and repeat every 7-10 days through spring.

• Improve air flow – Space viburnums properly, and selectively thin inner branches to promote good air circulation and quick foliage drying.

• Water at base – Avoid wetting foliage, and water deeply at soil level to discourage splashing infections.

What do Bacterial Leaf Spots Look Like?

Bacterial leaf spot also first appears in spring. Look for very small, angular brown spots that seem to follow leaf veins. The spots may be surrounded by a narrow yellow halo. As they enlarge, the centers dry out and drop away, leaving shot-holes. Badly infected leaves turn entirely brown and crinkly before dropping off.

To confirm bacterial leaf spot, look for bacterial ooze early in the season. Later, hold diseased leaves up to the light to see translucent spots. Check undersides for shiny brown exudates that rub off.

Treating Bacterial Leaf Spot of Arrowwood

Since bacteria can overwinter in twigs and buds, treatment takes persistence:

• Remove and destroy heavily infected branches and leaves.

• Disinfect pruning tools after each cut to avoid spreading bacteria.

• Apply copper-based bacteriacide sprays every 7-14 days through spring, especially after wet weather.

• Improve air circulation through proper spacing and selective thinning.

• Water at the base, avoid overhead irrigation, and space plants for quicker drying.

Preventing Brown Spots on Arrowwood Viburnum

Prevention is the best medicine when it comes to fungal and bacterial diseases. Promote vibrant arrowwood shrub health through smart cultural practices:

• Select disease-resistant arrowwood varieties like Viburnum dentatum ‘Blue Muffin’ and ‘Morton’.

• Give viburnum optimal growing conditions including well-drained, fertile soil and appropriate sun exposure.

• Water and fertilize judiciously – overdoing it weakens plants and invites infections.

• Prune annually to improve air circulation and light penetration.

• Remove nearby weeds that harbor diseases and compete for resources.

• Clean up and dispose of all fallen debris promptly to lower spore levels.

• Avoid overhead watering and allow proper spacing for air flow and quick drying.

When to Call a Professional

If brown spots cover more than 30% of the leaves or defoliation is excessive, call a professional arborist. They can assess if a more aggressive treatment plan or restoration pruning is needed to save the shrub.

For mildly affected shrubs, diligently applying the at-home treatments above for anthracnose and bacterial leaf spot should restore their beauty. Be sure to properly diagnose the cause and treat accordingly. Then reinforce prevention to help your arrowwood viburnum thrive spot-free in seasons to come.