Verbenas are annual or perennial plants with purple, pink, red or white flowers. They bloom from spring to summer but may not flower as heavily during periods of extreme heat.
Perennial verbenas grow in zones 7 to 11. They are 6 to 12 inches tall and spread up to 6 feet across. Annual verbenas are more upright, growing 6 to 12 inches across and 12 to 18 inches tall.
The first signs of powdery mildew on Verbenas are dusty gray or white circular patches on the lower leaves. If left untreated, the patches join together and cover entire leaves. The leaves turn yellow, become necrotic and fall off. Leaves, stems and buds grow distorted as the disease progresses and the plant becomes distorted.
Some varieties of Verbena are resistant to powdery mildew, such as the Aztec, Laskar and Tukana series.
Alternaria Leaf Spots
The fungus Alternaria causes small water-soaked spots on the lower, older leaves of Verbenas. As the spots mature, they begin to sink and turn brown. There may be a yellow halo or concentric rings around the spots. As the spots merge, the leaves become chlorotic and fall off.
Anthracnose Leaf Spots
Leaf spots caused by the Anthracnose fungus begin as small round spots on the leaves and petals of Verbenas. The spots merge into large areas of necrotic tissue. A close examination of the spots reveal pinkish to orangish sporulation oozing out of the lesions.
Cercospora Leaf Spots
Cercospora is a fungus that causes light-green sunken spots on the leaves of Verbenas. The spots turn gray and then darken. They appear raised as spores are produced. The spots may merge into a V-shaped area. Heavily infected leaves may fall off.
Preventive Measures and Treatment
Fungal diseases are the most common diseases that affect Verbenas. To help prevent fungal disease, adequate air circulation is necessary. Place annual Verbenas at least 12 inches apart. Perennial Verbenas should be planted the recommended distance apart according to variety.
Avoid overhead watering, or water in early morning to allow plants to dry before damp evening hours. Remove any infected plant parts and debris from around plants and destroy.
Apply a fungicide for the targeted causal agent that is approved for Verbenas. Use at the rate recommended by the manufacturer.