Verbena x hybrida has 250 different species. It is a flowering annual, or a plant that completes its life cycle in one season, in the U.S. Department of Agriculture's (USDA) zones 2 to 11. They have 2 to 3 inch wide, small dome shaped heads of blossoms that flower from summer to frost. The plant grows 6 inches to 1 foot tall. It prefers full sun and fertile, well drained soil. The verbena can be prone to several diseases
Powdery mildew is caused by the fungus Golovinomyces sp. This fungus prefers high humidity and dry leaves. The actual injury to the plant depends on the species. However, some of the symptoms include a white coating over leaf surfaces. The fungus generally resides on the underside of leaves, and the bottom leaves are the first to show symptoms. It starts as small circular spots that expand and produce a matt of mildew.
Oedema is a disease of the roots that occurs when they take up water faster than the plant can use it. The pressure from the water builds in the cells, causing swelling and blister-like spots on the lower leaves. These blisters can harden to form white, tan or brown bumps on the leaves. In plants that are severely infected, the spots can form on the petals and stems, too. Eventually leaves turn yellow and fall off. It is most common in later winter or during times of cool, cloudy weather. The right conditions for oedema to occur are warm moist soil and cool moist air.
Bacterial Leaf Spot
Many plants are vulnerable to bacterial diseases. The common symptoms include leaf spots, blight, wilting, tip burns, leaf spots, rot or wilt. The most severe leaf spot diseases of plants like verbenas are caused by the bacteria Erwinia, Xanthomonas and Pseudomonas.
Bacterial Leaf Blight
Bacterial leaf blight symptoms include see-through wounds at leaf tips. The wounds may widen and extend into the middle of leaves. These wounds or lesions are green at first, then yellow and then brown.The diseased area contains a yellow border that separates it from the healthy part of the leaf.
Powdery mildew can be controlled by cultural or chemical control. Cultural control involves planting appropriately, such as leaving good spacing for air circulation. It also involves picking up fallen leaves and destroying any plant debris. Chemical control involves fungicides that should be applied before symptoms occur. Some examples include bicarbonate-based products, Compass O 50 WDG and Cygnus 50 WG. It is best to consult a local Cooperative Extension Office or garden center before using these products.
Oedema can be controlled by avoiding overwatering and improving air circulation between plants. Infected plants can recover with proper growing conditions.
Leaf spot and leaf blight can be controlled through prevention. Optimum growing conditions are an important preventive measure. High humidity, overcrowding and poor air circulation should be avoided.
An infected verbena should be isolated from other plants and the sick leaves removed. If the disease is systemic, or has spread throughout the whole plant, it will not recover. It is best to destroy the plant to help prevent the spread of the bacteria.