A I am glad you brought a clipping of the leaves to one of the plant clinics we conduct twice a month. The spots in my photo have a watery edge around the spot which indicates the possibility of bacterial blight. Generally bacteria will stay confined within the net-like veins of the leaves. Bacteria are normally present on plant surfaces and will only cause problems when conditions are favorable for their growth and multiplication. These conditions include high humidity, crowding, and poor air circulation around plants. Over-head irrigation of plants can provide a film of water on the leaves where bacteria can multiply. Too much, too little, or irregular watering can put plants under stress and may predispose them to bacterial infection. Other conditions that produce stress include low light intensity, fluctuating temperatures, poor soil drainage, heavy layers of mulch, deficient or excessive nutrients. Prune infected leaves, but avoid excessive handling of diseased plants. Clean pruning shears between each cut otherwise the disease will be transferred from one plant to another. If more than one third of the plant is involved, prune infected leaves over a period of time, since removing too many leaves at one time will put the plant under further stress. If the disease is systemic and has spread throughout the plant, affecting the stems as well as the leaves, the plant will most likely not recover. We recommend destroying the plant to prevent spread of the bacteria to healthy plants.
Q: What is causing these spots on my viburnum shrub?