Black Leaves on Aucuba Shrubs
Aucuba japonica is an evergreen shrub that's also known as the gold-dust plant or as Japanese aucuba. It produces tiny maroon flowers and dark green, toothed leaves. Sometimes the leaves have yellow spots, but black leaves are an indication that your shrub is either suffering from too much sun, or from a disease.
Exposure to direct sunlight causes young aucuba leaves to turn black. These shrubs require shade, and do well even when planted in deep shade. Shrub varieties that have patterned, or different coloured leaves, are especially prone to scorching. Leaf scorch can resemble anthracnose, but while browning from anthracnose occurs along the veins, leaf scorch burns the leaf edges.
Anthracnose is a fungal infection that causes grey, tan or dark brown spots to appear on leaves, stems, fruit and twigs. The spots grow larger and may cover the entire leaf, causing it to wither and fall prematurely. Other symptoms include cankers and branch dieback. Control anthracnose by raking up leaves and destroying them. Prune dead branches to keep the fungus from overwintering in them. Chemical control may also be necessary.
Leaf spots are caused by several different fungi, including alternaria, phyllosticta and colletotrichum. Symptoms include the appearance of large brown and black spots along the margins. Leaf spot fungus diseases often attack shrubs that are suffering from other problems like lack of fertility, root diseases or poor drainage. To control leaf spots, improve drainage, test soil for nutrients and pH and apply an ornamental fungicide.
Sooty-mould fungus coats the leaves, fruits and branches of aucuba shrubs with a charcoal black coating. The mould grows on honeydew, which is a sweet, sticky substance secreted by certain insects like aphids and whiteflies that have infested the plant. Sooty mould interferes with a plant's ability to photosynthesise and may end up killing your shrub. Although sooty mould can be washed off plants, it will return unless you control the insects responsible for excreting honeydew. Consult your local county extension for help with specific ways to control insects, since insecticides aren't always effective and can make some infestations even worse.
Although only the tips of the leaves usually blacken when an aucuba shrub is suffering from root rot, sometimes the entire leaf can look burnt. Root rot is caused by several different fungi that live in the soil. Additional symptoms include yellowing leaves and rotting roots. Root-rot problems are worse when shrubs are planted in poorly draining soil. Improve drainage and apply soil fungicides to control this problem (see Resources section).