Holly plants range in size from shrubs of just over a foot to full-sized trees that reach 50 feet or higher, and they produce sharp-edged dark green leaves that offset their bright red berries. However, yellow leaves with black spots are an unattractive symptom of disease and illness in holly plants.
According to the Clemson Cooperative Extension website, tar spot is one disease that affects the leaves of holly plants. Tar spots start as yellow or light brown fungal leaf spots, but turn black and tarry as the disease matures and progresses. Eventually the host leaves turn yellow and drop prematurely from the holly; serious infections have the potential to defoliate entire trees or shrubs. Treat tar spot with good cultural maintenance. Remove infected leaves from the holly and the growing area if they fall from the tree.
Leaf scorch is caused by water that sits on the surface of the leaves and contributes to burning or scorching the surface of the leave. According to the Penn State University Extension website, a fungus attacks the scorch spots and causes small brown or black spore-producing spots to form. Severely scorched leaves turn yellow and die, which leaves them yellow and carrying black fungal leaf spots. Remove damaged leaves from the holly plant to protect it from further disease.
Phytophthora Root Rot
Phytophthora cinnamomi is a fungus that causes Phytophthora root rot in holly plants. As the fungi progress and mature, the roots of the holly plant die and stop transporting water and nutrients to the upper portions of the tree or shrub. Leaves turn yellow and develop brown or black fungal spore spots before they drop from the tree. Dieback of small branches also accompanies Phytophthora root rot. There is no treatment for this disease, and infected plants should be removed and destroyed.
Web blight is a disease that affects hollies beginning at the lower interior portions of the plant and spreads up and out to the canopy. Leaves develop brown spots that become large black spots or blotches that may cover entire leaves. Leaves that are not covered by the spots turn yellow before falling from the plant. This disease is common in container hollies and dwarf varieties, according to the Alabama Cooperative Extension. Treat web blight by spacing plants properly and spraying chlorothalonil.