You'll notice the brown spots immediately if they appear on the glossy green leaves of your privet plant (Ligustrum spp.). Whether your shrub is deciduous, semi-evergreen or evergreen, it's the attractive leaves that hold your eye. Fungal leaf spot marks those pretty leaves with giraffe-style spots, and the disease is easier to prevent than to cure.
Not Exactly Mannerly
"Privet" sounds like a neat plant used in hedges in cottage gardens, but that impression is not entirely correct. Common or European privet (Ligustrum vulgare) is a wild child, escaping cultivation in U.S. Department of Agriculture plant hardiness zones 4 through 7 and earning the label "invasive." Other privets are more mannerly, like Japanese privet (Ligustrum japonicum), that presents as a compact, evergreen bush growing to 10 feet in USDA zones 7B through 10, an ideal hedge plant that responds well to shaping. Even this species, however, has been known to escape cultivation in the South, as has glossy privet (Ligustrum lucidum) in USDA zones 8 through 10. All these species, and most other privets, are vulnerable to fungal leaf spot.
Spotting Leaf Spot
When your privet leaves are marred by yellow or brown spots with distinct borders, the smart money's on Cercospora leaf spot, a common disease of privets. Fungal leaf spots like this one do not stop willingly after a few brown dots; left to their own devices, the spots increase, merge and kill the leaf; in time, the disease may seriously mar the plant. The spots are actually lesions that produce spores that are carried on the wind or water to infect new leaves. Much of the reinfection comes from diseased leaves that fall to the ground.
An Ounce of Prevention
Good cultural practices are a gardener's best bet against leaf spot diseases on a privet, starting with good watering habits. Fungal diseases appear during the wetter seasons of the year, and your privet's chances for infection increase if you get water on its foliage when you irrigate. Always water in the morning to give the shrubs a chance to dry out before nightfall. Use a drip system or a soaker hose or just be careful. Prune to increase the air flow between shrubs to help keep them dry. It's also important to remove all dead and fallen leaves from beneath the plants. Do a major clean-up in autumn, but also periodic clean-ups throughout the growing season. Leaf-spot reinfections often come from fallen leaves, not those hanging on the plant.
Last Resort: Fungicide
Most times, you can keep your privet safe and spotless with good cultural practices, and it's best if you do since Cercospora leaf spot resists fungicides available for home use. Mild cases of leaf spot may not merit fungicide application, but if you decide that the health of your plant is at stake, you can call in an expert to apply either myclobutanil or azoxystrobin products. According to Clemson University experts, these professional-use-only fungicides work well against fungal leaf diseases in privets.