Diseases to Mock Orange Plants

A mock orange flower.

The mock orange is an ornamental plant grown primarily for its fragrant blossoms. It can be cultivated outdoors in USDA hardiness zones 4 through 7. It is a favorite for home gardening, as it easy to grow, drought-tolerant, and resistant to deer. While this plant is generally thought to be pest-free, there are several diseases which can affect it.

Bacterial Blight

One common mock orange disease is bacterial blight. This disease is caused by a bacterial infection that lives over winter on the stems and buds of the plants. This disease is more prevalent in years with a cool, wet spring. Bacterial blight can be identified by small, water-soaked spots on the leaves, which can quickly become necrotic (dead). In severe cases, leaf growth can become distorted resulting in loss of the leaf. This can eventually cause the plant's growth to become stunted. The best way to avoid this disease is to protect plants from rain or frost in the early spring. This can be done by covering them up with a plastic bag, although the bag should be removed immediately after the shower as it can detrimental to the health of the plant. It is also important to maintain adequate spacing for air flow between plants, and remove any leaves or shoots that you find to be infected.

Powdery Mildew

There are actually many different types of fungi that are commonly referred to as powdery mildew. This is because their habits and management methods are all virtually the same. They most commonly attack fruit trees as well as lilac, phlox and some rose varieties. This disease can be most easily identified by the dusty white to gray coating found primarily on leaf surfaces and sometimes on other plant parts. Symptoms typically manifest late in the growing season, and include distortion or stunting of leaves and plant parts, yellowing or death of leaves, and nutrient loss in the plant resulting in a general lack of growth and vigor. Powdery mildew grows best in high relative humidity or shade, and on plants growing closely together.

Fungal Leaf Spot

Fungal leaf spot is the result of a fungal infection on plant leaves. This disease overwinters on fallen leaves and debris on the ground. In the spring, spores splash onto the living plant during or after rain showers. Fungal leaf spot proliferates most easily on leaves which are covered with a thin film of water. For this reason, this disease is most prevalent during cool, wet growing seasons, and is rarely present during warm, dry spring seasons. Severe cases of this disease result in defoliation of the plant, although it is rarely terminal. Often the time when symptoms are observed is too late for chemical treatment, and most plants have some natural resistance to the disease.

References

• Oregon State University: Information on Mock Orange
• Better Home & Gardens Plant Dictionary: Mock Orange
• Cornell University: Powdery Mildew Fact Sheet
• Michigan State University: Fungal and Bacterial Leaf Diseases
• University of Connecticut: Leaf Spot Disease of Trees and Shrubs

Resources

• University of Missouri: USDA Hardiness Zones Map