Phomopsis arnoldiae

Phomopsis arnoldiae is a fungal pathogen that manifests as a plant disease, primarily affecting various woody ornamental species. It is a member of the diverse Phomopsis genus, known for its ability to cause necrotic lesions and systemic decline in susceptible hosts within home gardens and commercial nurseries.

The host range of this pathogen includes various species of conifers, such as Juniperus and Thuja, as well as several broad-leaved shrubs. The fungus is opportunistic, typically entering the plant tissue through fresh pruning wounds, winter frost cracks, or damage caused by pests and mechanical stressors.

Symptoms of an infection often begin with the browning and flagging of individual shoots or branch tips. As the pathogen advances, small, dark, pimple-like structures known as pycnidia develop on the bark. These structures release spores, which are the primary means for the disease to spread to adjacent branches or healthy nearby plants.

Development and spread of Phomopsis arnoldiae are heavily dependent on environmental conditions, particularly high humidity and prolonged moisture on foliage. During rainy periods, spores are splashed from infected tissues to healthy areas by raindrops or irrigation water. Poor air circulation within dense plantings acts as a catalyst, significantly increasing the infection rate.

Management of the disease requires an integrated approach to sanitation and cultural practices. Pruning out infected branches during dry weather is essential to reduce the inoculum level. Chemical control involving fungicides containing copper or other systemic active ingredients can be applied preventatively, especially during seasons with high humidity and active new plant growth.