Alternaria leaf spot of protea

The disease is caused by the fungal pathogen Alternaria proteae, which primarily targets plants within the Proteaceae family. As a significant foliar pathogen, it is responsible for causing necrotic lesions on leaves, stems, and occasionally flower bracts, significantly impacting the aesthetic and commercial value of these high-demand ornamental crops.

The host range is predominantly limited to the genus Protea. While wild populations can be affected, the disease is most prevalent in nursery settings and commercial greenhouses where high-density planting and controlled environments can inadvertently foster the growth of the fungus. The pathogen is disseminated primarily through wind-blown conidia and splashing water.

Initial symptoms are characterized by small, circular or irregular necrotic spots that appear on the leaves. These lesions typically expand over time, changing color from light tan to dark brown or black. A diagnostic feature is the presence of dark, velvety fungal growth on the surface of the lesions, especially during periods of high ambient humidity, which marks the production of new spores.

Environmental conditions are critical for the lifecycle of Alternaria proteae. High humidity levels above 80% and moderate temperatures are ideal for the rapid germination of conidia and subsequent infection. Poor air circulation in greenhouse environments acts as a major catalyst for disease spread, as moisture remains trapped on leaf surfaces, creating a perfect infection bridge.

The damage caused by this fungus can be severe, leading to premature senescence and defoliation of the plants. Effective disease management requires an integrated approach that includes:

• Sanitation: immediate removal and destruction of infected plant debris.
• Environment management: lowering humidity and improving airflow.
• Irrigation practices: avoiding overhead watering to keep foliage dry.
• Chemical control: applying protective and systemic fungicides, such as chlorothalonil or strobilurins, during high-risk periods.