Cercospora leaf spot

Pathogen and type of disease. The disease is caused by the fungus Cercospora ipomoeae, a species of ascomycete fungus. It is categorized as a foliage disease, primarily manifesting as leaf spots, which can lead to significant damage to the photosynthetic apparatus of host plants.

Host range. The fungus primarily attacks members of the Convolvulaceae family, most notably ornamental Ipomoea species (morning glory) and the Ipomoea batatas (sweet potato). Weeds in the same family can act as alternative hosts, facilitating the survival and transmission of the pathogen during the off-season.

Symptoms and signs. Symptoms typically begin as small chlorotic spots on the leaves, which gradually darken to brown or greyish-brown as the tissue becomes necrotic. A distinct yellow halo often surrounds these spots. Under conditions of high humidity, a velvety layer of conidia may be visible on the underside of the infected leaves.

Development and spread. The fungus thrives in warm, humid climates, with optimal infection occurring between 22°C and 28°C. Spores are primarily dispersed through splashing rain, wind, and movement of gardening equipment. Poor air circulation in greenhouse environments significantly accelerates the development and secondary spread of the infection.

Damage and control measures. Severe infections lead to premature defoliation, which stunts plant growth and reduces the quality and yield of sweet potato roots. Integrated management practices include:

• Sanitation by removing and destroying all infected plant debris.
• Providing adequate spacing between plants to improve airflow.
• Avoiding overhead irrigation to keep foliage as dry as possible.
• Application of protective and systemic fungicides containing copper or azole compounds when conditions are favorable for disease development.