Alternaria leaf spot of stevia

Alternaria leaf spot of stevia is a destructive fungal disease caused by the pathogen Alternaria steviae. This disease poses a significant threat to stevia cultivation, as it directly impacts the foliage quality and total biomass yield of this economically valuable plant, which is grown primarily for its natural sweeteners.

The disease is characterized as a fungal infection that targets the aerial parts of the stevia plant. The pathogen invades the host tissue by releasing enzymes and toxins, which break down plant cells and lead to the formation of localized necrotic lesions. Left unmanaged, the infection can significantly reduce the photosynthetic capacity of the entire plantation.

Symptoms typically begin as small, dark brown or black circular spots on the leaves. As the disease progresses, these spots often expand and develop concentric rings, giving them a target-like appearance. In conditions of high humidity, the lesions become covered with dark, velvety mycelium and conidia, which are easily disseminated by wind and rain splashing to neighboring healthy plants.

Environmental conditions play a critical role in the epidemiology of Alternaria steviae. The fungus thrives in warm, moist climates where temperatures stay between 20°C and 28°C and relative humidity levels are high. Stagnant air within dense plant stands and poor soil drainage create microclimates that further accelerate the development and spread of the pathogen throughout the growing season.

Preventative and management strategies are vital to maintaining healthy stevia yields. Recommended practices include:

• Implementing strict crop rotation to reduce inoculum levels in the soil.
• Removing and burning infected plant debris after harvest to break the disease cycle.
• Ensuring adequate plant spacing to promote airflow and foliage drying.
• Selecting disease-resistant cultivars whenever available.
• Applying appropriate fungicides at the onset of disease symptoms to prevent widespread infection.