Azalea leaf spot

The causal agent of this disease is the fungus Sphaerulina azaleae (synonym Cercospora handelii), which belongs to the class of Ascomycetes. It is a pathogenic fungal disease primarily affecting ornamental shrubs of the Ericaceae family. The disease is often focal in nature and is most commonly encountered in nurseries, greenhouses, and gardens with high humidity.

The main crops susceptible to infection are species of the Rhododendron genus, particularly Indian and Japanese azaleas. The pathogen affects both leaf blades and young shoots of the plant, which, if left uncontrolled, leads to a significant decline in decorative appeal and the overall health of the shrub.

Symptoms of the infection manifest as circular or irregular spots on the leaf surface. Initially, they are brown or reddish in color, but over time, the center of the spot lightens, and a dark border forms along the edges. Under conditions of high humidity, fungal sporulation appears on the underside of the leaf as small dark dots.

The development of the disease is closely linked to agro-climatic conditions: prolonged rainfall, high air humidity (over 80%), and moderately warm temperatures (18–25°C) create an ideal environment for spore germination. The infection is transmitted via raindrops, wind currents, the use of contaminated gardening tools, or the purchase of infected nursery stock.

The destructiveness of Sphaerulina azaleae lies in premature leaf drop, which leads to defoliation and disruption of photosynthetic processes. Chronic infection weakens the plant, reducing its cold hardiness and flowering potential. Control and prevention measures include:

• Regular removal and disposal of fallen foliage.
• Ensuring good air circulation around the bushes through proper pruning.
• Application of copper-based, mancozeb-based, or systemic fungicides at the first sign of infection.
• Avoiding overhead irrigation during periods of active growth.