Septoria leaf spot of snapdragon

Septoria leaf spot, caused by the fungus Septoria antirrhini, is a common and destructive disease affecting snapdragons (Antirrhinum). This pathogen targets the foliage, causing significant defoliation and weakening the plant's overall health and aesthetic appeal.

The disease primarily affects Antirrhinum majus. It is a persistent issue in both professional greenhouse production and home gardens, where it can spread rapidly if environmental conditions are conducive to fungal growth and spore transmission.

Symptoms typically begin on the lower leaves as small, circular or irregular spots that are initially tan or gray with a dark purple or brown border. As the lesions expand, tiny black, pepper-like fruiting bodies called pycnidia appear in the center of the spots. Severely infected leaves eventually turn yellow, wither, and drop prematurely.

The development of Septoria antirrhini is strongly linked to moisture and temperature. Spore production and infection occur most readily during cool to moderate, humid weather with frequent rain or overhead irrigation. High plant density and poor air circulation within the canopy significantly accelerate the spread of the fungus.

Effective management requires an integrated approach to minimize the spread and severity of the disease:

• sanitizing the garden by removing and destroying infected plant debris;
• practicing proper watering habits, such as drip irrigation or watering at the base of the plant;
• increasing plant spacing to improve air circulation and reduce humidity;
• applying preventative fungicide sprays containing copper or mancozeb when weather conditions are favorable for infection;
• selecting disease-resistant cultivars when available and ensuring the use of pathogen-free seeds or seedlings.