Alternaria leaf spot of cineraria

The fungus Alternaria cinerariae is the primary causal agent of leaf spot disease in various ornamental species, specifically targeting the genus Senecio. This pathogen is categorized as a necrotrophic fungus, which colonizes plant tissue by secreting toxins that cause localized cell death.

The main symptoms include the appearance of dark brown to black spots on leaves, which typically display a concentric, target-like pattern as they enlarge. Under humid conditions, a characteristic velvety or fuzzy dark olive-colored mass of spores develops on the surface of the lesions, especially on the underside of the leaves.

Disease development is heavily influenced by environmental factors, particularly relative humidity levels exceeding 80% and moderate temperatures. The fungus produces large quantities of conidia that are easily dispersed by wind, splashing water from irrigation, or human activity within the nursery or garden environment.

The impact of this disease on ornamental production is significant due to the loss of aesthetic value, which renders the plants unmarketable. Severe infections lead to extensive defoliation, stunted growth, and in sensitive seedling stages, it can cause rapid decline and eventual plant death.

Management strategies focus on sanitation and environmental control, including removing and destroying infected plant debris to reduce inoculum sources. Growers should also prioritize ventilation to keep foliage dry. Effective chemical control involves the application of fungicides such as chlorothalonil, mancozeb, or specific systemic strobilurins to prevent spore germination.