Red blotch of amaryllis

Red blotch, caused by the fungus Didymella curtisii (also classified as Stagonospora curtisii), is a significant fungal disease that specifically affects plants in the Amaryllidaceae family. It is a persistent pathogen that can severely damage the aesthetic and physiological health of ornamental bulbous plants if left untreated.

The primary hosts of this pathogen include Hippeastrum, Amaryllis, Clivia, Crinum, and Narcissus species. The fungus infects all parts of the plant, including leaves, scapes, and bulb scales. It is particularly problematic in greenhouses and indoor gardens, where high humidity and temperature regimes often favor its rapid proliferation.

Symptoms are easily identifiable as small, reddish-brown spots or streaks appearing on leaves and flower stalks. As the disease progresses, these spots coalesce, causing significant tissue necrosis. The flower stalks often become misshapen, bent, or brittle, which significantly reduces the quality of blooms. On the surface of these spots, pycnidia (fruiting bodies) may appear, releasing spores.

The pathogen spreads rapidly through water droplets, such as during overhead irrigation, which splashes spores from infected leaves to healthy ones. The disease thrives in conditions of high humidity, poor ventilation, and stagnant air. Furthermore, the fungus is often introduced into new collections through the purchase of already infected, asymptomatic bulbs.

Control measures must focus on both sanitation and chemical protection. Growers should strictly inspect all bulbs before planting and remove any showing signs of lesions. Fungicides containing copper or triazole active ingredients are effective in preventing and treating the infection. Maintaining dry foliage and ensuring adequate ventilation are critical cultural practices to minimize the risk of a new outbreak.