Gladioli (Gladiolus), also called gladiola and glads, are perennial plants of the Iris family (Iridaceae). Gladioli boom tall spikes of large flowers in a wide variety of colors. These easy-to-cultivate plants grow from bulb-like structures known as corms. Gladioli corms are susceptible to various fungal, bacterial and viral diseases.
Scab, a bacterial infection (Pseudomonas marginata), causes the formation of black-edged lesions on gladioli corm husks. The first sign of infections is the appearance of water-soaked, pale yellow spots. These spots turn into sunken brown lesions with raised margins. Scab usually infects the base of the gladioli corm. The scab pathogen spreads by bulb mites and splashing water.
Fusarium Rot, Yellows and Wilt
Fusarium rot, yellows and wilt are all infections caused by the Fusarium oxysporum fungus. Corm rot is generally restricted to the base of the corm, which develops dark surface spots. Infected corms cause their plants to suffer from yellowing leaves, faded flower colors and arching stalks. The affected plants are sometimes stunted and don't bloom at all. Infected corms and their plants should be removed to avoid spreading the infection.
Storage rot is an infection caused by a blue mold fungus (Penicillium). Corm surfaces typically form rough, concentric wrinkles or red-brown sunken spots. Green-blue masses of fungal growth often develop on the lesions. Storage rot is often associated with improperly curing the corm before storage.
Botrytis Corm Rot
The Botrytis fungus often causes a stem infection that results in gladioli corm rot. Small, hard, black structures, called sclerotia, develop on the corms. These sclerotia can house the fungus for years. Infected corms also form round, sunken, greenish-brown lesions. The corm's interior might have a spongy decay or rotting, brown strands radiating out from the center. The Botrytis fungus is commonly managed with applications of protective fungicides.
Corm Dry Rot
Corm dry rot is a fungal disease (Stromatinia gladioli) most prevalent during periods of wet, cool weather. Affected gladioli corms develop small, sunken red-brown lesions on the surface. The inside of the infected corm has dark streaks radiating out from the center. The leaves turn yellow and decay, while the leaf bases become brittle. The corm dry rot pathogen overwinters in infected gladioli tissue.
Sclerotinia Dry Rot
This dry rot disease is caused by the Sclerotinia gladioli fungus. This disease is more prevalent during wet seasons on gladioli planted in poor-draining locations. The affected corm's core has no discolorations, while the surface typically develops dark brown to black, sunken spots or lesions. The spots sometimes run together, forming a large, irregular area.