Michelle Grabowski, University of Minnesota Extension Educator
This time of year gladiolus blossoms are a welcome sight, whether left in the garden or brought inside as a cut flower. Unfortunately not all gardeners will have gladiolus blossoms this year. A corm rot disease of gladiolus, known as Fusarium wilt has been found in Minnesota. This disease is caused by the fungus Fusarium oxysporum f.sp. gladioli. The last part of the fungus's name 'f.sp. gladioli' indicates that this fungus has a very narrow host range and will only cause disease in gladiolus and very closely related plants. This is good news for gardeners who have infected gladiolus plants mixed with other flowering annuals and perennials.
Gladiolus plants infected with Fusarium wilt are often stunted and do not produce flowers. The leaves first yellow, then turn completely brown and fall over. The fungus infects the corm of the plant, often starting at the basal plate where the roots attach. Gardeners may see dark spots on the outside of infected corms. When cut open, a reddish brown rot can be seen within the corm.
If Fusarium wilt occurs, completely remove all infected plants, including the corm. Destroy infected plants. Do not compost them. Save only firm healthy corms for next year. Rot can continue in storage so inspect corms before and after storage. Discard any discolored soft or crumbling corms. When purchasing new corms, buy plants from a reputable nursery and inspect them before planting. Varieties of gladiolus vary greatly in their susceptibility to Fusarium wilt. Whenever possible select disease resistant varieties.
Although the fungus is often first brought into the garden on infected corms, once introduced it can survive for years in the soil. Avoid planting gladiolus in garden areas that have had previous problems with Fusarium wilt. Soil that is neutral to slightly acidic (pH 6.6-7.0) will favor the plant over the pathogen and can help to reduce incidence of Fusarium wilt. A soil test (link to http://soiltest.cfans.umn.edu/) can be used to determine the pH of your soil. In addition, wounding of corms has been shown to increase disease problems so take care not to scratch or injure corms when planting.
Photos by Michelle Grabowski