Fusarium wilt caused by the fungus Fusarium oxysporum f.sp. chrysanthemi is a vascular
wilt disease of chrysanthemum, and one I worked on for my Master’s thesis. The fungus is soil-borne or spread through vegetative cuttings, and invades the stem. The fungus plugs water conducting tissue (xylem) with mycelium and spores. Fusarium wilt symptoms are often confused with root rot but plants infected with Fusarium may show wilt only on one side and roots often appear healthy. Early symptoms of Fusarium wilt are wilting leaves on one side of the plant followed by yellowing and browning of the leaves. Infected plants are stunted, fail to produce flowers, and vascular browning can be seen on cut stems. Symptom development is favored by temperatures above 24 C (75 F) and high humidity. Although specific to chrysanthemum, the fungus can survive in soil or debris for years and is difficult to control if established in a field or bed.
Managing Fusarium wilt includes use of disease-free (culture-indexed) cuttings and pathogen-free media and materials. Avoid highly susceptible cultivars such as ‘Bravo’, ‘Cirbronze’, ‘Illini Trophy’, ‘Orange Bowl’, ‘Royal Trophy’, Allegra and ‘Yellow Delaware’. Other management practices include maintaining soil pH between 6.5 and 7.0, utilizing the nitrate form of nitrogen in fertilizers and drenching with fungicides. Effective fungicides include azoxystrobin (Heritage), pyraclostrobin plus boscalid (Pageant) and Trichoderma harzianum (PlantShield). Fungicides containing thiophanate methyl (Cleary’s 3336) and fludioxonil (Medallion) have been reported to suppress Fusarium.
In studies at the Long Island Horticultural Research and Extension Center, Cornell University, yellow-flowered mums were evaluated for susceptibility to Fusarium wilt. Wilt developed three weeks after inoculation in ‘Allegra’ (susceptible check), and the cultivars ‘Conaco’, ‘Florina’, ‘Elena’, ‘Malmo’ and ‘Golden Helga’ developed wilt. Symptoms did not develop and the fungus was not re-isolated from cultivars ‘Bernadette’, ‘Bethany’, ‘Diana’, ‘Mary’, ‘Mildred’, ‘Mouria’, ‘Novare’, ‘Castor’ or ‘Sundance’.