Fusarium oxysporum causes wilting in a wide variety of crops. For every crop, there is a strain or 'forma specialis’ of the fungus that incorporates the name of the specific host. In tomatoes, for example, Fusarium wilt is caused by F. Oxysporum f. sp. radicis-lycopersici, and in chrysanthemums by F. Oxysporum f. sp. dianthii, etc. F. Oxysporum lives in soil and substrate. The fungus overwinters as spores or mycelium in crop residue, but also forms thick-walled asexual chlamydospores that are resistant to dehydration. Survival can be very long-term and very deep in the ground. From the mycelium or spores, the fungus infects the roots of the host plant at the point where the lateral root emerges from the primary root or in wounds. The fungus grows between the root cells and up to the stem. Numerous microconidia are formed in the plant that are passively spread up the plant through the sap flow. Once the vessels have been reached, the fungus grows into the vessels and, due to the mycelium, spores and gum secreted by the plant as an immune response, the vascular tissue becomes blocked, resulting in the typical wilting. The fungus keeps growing and sporulates on dead tissue, producing the crescent-shaped pinkish red macroconidia that are typical of Fusarium, and which are then dispersed. The fungus can also be transmitted via seed. This need not necessarily be due to seed infection; it can also result from using infected tools or other materials. Fusarium is dispersed in soil and substrate by water. External spores on parts of the plant above the soil line are dispersed by the wind, water, people and materials, and by moving soil particles containing the fungus.
Most vegetables and flowers, cotton, tobacco, bananas, coffee, sugar cane. In the Netherlands, it is particularly common in flower bulb cultivation (including 'acid’ in tulips), tomatoes, carnations and chrysanthemums.
This fungus causes yellowing and wilting of the leaves, often only on one side of the stem. Cutting the stem base may reveal brown discolouration round the vascular bundles.
The optimum temperature range for germination of chlamydospores and conidia is 25-28 °C, but this varies per forma specialis: for F. oxysporum f. sp. radicis-lycopersici, the optimum range is 18-20 °C, and for F. Oxysporum f. sp. cucumericum it is below 20 °C. The fungus survives longest in fairly dry soil with a pH < 7.
- growing resistant strains or resistant rootstock;
- clean base materials;
- hygienic practices, disinfecting machines and materials;
- hot water seed treatment;
- avoiding soil with high phosphorus and magnesium levels;
- fertilising with nitrate rather than ammonium;
- in greenhouses;
- soil disinfection;
- clean feed solution and disinfecting recirculation water.