Fusarium wilt is a soil-borne disease that leads to wilting, yellowing, and sometimes death of various plant species. This disease causes significant crop losses worldwide and is disastrous under warm conditions.
Losses associated with this disease are economically substantial, particularly in developing countries. The disease is mainly encountered in crops such as tomatoes, bananas, cotton, potatoes, etc., but some other species are also susceptible.
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Causal Organism of Fusarium Wilt
The disease-causing organism is a fungus named Fusarium oxysporum. It is a soil-inhabiting fungus belonging to the order Hypocreales under the class Sordariomycetes under the phylum Ascomycota.
The morphological features of Fusarium oxysporum include non-septate micro-conidia, septate macro-conidia, and chlamydospores. The chlamydospores are resting spores that can survive in the soil for several years outside the host.
Symptoms of Fusarium Wilt
Symptoms may vary from host to host and according to environmental conditions. The general symptoms that can be seen are as follows-
Dropping and yellowing of leaves– Wilting of leaves are the general symptoms. During the initial stage, wilting of leaves takes place at the hottest time of the day and revives at night, but eventually, as the disease advances, the leaves permanently wilt. The yellowing of leaves takes place from the base of the stem and progresses upward.
Discoloration of vascular bundles– The infected plant shows dark-brown streaks in the xylem vessel. This can affect the vascular bundles of stem, roots, petioles, and branches. It is the most diagnostic feature of Fusarium wilt.
Infected plants also show reduced growth when compared to healthy plants.
Etiology of Fusarium Wilt
The pathogenicity varies according to
Environmental conditions- Fusarium grows well in warm and moist environments. The optimum temperature ranges between 24°-30° C. The pathogenicity is more enhanced in poorly draining soils where in the soil, water movement is limited and allows further spreading of the disease through vascular bundles of the plant.
Host-specific interactions– Different sub-species of Fusarium cause disease to specific plant species. Example Fusarium oxysporum f. sp. Lycopersici infects tomatoes whereas Fusarium oxysporum f. sp. cubense causes banana wilt. Host specificity is controlled by genetic factors of both hosts and pathogens.
Virulence factors– The Fusarium species produce a variety of virulence factors. Amongst these, the enzymatic degradation of cell walls, toxins damaging the cell organelles, and inhibitory protein inhibiting the defense mechanism of hosts are involved. These factors facilitate the pathogen to destroy host tissue and establish infection.
Genetic variability– Fusarium possess a high degree of genetic variability that allows them to survive and overcome host resistance mechanisms and adjust according to the changing environment.
Disease Cycle of Fusarium Wilt
Soil survival– The fungus can survive in soil for many years in the form of chlamydospores or within the plant debris.
Infection– The fungal spores germinate and enter into the host through root hairs wounds or openings. They start to colonize the cortical region and gradually invade some of the xylem vessels of the host. This phase is also known as the parasitic phase of Fusarium.
Vascular colonization– As the fungus grows, it produces mycelium and spores in the xylem vessels. It interferes with water transport causing wilting and brown- streaks on the xylem vessel. The fungus ascends the xylem, gradually assuming a large share of the vascular tissue.
Sporulation– During this stage, the fungus produces macro-conidia and micro-conidia which are dispersed through the soil, water, and plant debris. They can either infect other plants or remain dormant in the soil waiting for favorable environmental conditions.
Death and plant residue– Once the plant dies, the pathogen persists in dead plant residue. This stage is also known as the saprophytic stage of Fusarium. Later, the dead plant decomposes and the mycelia reverts into its chlamydospore stage where it is ready to infect other healthy plants during the next favorable season.
The micro-conidial, mega-conidial, and chlamydospore stages are known as the dormant stage of Fusarium.
Disease Management of Fusarium Wilt
Fusarium wilt is a difficult disease to manage because the fungus persists for a long in the soil and infects almost all plant hosts. Therefore, effective control will depend on an integrated management approach.
Cultural Practices
Crop rotation: Although the pathogen is soil-borne, it can be minimized through crop rotation with crops that do not act as hosts to the pathogen. However, this method remains restricted because Fusarium survives in the soil for long periods.
Soil sterilization: Soil fumigation and solarisation reduce the populations of the Fusarium in the soil. Soil solarisation is performed when the soil is covered by clear plastic during the hottest months, allowing the sun to heat it up to temperatures that kill the pathogen.
Clean planting material: Disease-free seeds or planting material should be introduced to the fields that do not introduce the pathogen in the field.
Sanitation: Cleaning of farm tools and machinery could prevent the transmission of the pathogen from one field to another.
Resistant Varieties
Plant breeding for resistant varieties is the best control method against Fusarium wilt. There are numerous tomato and banana cultivars bred using traditional methods or modern genetic engineering techniques and resistant to Fusarium wilt.
Examples
Tomatoes: Several tomato cultivars have been developed that have resistance to Fusarium oxysporum f. sp. lycopersici.
Bananas: The Cavendish bananas were bred for resistance to F. oxysporum f. sp. cubense Race 1 of the disease that wiped out the Gros Michel variety. Yet more virulent strains, such as Tropical Race 4, seem to threaten the Cavendish varieties.
Biological Control
Biological control, using useful microorganisms such as the Trichoderma spp., can inhibit or suppress pathogens of Fusarium wilt in the soil. The agents of biological control work either by competing with the pathogen for available resources in the host plant or by producing compounds that inhibit the growth of Fusarium.
Mycorrhizal fungi: These are symbiotic fungi that form associations with the roots of plants to enhance their ability to fight off attacks from Fusarium. These fungi enhance nutrient uptake and activate mechanisms of defense in plants against Fusarium.
Chemical Control
Fungicides are of little value in controlling Fusarium wilt since the pathogen lives well within the vascular tissue of the plant. However, where other secondary pathogens may be involved or seed-borne inoculum is the problem, fungicides can be of use. Soil fumigants such as methyl bromide have been used commercially, but have largely been withdrawn from use globally for environmental reasons.
Soil Amendments
The addition of organic matter, compost, or biochar competes with the pathogen by a healthy population of microbe, thereby inhibiting the growth of Fusarium. Some amendments affect the soil pH or nutrient balance, making the environment less favorable for the pathogen.
Fusarium wilt indeed is a complex, persistent disease requiring careful management. Its survival in soil for extended periods, wide host range, and genetic variability make it one of the most challenging plant diseases to control.
References
- The Editors of Encyclopaedia Britannica. (1998, July 20). Fusarium wilt | Description, Symptoms, & Treatment. Encyclopedia Britannica. https://www.britannica.com/science/fusarium-wilt
- Basal rot Fusarium oxysporum Tomato Koppert. (n.d.). https://www.koppert.com/plant-diseases/fusarium-wilt/
- Vedantu. (n.d.). Fusarium wilt. VEDANTU. https://www.vedantu.com/biology/fusarium-wilt
- Okungbowa, F. I., & Shittu, H. (2014). Fusarium Wilts: An overview. ResearchGate. https://www.researchgate.net/publication/292243135_Fusarium_Wilts_An_Overview
- Fusarium Wilt of Tomato | NC State Extension Publications. (n.d.). https://content.ces.ncsu.edu/fusarium-wilt-of-tomato
- Fusarium wilts / RHS. (n.d.). Royal Horticultural Society. https://www.rhs.org.uk/disease/fusarium-wilts
- Dita, M., Barquero, M., Heck, D., Mizubuti, E. S. G., & Staver, C. P. (2018). Fusarium Wilt of Banana: Current knowledge on epidemiology and research needs toward sustainable disease management. Frontiers in Plant Science, 9. https://doi.org/10.3389/fpls.2018.01468
- Fusarium wilt | Infonet Biovision Home. (n.d.). https://infonet-biovision.org/plant_pests/fusarium-wilt
- Planet Natural. (2018, March 30). Fusarium Wilt: Symptoms, treatment, and control | Planet Natural. https://www.planetnatural.com/pest-problem-solver/plant-disease/fusarium-wilt/
- GmbH, P. (2019, February 20). Controlling fusarium wilt. Plantix. https://plantix.net/en/blog/controlling-fusarium-wilt/
