Verticillium wilt is a destructive plant disease affecting over 300 plant species and causes major yield losses. The host plants include trees, shrubs, field crops, vegetables, ornamental plants, and weeds. However, gymnosperms and pteridophytes are immune.
Verticillium wilt is very similar to Fusarium wilt since both pathogens disrupt the vascular system of plants, eventually causing wilting, yellowing, stunting, and death. However, the biology and characteristics of pathogens are different.
Verticillium wilt is prevalent in temperate zones and in areas with low soil temperatures, whereas fusarium wilt is found in warm climatic conditions.
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Causal Organism of Verticillium Wilt
Several species belonging to the genus Verticillium cause verticillium wilt. These are V. albo-atrum, V. dahliae, V. longisporum, V. nubilum, V. tricorpus, and V. theobromae. Out of these species, Verticillium dahliae and Verticillium albo-atrum are the most common ones.
The genus belongs to the order Hypocreales under the class Sordariomycetes under phylum Ascomycota.
These pathogens are soil-borne and can remain viable in the soil for a long period of time, regardless of the presence and absence of plants.
Verticillium dahliae– The fungi possess septate hyaline hyphae responsible for colonizing the vascular bundles of the host. The pathogen produces microconidia and melanised structures called Microsclerotia. Microsclerotia are small-sized, dark to black-colored, irregularly shaped structures that are the major inoculum for infections.
Verticillium albo-atrum– The species also produces conidia, microsclerotia, and melanized hyphae that can survive cool temperatures and are more tolerant than V. dahliae. Its survival in perennial crops makes it more harmful in temperate climates.
Symptoms of Verticillium Wilt
Symptoms of verticillium wilt may vary in different host species and also within the species due to variations in the environmental conditions. The general symptoms include-
Wilting- Sudden wilting of small branches or drooping of leaves could be seen. The wilting may be temporary but as the disease progresses, it becomes permanent.
Yellowing of leaves- The discoloration of older leaves takes place. The yellowing is usually unilateral and starts from the margins and advances inwards.
Lesions– V-shaped, yellow-colored chlorotic lesions are formed at the edges of leaves in hosts such as tomatoes and potatoes. The lesions are often followed by necrosis.
Vascular discoloration– Brown or black streaks are seen on the vascular bundles, especially the xylem. This discoloration is caused by to colonization of the xylem by the fungus causing blockage in water and nutrient transport which ultimately leads to wilting.
In some hosts, the disease can cause stunted growth and premature defoliation, and in severe cases, it may cause death.
Disease Cycle of Verticillium Wilt
Survival in soil- The pathogen survives in soil as microsclerotia. They can survive in the soil for several years during unfavorable environmental conditions and the absence of a host. Microsclerotia are resistant to abiotic stress such as temperature and moisture and nutrient deficiency.
Infection- When the pathogen comes in contact with suitable hosts, microsclerotia creates a germ tube that penetrates the plant roots via root hairs or wounds. Then it grows as mycelium and produces microconidia. The conidia can travel through the vascular system and eventually colonize the other parts of the plant.
Vascular colonization- The fungus produces mycelium in the vascular bundles, mainly the xylem that causes the blockage in water transport making the host nutrient and water deficient thus causing wilting and yellowing.
If the pathogen proliferates uncontrollably, it eventually leads to the death of the host plant.
Sporulation and dissemination- Verticillium produces conidia within the host. These are transferred in the soil after the death of the host which again germinates after coming in contact with the susceptible host, thus restarting the cycle.
Factors Affecting Disease Development
Several factors affecting the disease development are as follows-
Temperature– Two major disease-causing species have different optimum temperature requirements. V. dahliae can survive at little warmer temperatures than V. albo-atrum. Verticillium species survive well in temperate zones.
Moisture– High soil moisture is favorable for the disease outbreak. The pathogen requires moisture to interact with the root hairs of the host, and thus moisture helps in germination. Poorly drained soils can cause severe outbreaks.
Host susceptibility– Stressed plants are more susceptible to verticillium wilt. Certain plants have inherent defense mechanisms for the disease, while many plants have been bred for genetic resistance.
Disease Management of Verticillium Wilt
Some strategies for management of Verticillium wilt are as follows:
Cultural practices
Rotation of host crops with non-host crops for several years can help in reducing the population of microsclerotia in the soil. However, this method is less effective as pathogens can survive in the soil for several years.
Soil solarisation is covering the soil with clear plastic during warm days so that soil temperatures can be increased to kill the pathogen with the help of the sun’s heat. This may reduce the population of pathogens in the soil.
Cleaning of farm equipment helps in controlling the spread of disease. Infected debris should be disposed of well so that microsclerotia are not released into the soil.
If the soil contains pathogens, then planting susceptible hosts should be avoided.
Resistant varieties
Breeding for resistant varieties is one of the effective methods for controlling Verticillium wilt. Many economically significant crops such as tomatoes, potatoes, etc. have been bred for disease resistance.
Grafting between susceptible scions and resistant stocks is also considered one of the most effective methods in disease management.
Biological control
Many bio-control agents have been discovered to decrease the population of Verticillium through competition with the pathogen for resources and the production of inhibitory compounds. Mycorrhizal fungi can form symbiotic relationships with the roots of plants for increasing resistance.
Chemical control
Soil fumigation with methyl bromide was previously used as a disease management strategy. Mixing a few organic by-products in the soil can result in the discharge of natural by-products in the soil that helps suppress the growth of pathogens. Organic matter can be added to manure for improving soil quality and drainage.
Quarantine measures
Quarantine measures may be developed to prevent the spread of pathogens to uninfected areas. Quarantine methods can involve restrictions for infected plant material, soil, and equipment.
References
- Berlanger, I., & Powelson, M. L. (2000). Verticillium wilt. The Plant Health Instructor. https://doi.org/10.1094/phi-i-2000-0801-01
- Care, T. (2022, April 30). Verticillium Wilt: identification, symptoms, and control. Hendricksen Tree Care Services. https://www.rwhendricksenco.com/verticillium-wilt-identification-symptoms-control/
- Farmonaut. (2024, September 30). Verticillium Wilt: Symptoms, treatment, and control for 20+ affected plants. https://farmonaut.com/blogs/verticillium-wilt-symptoms-treatment-and-control-for-20-affected-plants/
- Jacobs, K., Wingfield, M., & Gibbs, J. (2004). PATHOLOGY | Vascular wilt diseases. In Elsevier eBooks (pp. 766–772). https://doi.org/10.1016/b0-12-145160-7/00065-x
- Keykhasaber, M., Thomma, B. P. H. J., & Hiemstra, J. A. (2017). Verticillium wilt is caused by Verticillium dahliae in woody plants with emphasis on olive and shade trees. European Journal of Plant Pathology, 150(1), 21–37. https://doi.org/10.1007/s10658-017-1273-y
- The Editors of Encyclopaedia Britannica. (1998, July 20). Wilt | Description, Causes, & Examples. Encyclopedia Britannica. https://www.britannica.com/science/wilt
- The Editors of Encyclopaedia Britannica. (n.d.). Verticillium wilt | plant disease | Britannica. Encyclopedia Britannica. https://www.britannica.com/science/wilt
- Vedantu. (n.d.). Wilt. VEDANTU. https://www.vedantu.com/biology/wilt
- Verticillium Wilt – Oklahoma State University. (2021, April 15). https://extension.okstate.edu/programs/digital-diagnostics/plant-diseases/verticillium-wilt.html
- Verticillium wilt / RHS. (n.d.). Royal Horticultural Society. https://www.rhs.org.uk/disease/verticillium-wilt
