Verticillium Wilt

How to control this devastating plant pathogen.

Verticillium Biology

Verticillium wilt, caused by fungi in the genus Verticillium, is a common and devastating plant pathogen that is most often found in temperate regions, though it also occurs in subtropical and tropical regions. In the genus Verticillium, there are six species which are most problematic for agricultural crops. These species are V. dahliae, V. albo-atrum, V. nigrescens, V. nubilum, V. tricorpus, and V. theobromae. Of minor concern are V. nigrescens, V. nubilum, and V. tricorpus, which can cause Verticillium wilt but are not highly pathogenic. V. theobromae causes fruit rot in bananas. V. dahliae and V. albo-atrum are of most importance because they are polyphagous and cause Verticillium wilt in a wide range of plant hosts; V. dahliae has well over 200 plant hosts.

Verticillium dahliae and V. albo-atrum are closely related and morphologically similar, and the taxonomic status of the two species has been hotly debated for nearly a century. Given that V. albo-atrum and V. dahliae are difficult to distinguish from one another, a commonly used rule of thumb is if the average ambient summer temperature is above 25℃, the pathogen causing the wilt is likely V. dahliae, as this species thrives in warmer temperatures. The main distinguishing characteristic is the formation of microsclerotia: small brown or black pods full of spores. V. dahliae produces these microsclerotia while V. albo-atrum does not. V. albo-atrum produces melanized (melanin-containing) mycelium which acts as a similar survival structure, although microsclerotia are capable of surviving much longer periods of time. Both species are able to survive over the winter within perennial plant hosts.

Photo of Verticillium wilt symptoms on hops. Image from Wikipedia.

Verticillium fungi infect plants by first colonizing their roots, entering through the developing root tips or through openings in damaged roots. After entering the root, Verticillium begins to grow up the xylem of the plant. As the mycelium of the fungus grows in the xylem, it obstructs nutrient and water transport. The fungus releases mycotoxins which cause the infected plant to wilt quickly. Many of these mycotoxins degrade pectin, weakening the plant’s cell wall and allowing Verticillium to spread through the plant’s anatomy more easily. These mycotoxins can cause chlorosis and necrosis in many crops. One of the primary mycotoxins is an enzyme called polygalacturonase.

Within the xylem, Verticillium species also form conidia, a type of asexually-produced spore. These conidia break off and travel through the plant, and then begin to grow in new locations, hastening colonization. Once the plant has been thoroughly colonized, the fungus enters a saprophytic stage and begins to digest the plant tissues, causing necrosis. At this stage, V. albo-atrum is capable of producing conidia on plant tissues, which can be dispersed by wind, allowing the spores to travel and spread to new locations. The reproductive structures produced by Verticillium fungi can make them difficult to get rid of, as they are capable of surviving in the soil for multiple years. Microsclerotia are a long-term survival structure of the fungus and allow V. dahliae to persist in soils for several years without a viable host.

Symptoms of Verticillium Wilt

Given that Verticillium wilt can affect such a broad range of host species, the symptoms it causes can vary, and some plants will display no symptoms at all. Regardless of the visible symptoms, infection by Verticillium fungi often significantly reduces crop yields and biomass production. The primary visible symptom is wilting, but in some cases wilting does not occur. Wilting or loss of turgor typically presents on the lowest leaves first, and then spreads to the rest of the plant. Wilting often progresses into chlorosis, necrosis, and lesions on leaves. Roots of infected plants may also display brown patches or lesions where the fungus first entered the plant. When sliced open, roots and stalks may have brown streaking or discoloration within the vascular tissue. There are also strains of the pathogen V. dahliae which defoliate their hosts, causing the plant to drop all of its leaves and fruits.

Photo of Verticillium wilt symptoms on sunflower. Image from Wikipedia.

Products to Treat Verticillium Wilt

Apical Crop Science provides a range of Bacillus subtilis solutions that effectively combat Verticillium wilt. These soil bacteria quickly establish a dense biofilm around plant roots, acting as a protective barrier against soil-borne illnesses. Additionally, they produce antibacterial and fungicidal metabolites during root colonization, which can help prevent disease. By applying Bacillus subtilis during planting or chemigation, a disease protection zone can be created around the seed or transplant, leading to higher yields. Alternatively, applying Bacillus subtilis as a Foliar spray can safeguard crops against diseases.

Controlling Verticillium Wilt

While spores of V. albo-atrum can be dispersed by wind, long distance transport of Verticillium fungi is primarily carried out by contaminated farm equipment, soil, seeds, and root stock. Properly cleaning all equipment before relocating it should always be a priority and will reduce the spread of the pathogen. Cleaning seeds and root stock is more difficult, which is why sourcing clean, healthy stock is critical. When infected seeds are planted, Verticillium is introduced to the soil and can infect other nearby plants. Sterilizing seeds in dilute bleach solutions can significantly reduce the incidence of Verticillium but it is difficult to completely eradicate it, which is why clean stock is so important. However, even with the most diligent cleaning and management, Verticillium wilt can make an appearance, because the fungus is so ubiquitous and creates such long-lasting survival structures.

Verticillium-resistant crop genetics are one method of controlling Verticillium wilt. That said, they may not be a solution for every grower, and real-world applications of resistant genetics are still being researched and developed. Also, many Verticillium-resistant plants are able to be colonized by Verticillium to some degree. However, these resistant plants are able to cope with the mycotoxins and display less severe symptoms.

One of the most effective means of controlling Verticillium in infested soils is the use of solarization. Solarization is the process of covering soils with transparent plastic sheeting and allowing sunlight to raise the soil temperature. Extensive research has been conducted on the use of solarization to control Verticillium wilt and other pests, and the results are quite promising. In multiple studies, solarization reduced living Verticillium and microsclerotia to undetectable levels and significantly increased crop yield. This effect lasted for several seasons before Verticillium wilt appeared again. However, solarization is a lengthy process: most research has involved 8-12 weeks of complete soil cover throughout June, July, and August, when temperatures are highest. Soils must reach a temperature of 45-48 ℃ for effective control of Verticillium. The process is most effective when soils are pre-irrigated or already moist. Solarization may also promote the growth of other microbes which are antagonistic to Verticillium.

A 2017 study investigated the effects of Bacillus subtilis, Pseudomonas fluorescens, compost addition, and soil solarization on Verticillium wilt in strawberry plants. As discussed earlier, solarization is recognized as an effective means of suppressing Verticillium in the soil. The authors found that individually, soil solarization had the most significant effect on reduction of wilt symptoms. However, a combination of soil solarization, and inoculation with B. subtilis and P. fluorescens inhibited Verticillium fungi to such an extent that no symptoms were visible. This also led to the greatest increase in fruit production and quality.

Anaerobic soil disinfestation (ASD) has been used to reduce the incidence of Verticillium wilt as well. ASD requires incorporating large amounts of carbon into the soil, completely saturating it with water, and then covering the soil with plastic tarps to create an anaerobic (oxygen-free) environment for about three weeks. As anaerobic microbes consume the carbon source, they produce organic acids and other metabolites which suppress the growth of Verticillium and other pathogens in the soil.

A 2021 field study compared the effects of ASD, mustard seed cake, and crop rotations on the incidence of Verticillium wilt in strawberries. For ASD, rice bran was used as a carbon source at a rate of 22 tons per hectare. The authors found that a 21 day treatment of ASD combined with mustard cake increased the marketable yield of strawberries by 34.2% compared to an untreated control, and increased the yield 9.9% compared to ASD treatment alone. The disease severity was lower and the net return was greater in all ASD-treated plots, regardless of the high initial cost of treatment. However, ASD soil treatment is most effective when soil temperatures are 20 ℃ or above, which is not achievable in every situation. The authors found that crop rotations had no significant effect on yield or disease severity. That said, other studies have found crop rotations to be a useful tool in the management of Verticillium wilt. In strawberries, using a four-year crop rotation significantly lowered the incidence and severity of Verticillium wilt and generated higher net returns.

Many organic amendments have been reported to have suppressive effects against Verticillium. Among other organic amendments, crab shells (a source of chitin) and soybean stalk and alfalfa seem to have the most significant effect on the control of Verticillium. Amendments which contain chitin can encourage soil bacteria to produce chitinase, which degrades the cell wall of Verticillium and other pathogenic fungi, weakening them. The ammonia and nitrous acid created during the decomposition of high-nitrogen organic amendments like poultry manure can kill the microsclerotia of V. dahliae.

Verticillium wilt has been somewhat effectively managed by means of biological control. In the case of Verticillium, biological control is achieved by increasing root colonization with plant growth-promoting rhizobacteria (PGPR) and arbuscular mycorrhizal fungi (AMF), which may release antifungal compounds and prevent root penetration by the pathogen. Bacillus and Pseudomonas bacteria have both been recognized as Verticillium antagonists and plant growth-promoters.

In a 100-day greenhouse study in 2013, the authors investigated the effects of the antagonistic Bacillus subtilis HJ5 strain on Verticillium wilt in cotton plants. The authors made a 1:1 mixture of amino acid fertilizer and pig manure compost, and called this their organic fertilizer. They then took this organic fertilizer, inoculated it with B. subtilis HJ5, and allowed it to ferment and colonize with the bacteria. This inoculated organic fertilizer was named their bio-organic fertilizer. Cotton plants were raised in untreated control soils, soils amended with the non-inoculated organic fertilizer, or soils amended with the bio-organic fertilizer. Verticillium wilt incidence was near 70% in the control group and 80% in the plants amended with organic fertilizer, compared to a shockingly low 8.3% in the plants raised in bio-organic fertilizer inoculated with B. subtilis. In addition, the cotton yield for the bio-organic fertilizer group was about 2.5 times higher than the control group. This indicates that the microbe could be a very effective tool for Verticillium control when properly implemented.

Further Reading

Abada, K. A., Hassan, E. O. (2017). Effect of the combination among compost, bioagents and soil solarization on management of strawberry Verticillium wilt. International Journal of Scientific & Engineering Research, 8(2)

Cai, Y., Xiaohong, H., Mo, J., et al. (2009). Molecular research and genetic engineering of resistance to Verticillium wilt in cotton: A review. African Journal of Biotechnology.

Davis, J. R., Huisman, O. C., Westermann, D. T., et al. (1996). Effects of green manures on Verticillium wilt of potato. Disease Control and Pest Management

du Toit, L. J., Derie, M. L., and Hernandez-Perez, P. (2005). Verticillium wilt in spinach seed production. Plant Disease.

Fradin, E. F. and Thomma, B. P. H. J. (2006). Physiology and molecular aspects of Verticillium wilt diseases caused by V. dahliae and V. albo-atrum. Molecular Plant Pathology.

Huang, J., Li, H., and Yuan, H. (2006). Effect of organic amendments on Verticillium wilt of cotton. Crop Protection.

Li, S., Zhang, N., Zhang, Z.., et al. (2013). Antagonist Bacillus subtilis HJ5 controls Verticillium wilt of cotton by root colonization and biofilm formation. Biology and Fertility of Soils.

Michuda, A., Goodhue, R., Klonsky, K., et al. (2018). The economic viability of suppressive crop rotations for the control of Verticillium wilt in organic strawberry production. Agroecology and Sustainable Food Systems.

Pegg, G. F. and Brady, B. L. (2002). Verticillium Wilts. CABI Publishing.

Song, R., Li, J., Xie, C., Jian, W., and Yang, X. (2020). An overview of the molecular genetics of plant resistance to the Verticillium wilt pathogen Verticillium dahliae. International Journal of Molecular Sciences.

Zavatta, M., Muramoto, J., Milazzo, E., et al. (2021). Integrating broccoli rotation, mustard meal, and anaerobic soil disinfestation to manage Verticillium wilt in strawberry. Crop Protection.