Study confirms efficacy of Trichoderma against Verticillium (wilt disease)
The soil-born fungus Verticillium is a frequent harmful fungus in horticulture. This is especially the case in tree nursery production, where it causes mortality in numerous cultivars and ornamental fruit trees. Many areas become highly contaminated. An efficient, environmentally friendly, sustainable and non-chemical control concept is required. Bio-control agents such as Trichoderma show promising potential to bring Verticillium outbreaks under control and to reduce the mortality rate associated with the disease.
The most common Verticillium species harmful to plants are Verticillium albo-atrum and Verticillium dahliae. The latter species causes wilt disease. The harmful fungus blocks the pathways of the plant and it shows signs of wilting despite sufficient water supply.
In many cultures, Verticillium may be present without clear symptoms. In addition, the soil and crops can only partially be affected.
Verticillium dahliae persists in the soil for up to 10 years
The treacherous Verticillium dahliae – this species survives for many years (10-14 years) in the soil. The fungus develops melanised microsclerotia (small, hard survival structures) in unfavourable habitats (drought, high or low temperatures). The harmful fungus also remains in the soil in the form of hyphae or mycelium. The fungus penetrates the plant via the roots. Wounds (caused mechanically or by pests) are the preferred entry points. It grows in the vessels of the plant and generates spores there, which reach the upper parts of the plant via the vessels. The xylem vessels become clogged through tyloses (self-inflected) and the plant dies. Spores are then transferred to new hosts via wind. In older plant tissues, mycelia or microsclerotia also form. However, the spread of mycelial growth is limited. The distribution takes place via water, soil particles, tools, etc.
Optimal growth temperatures are 21 °C for V. albo-atrum, while V. dahliae prefers 25-27 °C. (The latter temperatures are also ideal for Trichoderma)
A study carried out in Andalusia on olive trees proves the effectiveness of Trichoderma for the organic control of Verticillium.
Here is a summary of the results:
- All tested T. asperellum strains could overgrow V. dahliae in dual-culture tests and reduce the colony size. There was no significant difference in efficiency against V. dahliae between the different Trichoderma asperellum strains.
- Different plantlets of olive trees showed different levels of colonization in the roots.
- No symptoms were observed in non-inoculated plants, while plants treated with the two Trichoderma strains (plants not in contact with Verticillium). Symptoms associated with Verticillium wilt appeared in plants inoculated with Verticillium only or when treated with T strains Bt3 and T25 first before inoculation with the pathogen. Symptoms in the untreated control were visible 29.5 days after transplantation to a Verticillium infected soil mixture and on the plant. Treatment with Trichoderma strains Bt3 or T25 prolonged the incubation period by 7-8 days.
- At the end of the trial 10 weeks after inoculation with Verticillium, 92.5% of non-Trichoderma treated plants showed symptoms averaging 3.3 in intensity on a scale of 0-4. Treatment with Trichoderma strains had no significant effect on disease frequency BUT significantly reduced the mean severity of symptoms and suppressed SAUDPC (Standardised area under disease-progress curve) by 43-48%.
- Infection with Verticillium reduced tree length growth by 51-69%. The stem of T25 even showed increased extension growth in Verticillium-infected soil – (83% (Trichoderma dry) and 141% (Trichoderma watered)) and control plants (without Verticillium, 126% (Trichoderma dry mixed with substrate) and 25% (Trichoderma watered)).
Conclusion of the study:
- Results of these experiments showed that the mycelial growth of Verticillium can be reduced by Trichoderma asperellum. The reduction in virulence was dependent on the Trichoderma strain used.
- The results also showed that previous root colonisation by Trichoderma could suppress the development of Verticillium. This potential did not allow the reduction in the frequency/occurrence of the disease, but led to a delay in the onset of the disease along with a reduction in symptom severity and the extent of disease outbreak.
- Infection of the soil substrate with Trichoderma achieved higher root colonisation than pouring a Trichoderma spore solution onto the soil.
- The increased growth (plant biomass) caused by Trichoderma may be due to increased root colonization and the production and transmission of auxins produced by Trichoderma.
Recommendation for Verticillium treatment with Trichoderma
MycoSolutions recommends a soil preparation with Avengelus granules (10 ml per m²) and carbon-enriched charcoal (vegetable charcoal supported by the function as water reservoir Trichoderma and protection against leaching) for the treatment of Verticillium in the soil. For previously existing rows of plants, the spreading of Avengelus granules (10 ml per m²) before or during rainfall is effective to flush the Trichoderma spores into the soil to combat the harmful fungus as well as to the roots of the plant. An annual, superficial application of ground plant carbon-enriched biochar (seeping into the soil over time) can also be advantageous for maintaining the Trichoderma population for a longer period of time.
The frequency of application depends on environmental conditions. If Trichoderma finds sufficient food (harmful fungi, dead organic material) and moisture, application can take place less frequently than with little moisture. In principle, we recommend application every 30-60 days from a temperature of 8°C.