The olive grove is facing an increasingly demanding scenario from a sanitary point of view, in which prevention and technical soil management play a decisive role. In this context, the olive verticillosis, caused by the fungus Verticillium dahliae, remains one of the most worrying diseases because of its persistence, the complexity of its control and the impact it can have on the development and productivity of plantations.

The fungus colonises the vascular system of the tree, specifically the xylem, which alters water transport and can lead to a wide range of symptoms, such as chlorosis, wilting, leaf curling, drying of shoots and branches, necrosis of inflorescences or mummification of fruit. In the most severe cases, associated with highly virulent isolates, the disease manifests itself as a rapid decline, characterised by intense defoliation and even death of the tree.

The agronomic importance of verticillium dieback lies not only in the direct damage it causes, but also in the difficulty of managing the disease once the pathogen has become established in the soil. The persistence of the inoculum, the influence of environmental conditions and the possibility of confusing its symptoms with those of other root or vascular problems make it necessary to work with integrated strategies and complementary tools to help reduce the risk of development and spread of the disease.

In recent years, interest in naturally occurring compounds with activity against soil pathogens has been gaining importance as part of integrated management strategies. Among them, an aqueous extract from onion has shown promising antifungal activity against V. dahliae, with the potential to inhibit fungal growth and reduce the inoculum present in the soil.

In our studies, this potential has been addressed from three complementary approaches: on the one hand, the evaluation of the ability to reduce the inoculum of artificially infested soil; on the other hand, the evaluation of the response of olive seedlings in growth chambers under controlled infection conditions; and, finally, its assessment in commercial olive orchards under real growing conditions.

Soil population reduction: a preventive approach to olive verticillium verticillosis

The reduction of soil inoculum is one of the most important lines of work in the management of olive verticillium, since the persistence of the inoculum in the soil is a key factor in the V. dahliae conditions the appearance and evolution of the disease. With the aim of evaluating the capacity of an aqueous extract of onion to reduce the inoculum of V. dahliae In this matrix, a test was designed on a sterilised substrate artificially inoculated with the pathogen. Different concentrations of the compounds were applied to this substrate and their effect was compared with an infested and untreated positive control.

Based on this experimental approach, the results showed that the application of these compounds significantly decreased the population of V. dahliae in the substrate, and a dose-dependent response was also observed. The highest concentrations generated the strongest effect, achieving very marked reductions in inoculum compared to the positive control. In particular, decreases of up to 5 log units in the pathogen population were observed at the highest dose.

Figure 1. Concentration of V. dahliae in substrate treated with aqueous onion extract at 100, 500 and 1000 mg/L versus positive control (C+). Values represent mean ± standard deviation. Different letters indicate significant differences (p < 0.05).

From an agronomic point of view, this result is particularly relevant because the reduction of soil inoculum in olive trees should not be understood only as a punctual measure, but as a preventive tool aimed at reducing the infection pressure of olive trees. V. dahliae. In a disease as complex as olive verticillium, reducing the pathogen load in the soil can be decisive in limiting new infections and reducing the impact of the disease on the crop.

Growth chambers: assessing the response of olive trees to Verticillium dahliae

In order to evaluate the response of the olive tree against V. dahliae under controlled conditions, a growth chamber trial was designed with olive seedlings of the Picual variety grown in a substrate artificially infested with the pathogen. The infested substrate was pre-treated with different doses of an aqueous extract of onion and, subsequently, the seedlings were transplanted into the substrate. In some treatments, this initial application was complemented with an additional application by irrigation, in order to assess its effect on the evolution of the disease.

The results showed that the application of aqueous onion extract reduced the incidence and severity of olive verticillium as compared to the untreated infected control. In addition, less vascular colonisation of the fungus was observed and a reduction of the density of V. dahliae in the soil, which shows a favourable effect on both the development of the disease and the presence of the pathogen in the root environment.

Figure 2. Incidence, severity, vascular colonisation and density of the pathogen in soil inoculated and treated with an aqueous extract of onion at 250 and 500 mg/L. Different letters indicate significant differences (p < 0.05).

The most favourable effects were recorded when substrate pre-treatment was combined with a subsequent application by irrigation, suggesting that the application strategy may have a relevant influence on the observed response. From an agronomic point of view, these results reinforce the interest of acting on early stages of infection and combining different application routes in the framework of preventive management of olive verticillium blight.

Commercial olive groves: response to Verticillium dahliae under real growing conditions

To test whether the effects observed under controlled conditions could be transferred to the field, the onion extract was evaluated in two commercial olive groves of the Picual variety with a high incidence of verticillium, located in Linares and Santaella. In both cases, the application by irrigation allowed to assess its behaviour under real growing conditions and under natural pressure of V. dahliae.

The results showed interesting effects on the physiological state of the olive tree. In leaf, the treatment was associated with a reduction in oxidative stress, with a significant decrease in malondialdehyde levels in Santaella and a trend in the same direction in Linares. In addition, in both farms a significant reduction in the load of V. dahliae in leaf, measured as gene copy number, which reinforces the interest of the extract against verticillium under real growing conditions.

From the production point of view, the most outstanding result was recorded in Santaella, where the treatment was associated with a significant increase in the fat content of the fruit. A positive trend was also observed in Linares, although without statistically significant differences. Overall, these trials in commercial olive orchards indicate that aqueous onion extract can provide benefits of agronomic interest by contributing to reducing pathogen load, improving the physiological response of the tree and favouring fruit quality parameters under certain conditions.

Conclusions

The results obtained show that the aqueous extract of onion is of interest against V. dahliae in olive from a preventive and applied perspective. Its effect was observed both in soil inoculum reduction and plant response under controlled conditions and in commercial olive orchards.

Overall, the results obtained reinforce the interest of the aqueous extract of onion as a complementary tool in the management of onion diseases. Verticillium dahliae and olive verticillium.

For further information on this line of work and to find out more about the solutions that DOMCA develops for agriculture, You can consult our website or send an email to informacion@domca.com.

References

Falcón-Piñeiro, A., Remesal, E., Noguera, M., Ariza, J. J., Guillamón, E., Baños, A., & Navas-Cortés, J. A. (2021). Antifungal activity of propyl-propane-thiosulfinate (PTS) and propyl-propane-thiosulfonate (PTSO) from Allium cepa against Verticillium dahliae: In vitro and in planta assays. Journal of Fungi, 7(9), 736.

Falcón-Piñeiro, A., Zaguirre-Martínez, J., Ibáñez-Hernández, A. C., Guillamón, E., Santander, K., Barrero-Domínguez, B., López-Feria, S., Garrido, D., & Baños, A. (2024). Evaluation of the Biostimulant Activity and Verticillium Wilt Protection of an Onion Extract in Olive Crops (Olea europaea). Plants, 13(17), 2499.