Plant Biostimulants: New Insights Into the Biological Control of Verticillium Wilt of Olive

Abstract

Verticillium wilt of olive (Olea europaea subsp. europaea L.) (VWO), caused by the hemibiotrophic soil-borne fungus Verticillium dahliae Kleb., is considered the major limiting factor of this crop in Mediterranean-type climate regions of the world. The absence of effective chemical treatments makes the control of the disease difficult. In this way, the use of biostimulants and host plant defense inducers seems to be one of the most promising biological and eco-friendly alternatives to traditional control measures. Thus, the main goal of this study was to evaluate the effect of 32 products, including amino acids, micronutrients, microorganisms, substances of natural origin, copper complex-based products, and organic and inorganic salts against the disease under controlled conditions. To this end, their effects on mycelial growth and microsclerotia (MS) inhibition of V. dahliae were evaluated by means of dual cultures or by sensitivity tests in vitro as well as on disease progression in planta. Wide ranging responses to the pathogen and disease reduction levels were observed among all the products tested, suggesting multiple modes of action. Copper-based products were among the most effective for mycelial growth and MS inhibition, whereas they did not show an important effect on the reduction of disease severity in planta. Phoma sp. and Aureobasidium pullulans were the most effective in disease reduction in planta with foliar application. On the other hand, two phosphite salts, one with copper and the other with potassium, were the most effective in disease reduction in planta when they were applied by irrigation, followed by A. pullulans and Bacillus amyloliquefaciens. This study will be useful to select the best candidates for future studies, contributing significantly to new insights into the current challenge of the biological control of VWO.

Elucidating the Effect of Nutritional Imbalances of N and K on the Infection of Verticillium dahliae in Olive

“…For inoculum preparation, 1 kg of cornmeal-sand mixture (CMS; sand, cornmeal and distilled water; 9:1:2, w:w:v) was placed in 2 L Erlenmeyer flasks. Subsequently, the flasks were sterilized following the protocol described by López-Moral et al [5]. For CMS pathogen colonization, 50 mycelial plugs (7.5 mm in diameter) of V. dahliae isolate V323 growing on PDA were introduced into each flask.…”

“…The flasks were incubated at 24 • C in the dark for 4 weeks and manually shaken once a week to favour a homogeneous colonization. Finally, the inoculum density of the colonized CMS was estimated by means of the serial dilution method on PDA and expressed as colony-forming units (CFUs) [5,18].…”

“…MS not only serve as resistance structures, but are also the primary inoculum of the pathogen. They germinate due to the biochemical stimuli from host root exudates, giving rise to infectious hyphae that penetrate the roots of the plants and grow into the xylem vessels [1,5]. The biology of the pathogen suggests that important attention should be paid to elucidating the effect of soil mineral nutrition into its life cycle in olive tree, and particularly in the infection process in this woody host.…”

“…Therefore, determining the influence of nutritional imbalances of one or several nutrients on the incidence and severity of VWO is essential to establish an adequate nutrient program preventing major V. dahliae infections. This issue is even more relevant in the case of VWO since adequate nutrition management must be taken into account within an integrated disease management strategy [2,4,5]. In addition, this last aspect is of major importance to be considered in olive groves under fertigation systems, where not only the imbalanced mineral nutrition, but also the water regimes, can affect the severity of VWO [15,16], while the interaction between irrigation and fertilization can act in favour of VWO epidemics [8].…”

Management of the soil-borne fungal pathogen – Verticillium dahliae Kleb. causing vascular wilt diseases

“…Application of beneficial microorganisms for the management of plant diseases have gained the attention of scientists and farmers in recent years (Alabouvette et al 2006;Copping and Duke 2007;Angelopoulou et al 2014;Ghorbanpour et al 2018;Millan et al 2021). The inhibitory effects of several bacterial and fungal antagonistic isolates in controlling and reducing Verticillium wilt have been documented in many studies (Berg et al 2006;Gizi et al 2011;Jorjani et al 2011;Eljounaidi et al 2016;Antoniou et al 2017;Deketelaere et al 2017;Abada et al 2018;Lόpez-Moral et al 2021). The search for biological methods for the destruction of propagation forms of V. dahliae, mainly microsclerotia, is aimed at inhibiting the development of these forms, reducing their survival rate, preventing them from germinating, protecting the roots from infection by sprouting microsclerotia.…”