The impact of Verticillium dahliae existence in crop production
The ascomycete Verticillium dahliae is characterized as a pathogen of great economic importance, because of its detrimental action to healthy plants, in combination of the large number of plant species that become vulnerable to it. More specifically, this fungus is detrimental to the health of almost all dicotyledons tree species, to the point that they may undergo universal wilting.
As mentioned above, the number of plant species affected by the pathogenicity of this fungus is large and in these specires are included plant species of great economic importance, such as olives and other cultivated plant species like cotton and potatoes. Therefore, these losses concern producers directly (reduced quantity and degraded quality of harvested products), but also indirectly, through the possible loss of established plants, especially in arboricultures. Also, the presence of this pathogen is worrying not only in the field, but also in nurseries where propagules are produced and the control of V.dahliae presents difficulties.
Spreading and control measures regarding Verticillium dahliae
The dissemination of V.dahliae takes place mainly through microsclerotia, structures that contribute to its further spread. These microsclerotia become contagious when secretions produced by the roots of host plants are present and reproduce on the dead tissues of the host in the final stages of the fungal biological cycle.
Also, the infectivity of this fungus is related to the anatomical traits of plants, at least as far as concerns tree species. More specifically, the intensity of the appeared symptoms is affected by the ability of plants to isolate and replace parts where they had been infected by the fungus. Therefore, the chances of survival of a tree capable of partitioning and timely repair all infected parts of the woody vascular system (xylem) are increased compared to other plant species, where the isolation and repair of these parts is carried out slowly or not at all.
As it becomes noticeable, the strategies involved in V.dahliae control if is entered in the field by any way, is focused on reducing the number of microsclerotia available to soil. In addition, despite the fact that some cultivation measures reduce their population on soil, their management is still problematic, due to their survival in this for a long time (at least a decade) and due to their survival regardless of host existence, even at great depth.
Due to the above mentioned problems and due to the lack of techniques aimed at healing the infected trees, control of V.dahliae is based on the observance of preventive measures. More specifically, the existing measures include the selection of healthy propagating material, planting of varieties that are resistant or even tolerant to the fungus, varieties that may result from cross-breeding of existing varieties with others from genetic material banks, and even from wild relative species.
In addition, measures focusing on the indirect suppression of further transmission of the pathogen, such as thorough cleaning of agricultural machinery as well as the burn of infected plants or the residues they leave, contribute significantly to reducing the spread of the infection.
Also, although crop rotation is considered by some as a measure to restrict V.dahliae spread is not considered economically viable, as the large number of host plants in combination with the longevity of microsclerotia make its implementation difficult. However, maintaining a history of previously cultivated plant species and species of pathogens that have previously appeared in the field would facilitate the selection of plants or varieties to be cultivated.
As previously mentioned, the control of this pathogen is carried out mainly by taking precautionary measures. However, scientists have focused on eradicating – or even limiting – the transmission of V.dahliae, either through the usage of fungicidal chemicals or through biological control methods, such as the use of biological preparations or composted materials.
Regarding the control of V.dahliae with the use of chemical preparations or in combination with using other methods on parallel, such as solarization, they showed remarkable results. More specifically, the use of prochloraz in combination with the addition of complexes in vitro and field conditions has been shown to be effective, at least in terms of the effectiveness of these substances in vitro conditions.
Also, in the past, phosphorous fungicides with systemic action were used, which are considered environmentally friendly, due to their decomposition through physiological processes. However, although these preparations have a more generic fungicidal action, in the case of V.dahliae they inhibited its growth to a considerable extent and therefore the conclusion is that they acted as fungistatics and not as fungicides in vitro.
In addition to the control measures taken against the pathogen using chemical substances, the use of biological preparations is being investigated in the context of organic agriculture. More specifically, in the case of insect pathogenic fungi solutions tested in vitro, it was observed that they not only suppress the growth of V.dahliae hyphaes, but also significantly inhibit the formation of microsclerotia and consequently the further spread of the pathogen. This effect is probably due to the production of secondary metabolites, which may suppress the growth of the pathogen (and several other pathogens). In addition, scientists claim that the effectiveness of these fungi, although affected by pH – a phenomenon observed in the action of various fungicides – is not affected by relatively high temperatures and ultraviolet radiation.
Finally, alternative ways of controlling V.dahliae may potentially be the colonization of fungi and bacteria competing with the fungus in seedlings, as well as the use of composted materials, which is mentioned below.
Regarding the biological control of this pathogen, the addition of composted materials to the soil, either individually or in combination with using various other materials, is also included. More specifically, remarkable results have been observed in terms of long-term control of the pathogen in sandy soils, when interventions involve the addition of chitin-rich material and/or the incorporation of fresh plant mass into the soil combined with the addition of thick plastic sheet.
In addition, in suppressing the growth of V.dahliae, they have been also used in the past in clay loam soils composted waste from fruit extraction, such as marcs or by-products from dairy production with remarkable – albeit temporary – results in the eradication of V.dahliae.
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