Natural Biostimulants Elicit Plant Immune System in an Integrated Management Strategy of the Postharvest Green Mold of Orange Fruits Incited by Penicillium digitatum.

Federico La Spada,Francesco Aloi,Santa Olga Cacciola,Antonella Pane,Maurizio Coniglione

doi:10.3389/fpls.2021.684722

Federico La Spada, Francesco Aloi + Show 3 more

Open Access

https://doi.org/10.3389/fpls.2021.684722

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Abstract

This study was aimed at testing the integrated use of a natural biostimulant based on seaweed (Ascophyllum nodosum) and plant (alfalfa and sugarcane) extracts and reduced dosages of the conventional synthetic fungicide Imazalil (IMZ) to manage postharvest rots of orange fruits. The following aspects were investigated: (i) the effectiveness of postharvest treatment with natural biostimulant alone or in mixture with IMZ at a reduced dose against green mold caused by Penicillium digitatum; (ii) the differential expression of defense genes in orange fruits treated with the natural biostimulant both alone and in combination with a reduced dose of IMZ; (iii) the persistence of the inhibitory activity of both biostimulant and the mixture biostimulant/IMZ against green mold; and (iv) the residue level of fungicide in citrus peel when applied alone or in combination with the biostimulant. Treatments with the chemical plant resistance-inducer potassium phosphite, alone or with a reduced dose of IMZ, were included for comparison. The mixture of natural biostimulant and IMZ at a low dose consistently reduced the incidence and severity of fruit green mold and induced a significant increase of the expression level of β-1,3-glucanase-, peroxidase (PEROX)-, and phenylalanine ammonia-lyase (PAL)-encoding genes in fruit peel, suggesting that the natural biostimulant elicits a long-lasting resistance of citrus fruits to infections by P. digitatum. Interestingly, the residual concentration of IMZ in fruits treated with the biostimulant/fungicide mixture was significantly lower than that of IMZ in fruits treated only with the fungicide at the same dose and by far below the threshold values set by the European Union. This study laid the foundations for (i) conceiving a practical and more eco-friendly alternative to the conventional postharvest management of green mold of citrus fruits, based almost exclusively on the use of synthetic fungicide IMZ, alone or mixed with potassium phosphite and (ii) providing a better insight into the mechanisms of disease resistance induction by biostimulants.

Highlights

Molds produce major postharvest losses of citrus fruits during storage, processing, transportation, and marketing (Eckert and Eaks, 1989)
To explore the possibility of minimizing the application of conventional fungicides to manage postharvest citrus diseases, this study investigated the effectiveness of a new natural biostimulant product, based on seaweed-extract and plantderivatives, both alone and in mixture with a notably reduced dose of fungicide, as resistance inducer in citrus fruits to green mold incited by P. digitatum and as means to control the pathogen directly on infected fruits (Figure 1)
The induced resistance in citrus fruits as a consequence of treatments with the substances tested (Table 1) was evaluated based on the expression levels of genes involved in the synthesis of enzymes related to the main plant defense pathways; these include PR proteins (i.e., β-1,3-glucanases-encoding gene), phenylalanine ammonia-lyase (i.e., PAL-encoding gene), and peroxidases (i.e., PEROX-encoding gene)

Summary

Introduction

Molds produce major postharvest losses of citrus fruits during storage, processing, transportation, and marketing (Eckert and Eaks, 1989). Green mold caused by Penicillium digitatum is the most damaging and widespread postharvest disease of citrus fruits worldwide (Ismail and Zhang, 2004; Youssef et al, 2014). The rapid infection progress leads, in a few days, to the complete decay of the fruits, which at the late stages appear to be completely covered by green conidia (Ismail and Zhang, 2004). Due to serious losses caused by the disease, the postharvest management of citrus green mold comprises specific practices to prevent it, such as washing and drenching with synthetic fungicides (Ismail and Zhang, 2004). Imazalil (IMZ) is the most commonly used fungicide for postharvest treatments of citrus fruits (Davé et al, 1989; Holmes and Eckert, 1999; Ismail and Zhang, 2004; Moraes Bazioli et al, 2019)

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