**1. Introduction**

The food issues worldwide result from the increased human population, besides plant pathogens resulting in whole or partial harm to crop yields [1]. Most roots of the *Solanaceae* family and other plants have suffered from soil fungal plant pathogens that cause harmful effects on morphological, physiological, molecular, and yield properties. Pepper (*Capsicum annuum* L.) is a hardy plant cultivated extensively worldwide. The annual production of Egyptian pepper is 623,221 tons, with a total cultivation area of 41,047 hectares [2]. Pepper crops worldwide, including in Egypt, are being destroyed by soil-borne diseases such as *F. oxysporum*, which causes significant losses in quantity and quality [3]. Despite the effectiveness of synthetic fungicides in eliminating *Fusarium* and minimizing the harmful effects, the ecological troubles and the increased fungal resistance

**Citation:** Abdelaziz, A.M.; Attia, M.S.; Salem, M.S.; Refaay, D.A.; Alhoqail, W.A.; Senousy, H.H. Cyanobacteria-Mediated Immune Responses in Pepper Plants against *Fusarium* Wilt. *Plants* **2022**, *11*, 2049. https://doi.org/10.3390/ plants11152049

Academic Editors: Carlos Agustí-Brisach and Eugenio Llorens

Received: 23 June 2022 Accepted: 19 July 2022 Published: 5 August 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

to these chemical fungicides are evident. We should not fail to note that the excessive use of pesticides has led to more serious problems than the disease itself, as it has negatively affected humans, animals, the environment, and healthy microbial communities in soil and plants [4–6]. Therefore, biological control of *Fusarium* wilt through different non-pathogenic microorganisms such as cyanobacteria, fungi, yeast, and bacteria are good techniques [7,8]. Plant resistance means preventing or limiting the progression of its damage, whether biotic or abiotic [9,10]. Biological agents can induce systemic pepper plant resistance. Inducers of resistance affect anatomical structures, morphology, or the making of certain chemical composites that obstruct the pathogen or minimize the severity of stress [11,12]. Structure and chemical weapons may be present in the plant regardless of whether a pathogen is attacking it. These weapons may also originate from an attack on the plant by a pathogen or stress [13]. The destruction of *F. oxysporum* in-vitro and in vivo is similar to the activity of mancozeb chemical fungicides through inhibition of fungal growth and sporulation [14].

Algae, including cyanobacteria, act as bio-protectants and bio-stimulants for crop enhancement [15,16] by destructing the structure or function of the membrane of plant pathogens, devastating pathogenic enzymes, and the defeat of protein synthesis [17]. *Desmonostoc muscorum* is an effective bio-fungicide to control some plant pathogens such as *Alternaria porri* in-vitro [18]. It also can inhibit the radial *Fusarium* mycelial growth [19]. On the other hand, *Arthrospira platensis* has antifungal activity against *F. oxysporum* through polyphenols production in in-vitro [20] and in-vivo against *Moringa Fusarium* wilt [21]. *Arthrospira platensis* extract contains phenolics resulting in antifungal activity [22]. *Anabaena*, when applied to seeds, resulted in the protection of root diseases from fungal pathogens such as *Fusarium* [23] through different mechanisms such as phosphate solubilization and indole-3-acetic acid (IAA), ammonia, hydrogen cyanide (HCN), and enzyme production [24]. Today, the risk of fungal plant disease is one of the most urgent issues [25,26]. In light of our increased understanding of environmental issues, we must look for feasible and easy-to-use solutions to *Fusarium*'s harmful effects on plants. One of the most famous pathogens of fungal diseases, *F. oxysporum*, hurts crops, especially vegetables [8,27]. Cyanobacteria could be a viable alternative to synthetic fungicides in the fight against phytopathogenic fungi because they produce bioactive metabolites with high antifungal efficiency, particularly polyphenols and flavonoids [28,29]. The use of growth-stimulating cyanobacteria is a common strategy for researchers to enhance and improve the defense capacity and physiological immunity of plants as well as the bioavailability of minerals in the soil [30].

*Anabaena* sp. and *Oscillatoria nigro-viridis* recorded the highest phenolics and flavonoid levels and the most elevated antifungal activity among the investigated cyanobacterial strains in recent research [31]. Accordingly, we examined phenolics and flavonoid levels in the three cyanobacterial strains used in this study to determine which cyanobacterial strains are best for reducing the risk of pepper plant infection with *Fusarium oxysporum*. However, this study's major purpose was to explore cyanobacteria's activity to minimize the harmful effect of pepper *Fusarium* wilt by enhancing pepper plant immunity.
