*Article* **Cyanobacteria-Mediated Immune Responses in Pepper Plants against** *Fusarium* **Wilt**

**Amer Morsy Abdelaziz 1,\*, Mohamed S. Attia 1,\*, Marwa S. Salem 2, Dina A. Refaay 3, Wardah A. Alhoqail 4,\* and Hoda H. Senousy <sup>5</sup>**


**Abstract:** Research in plant pathology has increasingly focused on developing environmentally friendly, effective strategies for controlling plant diseases. Cyanobacteria, including *Desmonostoc muscorum*, *Anabaena oryzae*, and *Arthrospira platensis*, were applied to *Capsicum annuum* L. to induce immunity against *Fusarium* wilt. Soil irrigation and foliar shoots (FS) application were used in this investigation. The disease symptoms, disease index, osmotic contents, total phenol, Malondialdehyde (MDA), hydrogen peroxide (H2O2), antioxidant enzymes (activity and isozymes), endogenous hormone content, and response to stimulation of defense resistance in infected plants were assessed. Results demonstrated that using all cyanobacterial aqueous extracts significantly reduced the risk of infection with *Fusarium oxysporum*. One of the most effective ways to combat the disease was through foliar spraying with *Arthrospira platensis*, *Desmonostoc muscorum*, and *Anabaena oryzae* (which provided 95, 90, and 69% protection percent, respectively). All metabolic resistance indices increased significantly following the application of the cyanobacterial aqueous extracts. Growth, metabolic characteristics, and phenols increased due to the application of cyanobacteria. Polyphenol oxidase (PPO) and peroxidase (POD) expressions improved in response to cyanobacteria application. Furthermore, treatment by cyanobacteria enhanced salicylic acid (SA) and Indole-3-Acetic Acid (IAA) in the infected plants while decreasing Abscisic acid (ABA). The infected pepper plant recovered from *Fusarium* wilt because cyanobacterial extract contained many biologically active compounds. The application of cyanobacteria through foliar spraying seems to be an effective approach to relieve the toxic influences of *F. oxysporum* on infected pepper plants as green and alternative therapeutic nutrients of chemical fungicides.

**Keywords:** pepper; *Fusarium*; cyanobacteria; antioxidant enzymes; plants immunity; salicylic acid
