Extracts from the brown seaweeds Cystoseira myriophylloides, Laminaria digitata and Fucus spiralis were evaluated as plant defence inducers against the wild fire disease of Nicotiana benthamiana. Seeds’ imbibition in aqueous seaweed extracts (SE) results in plants with reduced disease severity. In addition, bacterial populations were significantly reduced in these plants when compared to those germinated in sterile distilled water. They primed N. benthamiana for H2O2 accumulation and for enhanced activity of several antioxidant enzymes including catalase, ascorbate peroxidase, guaiacol peroxidase and polyphenol oxidase. These results revealed that soaking of seeds in SE before sowing allows N. benthamiana to reduce pathogen attack and can be easily applied in practice.

Pepino mosaic virus (PepMV) is a highly infectious potexvirus, which has presently become a major pathogen for tomato crops worldwide. Since there is no effective method control for PepMV, strict preventive hygienic measures and cross‐protection have been established. However, the use of mild isolates to protect against aggressive isolates provides opportunities for interaction with other microorganisms, which under certain conditions may worsen disease symptoms. Thus, alternative control methods are needed. In this study we aimed at the development of chemical control against PepMV based on the use of heterocyclic compounds including symmetrical 2,5‐disubstituted 1,3,4‐oxadiazoles, symmetrical 2,5‐disubstituted 1,3,4‐thiadiazole and dihydrotetrazine derivatives. Results showed that spray application of three 1,3,4‐oxadiazole derivatives (OH‐Oxa, CH3‐Oxa, NO2‐Oxa) and of the derivative of 1,3,4‐thiadiazole (OH‐Thia) resulted in reducing the main symptoms of PepMV in tomato leaves, conversely to the dihydrotetrazine carboxylic acid. Double‐antibody sandwich enzyme‐linked immunosorbent assay (DAS‐ELISA) revealed that PepMV was not detected from tomato plants pre‐treated with the derivatives conferring elevated protection. Moreover, the protective ability of OH‐Oxa, CH3‐Oxa, NO2‐Oxa and OH‐Thia was closely related to the enhancement of the activity of antioxidant enzymes upon infection with PepMV.