Inhibition of pathogen and induction of resistance against early blight: Alternaria solani Sorauer in tomato plants ( Solanum lycopersicum L.) through external application of compounds from seaweed Chaetomorpha antennina Bory

Background The indiscriminate application of pesticides has depleted soil fertility. Marine algae are regarded as excellent sources of bioactive compounds that can stimulate plant growth and also enhance their resistance against biotic and abiotic stressors. Compounds derived from the marine algae Chaetomorpha antennina (Cladophoraceae) were analysed for their biocontrol efficiency against the tomato disease pathogen Alternaria solani as an alternate technique to preserve agricultural output sustainability. Results The active fraction from the methanol extract of C. antennina (CA-F8) proved effective against the pathogen in vitro by affecting A. solani 's spore germination (92.13%) along with their mycelial growth thereby decreasing their dry weight (44.71%). GC–MS examination revealed the presence of fatty acids (defence pathway activators), terpenes (antimicrobial compound) and antioxidants (deactivators of pathogen enzymes). Seaweed compounds also elicited polyphenol oxidase (PPO) (78.8%) and peroxidase (PO) (54.6%) synthesis, along with stimulating the accumulation of foliar phenols (67.5%) that are components of jasmonic acid defense pathway, higher compared to control plants. Salicylic acid was detected only in treated plants, thereby offering efficient disease control. Conclusions By triggering natural systemic defences, salicylate (SA) and jasmonate (JA) mediated pathways; the study demonstrates the elicitor potentials of chemicals from C. antennina that was efficient in controlling early blight (EB) disease. In addition to providing a different method for crop protection to reduce or replace the demand for chemical pesticides, this study reveals unique projections. Graphical Abstract