INDUCTION OF SYSTEMIC RESISTANCE TO FUSARIUM CROWN AND ROOT-ROT IN TOMATO PLANTS BY SEED TREATMENT WITH CHITOSAN

Chitosan, derived from crab-shell chitin, was applied as seed coating and substrate amendment prior to infection with the fungus Fusarium oxysporum f. sp. radicis-lycopersici. Experiments were performed either on a mixture of peat/perlite/vermiculite or on bacto-agar in petri dishes. In both cases, a combination of seed coating and substrate amendment was found to significantly reduce disease incidence as judged by the decreased number of root lesions and the healthier appearance of the root system. Although seed treatment alone could induce a delay in symptom development, it was not efficient enough to confer full seedling protection. Chitosan at concentrations ranging from 0.5 to 1 mg/ml was used for the ultrastructural and cytochemical investigations. Examination of the root tissues at sites of fungal penetration revealed that a pretreatment with chitosan was always associated with the expression of plant defense reactions. In the epidermis, cells showed typical signs of necrosis characterized by marked disorganization of the cytoplasm. Fungal growth was reduced in but was not restricted to this reacting area. Indeed, the pathogen was detected in the outer cortex where its development was halted. Fungal cells suffered from serious damage and were frequently encircled by an electron-dense material. In the noncolonized inner cortex, strong host reactions were detected that were mainly associated with the deposition of two types of material that differed in their electron density. Gold cytochemistry with a beta-1,3-glucanase and a laccase showed that the more electron-dense material was phenolic in nature whereas the other material, occurring either as deposits inserted between the phenolic aggregates or as globular structures lining the host cell walls, was made of beta-1,3-glucans. These observations bring further evidence that chitosan is an active inducer of plant defense reactions and, thus, has the potential of becoming a powerful alternative means of disease control.