Abstract Polysaccharides from fungal wastes were partially characterized and evaluated for their protective effects against bacterial spot caused by Xanthomonas gardneri on four tomato cultivars: Santa Cruz Kada, Natália, BRS Sena and Forty. The polysaccharides were extracted from spent mushroom substrate of Pleurotus ostreatus, residual brewery yeast (Saccharomyces cerevisiae), and basidiocarps discarded from Lentinula edodes production. These polysaccharides were characterized for total carbohydrates, phenolics and proteins content, pH, scatter intensity, conductivity, Zeta potential, DPPH scavenging assay and infrared spectroscopy. The effects of time interval between treatment and inoculation (4 or 7 days) and polysaccharide concentrations (0.5 or 1.5 mg.mL–1) were assessed for disease severity using a susceptible tomato cultivar. The polysaccharide action mode was investigated by determining the activity of peroxidases and phenylalanine ammonialyase and by quantifying flavonoids and total phenolics in the plants treated and challenged with X. gardneri. The polysaccharides obtained from Lentinula edodes (PSHII), Saccharomyces cerevisiae (PRC) and Pleurotus ostreatus (PSPO) (1.5 mg.mL-1) reduced bacterial spot severity by 50% on tomato cotyledons, leaflets and five-leaf plants. Furthermore, PRC and PSHII (1.5 mg.mL–1) could decrease disease severity in all tested cultivars. PSHII, the most effective, did not cause change in phenylalanine ammonia-lyase activity or flavonoid content on the cultivars Kada and Natália. However, an increase in peroxidase activity and total phenol content on cv. Kada was noted. The polysaccharides obtained from food industry wastes could provide protection against bacterial spot on tomato cultivars by inducing defense mechanisms and can be useful in formulating products with phytosanitary potential.

Bacterial spot, caused by Xanthomonas gardneri, is a disease of great importance for tomato crops, especially due to yield reduction. The objective of this study was to evaluate the effectiveness of low - and high-density chitosan for the control of bacterial spot and whether such control was due to activation of defense mechanisms in tomato. Tomato plants were treated with high-density (HD; 0.8 g/cm³) or low-density chitosan (LD; 0.4 g/cm³) at concentrations ranging from 0.0 to 3.0 mg/mL, and then inoculated with a bacterial suspension three days later. The effect of varying intervals (7, 5, 3 and 1 day) between the application of HD chitosan (3 mg/mL) and the inoculation of the pathogen was also assessed. The effect of chitosan on the in vitro growth of X. gardneri and the activities of peroxidases and polyphenol oxidases in leaves were also determined. HD chitosan (3.0 mg/mL) was able to greatly reduce disease severity by up to 85% compared to the control. The application of HD-chitosan at five days before inoculation showed larger reduction in disease severity and high level of peroxidase activity. The in vitro growth of X. gardneri was not affected by the treatment with chitosan, thus suggesting that the disease was controlled due to mechanisms related to induced resistance.