Abstract Apple is one of the most important temperate fruit to Brazil economy, and the use of synthetic chemicals has been the main method for reducing postharvest diseases, such as the blue mold, caused by Penicillium expansum. This work intends to evaluate the practical utilization of chitosan for blue mold control. For this purpose, fruits were treated in a preventive and curative way, immersing the fruits in chitosan solution (5 or 10 mg mL-1), or adding a single drop of this solution (10 mg mL-1) directly into the injuries. The eradicative effect of the polysaccharide was also evaluated in vitro and in vivo. Chitosan did not show a curative effect against the blue mold, and its eradicative effect was only evidenced on the higher concentration (10 mg mL-1). On the other hand, preventively, without the addition of adjuvants, chitosan reduced blue mold incidence in fruits by 24% and 93%, through the immersion or the single drop methods, respectively. Thus, it was found that, for long scale utilization, some improvements in the physico-chemical properties of the chitosan are needed, since it was only capable to prevent the infection by P. expansum when directly added on the fruit injury.

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.