Chitosan exhibits a variety of antimicrobial activities which depend on the type of chitosan (native or modified), its degree of polymerization, the host, the chemical and/or nutrient composition of the substrates, and environmental conditions. In some studies, oligomeric chitosans (pentamers and heptamers) have been reported to exhibit a better antifungal activity than larger units. In others, the antimicrobial activity increased with the increase in chitosan molecular weight, and seems to be faster on fungi and algae than on bacteria.
2. Against viruses
Chitosan was shown to inhibit the systemic propagation of viruses and viroids throughout the plant and to enhance the host’s hypersensitive response to infection. The level of suppression of viral infections varied according to chitosan molecular weight. Similar observations were reported with the potato virus X, tobacco mosaic and necrosis viruses, alfalfa mosaic virus, peanut stunt virus, and cucumber mosaic virus virus, peanut stunt virus, and cucumber mosaic virus ecrosis viruses, alfalfa mosaic.
3. Against bacteria
Chitosan inhibits the growth of a wide range of bacteria. The minimal growth-inhibiting concentrations vary among species from 10–1,000 ppm. Quaternary ammonium salts of chitosan, such as N,N,N-trimethylchitosan, N-propyl-N, N-dimethylchitosan and N-furfuryl-N,N-dimethylchitosan were shown to be effective in inhibiting the growth and development of Escherichia coli, especially in acidic media. Similarly, several derivatives of chitin and chitosan were shown to inhibit E. coli, Staphylococcus aureus, some Bacillus species, and several bacteria infecting fish.
4. Against fungi and oomycetes
Fungicidal activity of chitosan has been documented against various species of fungi and oomycetes. The minimal growth-inhibiting concentrations varied between 10 and 5,000 ppm. The maximum antifungal activity of chitosan is often observed around its pKa (pH 6.0).
5. Against insects
As more and more derivatives of chitosan (i.e., N-alkyl-, N-benzylchitosans) are made available through chemical synthesis, their insecticidal activities are being reported using an oral larvae feeding bioassay [37,38]. Twenty four new derivatives were shown to have significant insecticidal activity when administered at a rate of 5 g•kg-1 in an artificial diet . The most active derivative, N-(2-chloro-6-fluorobenzyl)chitosan, caused 100% mortality of larvae and its LC50 was estimated at 0.32 g.kg-1. All synthesized derivatives highly inhibited larvae growth as compared to chitosan by 7% and the most active derivative was the O-(decanoyl)chitosan, with 64% growth inhibition after 5 days of feeding on the treated artificial diet.
6. Applications of Chitosan in Plant Disease Control
Chitosan used to control plant pathogens has been extensively explored with more or less success depending on the pathosystem, the used derivatives, concentration, degree of deacylation, viscosity, and the applied formulation (i.e., soil amendment, foliar application; chitosan alone or in association with other treatments). For example, Muzzarelli et al.  tested the effectiveness of five Chemicallymodified chitosan derivatives in restricting the growth of Saprolegnia parasitica. Results indicated that methylpyrrolidinonechitosan, N-phosphonomethylchitosan, and N-carboxymethylchitosan, as opposed to N-dicarboxymethylchitosan, did not allow the fungus to grow normally.
7. Applied as seed coating agents
Guan et al. examined the use of chitosan to prime maize seeds. Although chitosan had no significant effect on germination under low temperatures, it enhanced germination index, reduced the mean germination time, and increased shoot height, root length, and shoot and root dry weights in two tested maize lines. In both tested lines, chitosan induced a decline in malonyldialdehyde content, altered the relative permeability of the plasma membrane and increased the concentrations of soluble sugars and proline, and of peroxidase and catalase activities.
8. Applied as foliar treatment agents
Foliar application of chitosan has been reported in many systems and for several purposes. For instance, foliar application of a chitosan pentamer affected the net photosynthetic rate of soybean and maize one day after application. This correlated with increases in stomatal conductance and transpiration rate. Chitosan foliar application did not have any effect on the intercellular CO2 concentration. The authors reported that the observed effect on the net photosynthetic rate is, in general, common in maize and soybean after foliar application of high molecular weight chitosan. Foliar applications of these oligomers did not, on the other hand, affect maize or soybean height, Root length, leaf area, or total dry mass.