Antimicrobial Activity of Different Molecular Weight Chitosans to Inhibit Some Important Plant Pathogenic Fungi

Abstract

The efficacy of different molecular weight (MW) chitosans to inhibit plant pathogenic fungi was investigated in vitro and in vivo. The first experiment studied the antimicrobial activity of different MW chitosans to inhibit the fungus Sphaceloma ampelinum in a liquid medium. The results revealed that a natural polymer chitosan (chitosan III), with a higher MW, exhibited more efficiency than oligomer and irradiated chitosans, with lower MWs, to inhibit the activity of the fungus. The efficiency of inhibition was increased with an increase in the concentrations of the chitosans. A concentration of 10,000 ppm was the most effective to inhibit the fungus S. ampelinum in vitro. The second experiment investigated the antimicrobial activity of different MW chitosans to inhibit nine plant pathogenic fungi in solid agar medium. The results indicated that lower MW chitosans exhibited higher mycelial inhibition than higher MW chitosans. Natural polymer chitosans showed 4.9% mycelial inhibition whereas lower MW chitosans ranged between 9.1 and 17.6% mycelial inhibition. The results also suggested that different plant pathogenic fungi showed different responses to different MW chitosans and concentrations. Both investigations concluded that different MW chitosans could exhibit different antimicrobial activity to plant pathogenic fungi when they were tested by different methods in vitro. The third experiment studied the efficacy of different MW chitosans to inhibit plant pathogenic fungi in vivo. Colletotrichum gloeosporioides, the causal or ganism of mango anthracnose, was used in this antimicrobial activity test in vivo. The results revealed that polymer and oligomer chitosans could not inhibit the incidence of the disease; however, oligomer chitosans tended to decrease the disease severity compared to polymer chitosans, which indicated that lower MW chitosans were more effective than higher MW chitosans to inhibit plant pathogenic fungi in vivo. The efficacy of chitosans to inhibit the infection of Helminthosporium turcicum, the causal agent of northern corn leaf blight, was investigated. The results suggested that lower MW chitosans might be more effective than higher MW chitosans to inhibit the fungus in corn plants.