PURPOSE:To provide the titled agent containing a chitosan having a low molecular weight as an active component and effective in preventing the outbreak of black spot of pear tree, especially NI-JU-SEIKI (20th Century) pear and restoring the tree infected by black spot. CONSTITUTION:The objective agent contains a low-molecular-weight chitosan having a molecular weight of =3,000 as an active component. The chitosan is a chitosan oligosaccharide of D-glucosamine, having a polymerization degree of 2-8 and easily and economically producible by decomposing a chitosan with a chitosanase originated from Bacillus No.7-M (FERM P-8139). Excellent effect can be attained by mixing the above active component with a carrier and an adjuvant and spraying to a pear tree in the form of liquid containing =0.05% active component. In case of using the component as a powdery agent, it is preferable to use in combination with a spreader.
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