The invention relates to a method for protecting plants from pests, in particular bacteria, fungi, and viruses, using a pesticide composition with adjustable persistence and degradability, having the following steps: providing a pesticide composition which comprises at least one poly(alkyl)guanidine with a monomer alkyl chain length ranging from C2 to C20 and a degree of polymerization n ranging from 2 to 600, and applying the pesticide composition to plant cultures for a specified time and in sufficient quantities in order to prevent or inhibit an adversive effect of the pests on the plants, wherein the length of the monomer alkyl chain and/or the degree of polymerization are set depending on at least one characterizing parameter of the soil or nutrient medium in which the plants are cultivated such that a specified or desired persistence and degradability of the pesticide composition on the cultures or in the soil or nutrient medium are obtained. In a special embodiment of the method according to the invention, the at least one characterizing parameter of the soil or nutrient medium is determined in a first step, and the parameter is preferably selected from the pH value, the humus content, the humin content, the content of microorganisms, the content of soil fungi, or the content of soil bacteria.
