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 invent
