Phytonematodes typically infect plants of various ages, but they are especially dangerous to woody plants in the first years of life. Plant infestations with phytohelminths are characterized by stunted growth, deformation of stems and shoots, shoot wilting, and chlorosis. Studying phytonematodes, their species, development conditions, and adaptation processes is important. The ability of phytonematodes to adapt to specific moisture levels allows them to be divided into the following major ecological groups: xerophylls, mesophills, hygrophiles, and eurybionts. Xerophylls comprise an ecological complex of nematode species confined to strictly arid environments. The mesophilic ecological complex includes nematode species confined to biotopes with moderate moisture levels. Hygrophiles comprise a complex of species confined to areas with excessive moisture due to the close proximity of groundwater. Hygrophila are divided into two subgroups: megathermic and mesothermic. Mesothermic hygrophila are confined to floodplain forests (tugai) of river valleys. Under irrigated agriculture, megathermic hygrophila can migrate to cultivated areas and cause significant damage to agriculture. Mesothermic hygrophila include nematode species confined to biotopes with excessive moisture and moderate or cold temperatures. Some species of mesothermic hygrophila are potentially dangerous parasites of agricultural plants grown under irrigated conditions, but cannot survive in dryland plantings. It should be noted that this unique habitat has a unique set of environmental factors, as well as a unique composition of plant nematodes.