The harmfulness of weeds is the ability of the weed component to reduce the yield of field crops and deteriorate the quality of the resulting products, to reduce the level of farming in general. The purpose of the study is to study the phytopathological state of corn crops depending on the degree of infestation. The studies were carried out on crops of the early ripening corn hybrid Krasnodar 291 AMB in the period 2020-2022. in the Grozny region of the Chechen Republic. In the agrocenosis of corn, weeds were identified, monitoring the number of which can be used to assess the phytosanitary safety of agricultural land. The type of contamination in the experiment is complex. With an increase in the number of weeds in corn crops, their weight increases from 125.0 to 2016.0 g/m2, and the prevalence of corn plant diseases also increases. Thus, against the background of minimal infestation, the incidence of bladder smut was only 0.16%, and fusarium cob disease - 0.13%. As the degree of contamination increases, these figures increase: 9.28 and 7.38%, respectively. 39.21% of the control yield was obtained with maximum weed infestation. The critical period for the harmfulness of weeds in corn sowing is the first 30 days from the moment of emergence. Moreover, in heavily weeded crops, due to the prevailing microclimate, favorable conditions are created for the spread of fungi - pathogens of corn diseases.