The occurrence of necrotrophic winter wheat and triticale pathogens in eight geographical regions of Poland was studied between 2015 and 2020. Over a period of six years, the incidence of the following pathogens was monitored: Parastagonospora nodorum, Parastagonospora avenae and Zymoseptoria tritici. The significant effect of meteorological factors on the incidence of pathogens was determined. The relationship between late-season and early-season factors associated with temperature and precipitation on the severity of diseases incited by the pathogens was statistically significant. Statistical models estimating the natural occurrence and severity of diseases caused by the pathogens were developed with the random forest (RF) algorithm based on 10,412 cases of the diseases. The data were randomly divided into training and test datasets and the accuracy of models was determined by the root mean squared error (RMSE) and Pearson correlation coefficient (r). The most promising model was developed for Z. tritici with the following test metrics: RMSE = 57.5 and r = 0.862. The model can be used to link disease severity to weather and predict low severity years and high severity years. Over the period of 2015–2020, the most significant winter wheat pathogen showed to be Z. tritici, while on winter triticale P. nodorum incited disease symptoms on the largest number of leaves. The occurrence of P. avenae f. sp. triticea on winter wheat and winter triticale was the least frequent and on average was below the economic threshold.