The growth of three Alternaria alternata isolates on Potatoe Dextrose Agar (PDA) nutrient supplemented with PEG 6000 in concentrations of 5%, 10% and 20% by volume was differentiated during 7 days of cultivation. The decrease of the weight of the osmotic factor from 72.8% to 29.8%, accompanied by the increase of the contribution of the fungal strain factors and the strain x PEG 6000 concentration interaction in the mycelium growth rate demonstrated the high capacity of the pathogen to adapt to water restrictions. At the same time, the low osmotic pressure (10%, 20%) caused the growth of the mycelium in the form of a film, thus affecting the growth of the air mycelium, but also the synthesis of melanin. PEG 6000 variants in 10% and 20% concentrations were distributed at the major Euclidean distance of 3.60–6.30 in separate cluster, compared to control and PEG 5% variants.