The resistance of mutant tomato lines to the main diseases upon the provocative infectious background was assessed. We used as experimental material 22 lines of the tomato genetic collection of the FSBSI ARRIBPP, Krasnodar. According to the results of assessing the damage by Alternaria sp. in the field it was found that line 41 exhibitted high resistance, there were no signs of disease damage. Mutant lines 80, 387, 434, 568, 620 showed resistance, the degree of the disease development varied within 1–8 %. Lines 172, 467, 509, 518 with a degree of development of 12–23 % were characterized by semi-resistance. Lines 41, 387 showed high resistance to P. infestans. No visible signs of damage by P. infestans were detected in these genotypes. Lines 341, 509, 518, 568 had semi-resistance to the pathogen, the degree of development varied from 10 to 18 %. The genotypes Mo 41 and 387 that showed high and relatively high resistance to Alternaria sp. and P. infestans are of the greatest interest for breeding and genetic work as sources of group resistance.