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Ten Phytophthora infestans isolates were analysed for maximal growth rate (μmax), sporulation capacity (K), ploidy and the ability to produce infective F1 progeny when combined in different crosses on tomato leaflets. Ploidy was determined using three different methods: simple sequence repeats (SSR, microsatellites), allele quantification at F382Y in the RNA polymerase, and flow cytometry. Eight out of 10 isolates were classified as diploid, the other two as triploid. There was no correlation between ploidy, growth rate and sporulation capacity although the latter was rather low for triploid isolates. Ten crosses were investigated in tomato leaflets (five 2n × 2n; four 2n × 3n; one 3n × 3n). Oospore production was observed in all crosses independent of parental ploidy. Germination and pathogenicity of oospores were investigated by measuring the number of infection sites (pathogenic F1 progeny) over a time period of 72 days. All crosses containing a triploid parent produced many fewer F1 isolates than 2n × 2n crosses. The generation of pathogenic F1 progeny isolates was best for 2n × 2n crosses. A total of 319 F1 offspring were produced and analysed for mating type and ploidy; a subset was tested for ploidy with all three methods. For 2n × 2n crosses, the majority of offspring were diploid, whereas in crosses containing one or two triploid parents, diploid, triploid and trisomic offspring were observed. The results suggest that less pathogenic F1 progeny isolates are produced if at least one parent is triploid.