| Summary: | The objective of this study was to determine the effects of potassium fertilization (applied to soil at 150, 250, and 350 kg K<sub>2</sub>O ha<sup>−1</sup>) and irrigation on the yield (fresh matter yield and dry matter yield of above-ground biomass and tubers) and the health status of tubers and leaves of three Jerusalem artichoke—JA (<i>Helianthus tuberosus</i> L.) cultivars (Topstar, Violette de Rennes, Waldspindel). The Topstar cultivar was characterized by the highest total tuber yield (60.53 Mg FM ha<sup>−1</sup>) and the highest above-ground biomass yield (65.74 Mg FM ha<sup>−1</sup>). An increase in the rate of potassium fertilizer to 350 kg K<sub>2</sub>O ha<sup>−1</sup> did not affect total tuber yields. The greatest increase in above-ground biomass yields was observed in response to the potassium fertilizer rate of 150 kg K<sub>2</sub>O ha <sup>−1</sup> (64.40 Mg FM ha<sup>−1</sup>). Irrigation increased tuber yields by 59% and above-ground biomass yields by 42% on average. Phytopathological analyses revealed that JA leaves were most frequently colonized by fungi of the genera <i>Alternaria, Fusarium</i>, and <i>Epicoccum</i>. <i>Alternaria</i> and <i>Fusarium </i>fungi were more prevalent in non-irrigated than in irrigated plots. A higher number of fungal pathogens was isolated from the leaves of cv. Violette de Rennes grown in a non-irrigated plot fertilized with 250 kg K<sub>2</sub>O ha<sup>−1</sup>. Tubers were most heavily colonized by fungi of the genera <i>Penicillium, Fusarium, Alternaria, Botrytis, </i>and<i> Rhizopus</i>. Fungal species of the genus <i>Fusarium</i> were isolated from tubers in all irrigated treatments, and they were less frequently identified in non-irrigated plots. Only the tubers of cv. Topstar grown in non-irrigated plots and supplied with 150 kg K<sub>2</sub>O ha<sup>−1</sup> were free of <i>Fusarium</i> fungi. The number of cultures of pathogenic species isolated from Jerusalem artichoke tubers had a minor negative impact on fresh and dry matter yield.
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