Screening diverse <i>Cannabis sativa</i> germplasm for resistance to <i>Golovinomyces ambrosiae</i>

Powdery mildew is one of the most common diseases affecting indoor hemp (<i>Cannabis sativa</i> L. <0.3% tetrahydrocannabinol) cultivation. The primary causal organism is the biotrophic fungus <i>Golovinomyces ambrosiae</i>, whose polycyclic nature and short latent phase allow it to rapidly produce asexual conidia that spread to other plants. Control strategies that rely on the use of fungicides are limited by the efficacy of products that are registered for use on hemp, thus the breeding of resistant cultivars is critical. Two resistance genes and one susceptibility gene involved in powdery mildew resistance have been described for <i>C. sativa</i>. In the present work, we evaluated the susceptibility to <i>G. ambrosiae</i> of 70 <i>C. sativa</i> entries in one year and 98 in a second year, including accessions from the United States Department of Agriculture - Agricultural Research Service germplasm repository, breeding lines, and commercial cultivars. Entries represented different geographic origins, levels of improvement, and market classes. The trials were inoculated with <i>G. ambrosiae</i>, and disease severity was rated on a weekly basis. A wide range of disease severity was observed among and within entries. Several entries displayed no or very low levels of disease across years, suggesting that they have genetic resistance to <i>G. ambrosiae</i>. This screening provides a foundation for further characterization of the mechanisms of powdery mildew resistance in hemp and contributes to broaden the publicly available phenotypic data that will better serve stakeholders who rely on germplasm collections.