Bigleaf hydrangea, Hydrangea macrophylla, is a popular ornamental shrub beloved worldwide for its large colorful inflorescences. As of 2019, bigleaf hydrangea topped $155 million in total sales in the United States. However, diseases impact the health, appearance and ultimately the salability of bigleaf hydrangea. One such disease is powdery mildew, caused by the fungus Golovinomyces orontii. Powdery mildew is a biotrophic obligate, which means that it will only grow on a living host, making it particularly hard to investigate. Inoculating plants via tapping infected leaves against clean leaves is the most common inoculation method; however, this method results in an unknown amount of inoculum and inconsistent infection. Quantifiable and consistent methods are needed to accurately and reproducibly study powdery mildew disease of bigleaf hydrangea. The purpose of this experiment was to compare two inoculation methods, an inoculation box and spray inoculation method, for inducing powdery mildew disease in bigleaf hydrangea. For the box inoculation, 6 infected leaves per plant were tapped above a 48-micron mesh and gently dusted through to land on the plant surface. A still air chamber was designed to be set over the box so that powdery mildew does not get disturbed by air currents. For the spray inoculation, 6 infected leaves per plant were combined into a spray by rinsing the leaves with deionized water, adding Tween 20 to aid with even dispersion and applied to the plant using a spray bottle. Powdery mildew was then evaluated on a scale of 0-100% and the area under disease progress curve (AUDPC) was calculated. The final disease severity of hydrangea inoculated via the box and spray method after 4 weeks of observation were 10.0 and 20.0%, respectively. These results will provide valuable information on more consistent inoculation methods using powdery mildew.
