In February, the Oregon State University, together with USDA, released a first report on the emergence of powdery mildew caused by Golovinomyces ambrosiae in Oregon.
Even though the majority of hemp is grown outdoors, it was a greenhouse grower who first noted some alarming signs on their hemp plants.
As the researchers point out, the first sign of the disease started as small powdery patches on leaves, until it got to the point of colonizing up until the buds. When morphological observations were carried out, researchers saw overlapping with several Golovinomyces spp, such as G. amborsiae, G. cichoracearum, and G. spadiceus.
Indeed, analyses confirmed such an identification. Moreover, pathogenicity was also confirmed by inoculation onto ‘Cherry Wine’ variety potted plants, which developed powdery mildew symptoms 14 to 21 days later.
The researchers conclude that this was the first known report of powdery mildew on hemp in Oregon. Even though powdery mildew mainly appears on hemp in protected cultivation, the widespread outdoor cultivation of hemp in Oregon might lead to an increased powdery mildew risk.