I did some more research this morning, and while writing this. I'm sure now this is generally known as
downy mildew, caused by a number of fuzzy-looking fungus or mold species. Downy mildew is a sub-classification of the wider group of fungal or fungal-like pathogens known as
leaf spots. The fuzzy appearance is the sporulation phase (producing spores).
Purdue University says, re: downy mildew, "Numerous
sporangiophores cause a down-like appearance on the underside of hemp leaves." (Sporangiophores are structures that produce spores.)
Cornell University says (2020), "Diverse leaf spots [on hemp] have been diagnosed in New York in association with several fungal genera including
Bipolaris, Boeremia (Phoma), Cercospora, Colletotrichum, Cristulariella, Leptosphaerulina, Phaeomycocentropora, Septoria, and
Stagonospora."
An Ontario, Canada, government website on specialty crops says (2020),
"Downy mildew of hemp (also known as cannabis downy mildew) has been identified in multiple hemp fields in southwestern Ontario. The disease first appeared in early September and was confirmed through microscopy and genetic tests to be caused by Pseudoperonospora cannabina....
Downy mildew is caused by an oomycete (water mold) and grows within the leaf. Spores of downy mildew are produced on the lower leaf surface and appear mostly in the early morning before they are released into the wind. While there is limited information on this pathogen, downy mildews typically prefer long periods of high humidity and moderate temperatures."
I was under the impression that the mold spots on my cannabis leaves were from mold infecting the underside of the leaves, but apparently this is not the case. The mold grows inside the leaf, causing chlorosis and necrosis on the upper surface, and then during sporulation the underside of the leaf shows dark spots. Magnified, these dark spots are patches of fuzzy, spore-producing structures (i.e. sporangiophores).
Here's some more info from the same Ontario group, comparing powdery mildew, septoria leaf spot, and downy mildew (2021):
"Downy mildew of hemp was first identified in the field in Ontario in 2020. It is caused by a hemp-specific species, Pseudoperonospora cannabina. It is favoured by humid conditions and extended periods of leaf wetness. Just like the other two foliar diseases, it usually begins once canopies begin to close and there is a humid environment in the centre of the plant.
Symptoms begin as small reddish-brown irregular spots that expand into roughly circular patches.... As they expand, they tend to become confined by the leaf veins and elongate.... Just like Septoria leaf spot, spots are usually surrounded by a yellow halo, but this halo is more irregular in downy mildew. Once lesions are present, spores are often produced on the underside of the leaf below the lesion each morning. This usually appears as a purplish/grey fuzz. The spores are spread by the first winds of the day and will not be present by midday. Downy mildew lesions lack the black pycnidia that are present in Septoria leaf spot lesions."
My best guess now is that my leaf spots are downy mildew, and possibly
Pseudoperonospora cannabina or a similar
oomycete (water mold) species. The above article states, the disease "...usually begins once the canopies begin to close..." which is consistent with what I'm seeing, because my leaf spots are happening almost exclusively during flowering.