The infection may have been caused by not one, but two fungal organisms; an epiphytic form of powdery mildew (possibly Podosphaera macularis) and endophytic false powdery mildew (probably Trichothecium roseum). More details and full update later.
Powdery Mildew Disease of Cannabis
Colorado Green Lab, in collaboration with Hempirical, is examining the life cycle, pathogenicity, and infection patterns in PM-resistant and PM-susceptible strains of cannabis.
Powdery mildew may cause substantial economic loss directly by negatively impacting plant growth and flower quality, and indirectly if treated plants are rendered unsuitable for human consumption by accumulation of unsafe levels of fungicide residue. This is particular relevant where microorganisms have developed high levels of resistance to common fungicides (Demethylation Inhibitors such as myclobutanil), and cultivators apply ever-increasing amounts of fungicide just to keep the mildew in check. By elucidating the underlying cellular and genetic mechanisms behind cannabis powdery mildew susceptibility (and resistance), we aim to develop more effective means of combating the pathogen and reduce cultivation losses.
Powdery mildew is not caused by a single organism, but by many different species of fungi. Most PM-species are highly specific to their preferred host and have evolved to attack a limited range of somewhat similar plants (e.g. cucumber mildew fungi cannot cause oak tree mildew, but can cause disease in squash plants). All PM-species are biotrophic, meaning they require living host tissue to grow and reproduce, and most share common infection mechanisms, with similar life cycles.
Powdery mildew fungi are predominantly epiphytic ("epi" = "upon, near to", "phyt" = "plant") and grow on and around the surface of the plant, with some variants that also produce full or partially endophytic ("endo" = "within") mycelium and hyphae inside the infected plant tissue.
Although the underlying mechanism behind resistance/susceptibility remains unclear, certain strains of cannabis are highly susceptible or highly resistant to infection by fungal organisms. The susceptible strain examined here is Blueberry, an Afghan indica hybrid (20% sativa / 80% indica).
Infection of Susceptible Afghan Indica
Powdery mildew infection of cannabis occurs on the adaxial (upper) surface of the leaf, which is coated with fine hairs (A-D) that may facilitate spore adhesion and subsequent infection.
Infection occurs in stages. Spores landing on the upper surface of the leaf (E) germinate and form appressorium (F), flattened appendages that facilitate penetration.
Appressorium form one or more haustorium (G), which are specialized appendages that punch a hole in the leaf surface, then act as feeding tubes that withdraw nutrients from the living plant.
In PM-resistant cannabis strains, stomata automatically close soon after spore attachment (not shown), possibly to shut down photosynthesis by the affected tissue and starve out the infecting fungus. In susceptible cannabis strains (e.g. "Blueberry"), stomata are locked open directly below the site of the infection (H), allowing the infected tissue to continue photosynthesis and direct nutrients to the invading fungal haustorium.
Following successful penetration of the leaf surface, filamentous mycelial growth invades the plant tissue, spreading throughout the interior of the leaf (I) and emerging from the lower (abaxial) leaf surfaces through stomata (J). Mycelium also forms on the exterior of the upper leaf surface and forms spore-bearing conidiophores (K), which release spores into the air and continue the cycle of infection.
Say hello to my little friend! Russet mite (probably...I'm not an entomologist) was found unexpectedly on the surface of a PM-infected leaf I was examining.
Leaves were destained with Visikol (http://visikol.com/) to improve visualization. For added contrast of fungal and plant structures, some samples (those with reddish hue) were stained with modified Lugol's reagent, followed by dilute sulphuric acid. Thanks and credit to Koroch et al for the useful staining technique described in the literature referenced below.
1. Appl Plant Sci. 2013 Jul 5;1(7).
Rapid staining method to detect and identify downy mildew (Peronospora belbahrii) in basil.
Koroch AR, Villani TS, Pyne RM, Simon JE.