Powdery Mildew

Powdery Mildew is arguably the most destructive Cannabis pest. It is an obligate biotroph that can vascularize into the plant tissue and remain invisible to a grower. It tends to emerge and sporulate 2 weeks into flowering thus destroying very mature crop with severe economic consequences.

It is believed to travel in clones and its is not known if it travels in seeds. Other stressful events like cloning can trigger Powdery Mildew spore formation. While many fungicides are known to treat Powdery Mildew, many states are outlawing these fungicides due to cannabinoid extraction techniques enriching fungicides (Russo, Raber). Early detection and eradication may be the safest approach.

Background

MacPartland et al. reported L.taurica and P.macularis as the organisms responsible for powdery mildew on cannabis. Multiple unsuccessful attempts to amplify DNA sequences from these organisms led us to whole genome shotgun sequence the Massachusetts derived organism. This sequence assembly presented a novel organism not found in NCBI.

Powdery mildews (PM) are a unique fungi. They tend to have 180Mb genomes (40Mb is more common in fungi) and they are 90% repetitive Transposable elements. As a result these are a challenge to sequence and identify. Since plant domestication, they have become highly adapted to each host so it is rare to find PMs that cross species and some cereals even have more than 1 PM species (Blumeria) that specialize on it.

Under ideal conditions, some powdery mildews have a 4-7 day past innoculation (dpi) window where it remains invisible as it builds a network internal to the plant (dpi work performed in arabidopsis).  Research performed in a close cousin to cannabis, (Humulus lupulus  or hops) has demonstrated incubation periods up to 49 days with P.macularis. The work described below was performed to test if the Cannabis derived powdery mildew vascularized network is detectable with a youPCR DNA based test prior to conidiospore generation.

Later stage powdery mildew infection and conidiospore generation results in rapid spreading of the fungus to other plants. This tends to emerge and sporulate 2 weeks into flowering thus destroying very mature crop with severe economic consequences.

It is believed to travel in clones and it is not known if it travels in cannabis seeds, however evidence of P.macularis transmission in Hops seeds has been reported. Detection of this pest prior to sporulation can help eradicate powdery mildew before it infests a grow with hearty spores. DNA based tools could facilitate early detection and rapid removal of infected plant materials or screening of incoming clones.

Powdery Mildews often promote other synergistic endophytes after they infect and there are known antagonistic endophytes like Bacillus that some growers use for treatment. If you try to petri dish plate plants with PM, usually another penicillium or mold will grow instead. Without DNA sequencing, you might not know this as the colonies don’t sprout with a taxonomic I.D and visual identification can be misleading.