Cannabis growers are divided when it comes to powdery mildew, one of the plant’s most notorious pathogens. In one camp, there are those who believe powdery mildew is a systemic pathogen that permeates via the plant’s vascular system. Others believe is it superficial pest that spreads across the plant’s surface.

As it turns out, there is a good reason for the debate. Based on the available literature and the tests we have conducted at Medicinal Genomics, it’s not clear whether powdery mildew is systemic. The science is not settled. But here is what we know so far:

The powdery mildew species that infects cannabis is a different organism than the powdery mildew that infects other plants, such as tomatoes or cucumbers.

We genetically sequenced infected cannabis plants from the United States and Canada and found that the DNA didn’t match any known powdery mildew sequences. Further complicating things is the possibility that there could be more than one powdery mildew species infecting cannabis. We need to collect more samples from Europe and Asia in order to determine that.

There has been no published research into how this novel powdery mildew organism infects the cannabis plant. For now, the best we can do is look to studies on how other powdery mildew species infect other plants for clues. However, until similar studies are conducted on cannabis, we can’t be sure exactly how the pathogen behaves.

Powdery mildew is an obligate biotroph, which is a fancy way of saying it cannot survive without taking nutrients from a host. A 2012 study showed how the Golovinomyces orontii species of powdery mildew (affects Arabidopsis thaliana) does just that. After dyeing an infected leaf, researchers observed that spores penetrated the leaf and the cell wall with a taproot called a haustorium, which serves as its primary feeding mechanism. The pathogen then forms secondary taproots into neighboring cells, creating a mycelium network over a period of several days before producing the tell-tale white powder.

Editor’s Note: The above picture demonstrates the formation of a haustorium and secondary taproots.

It’s important to note that Golovinomyces orontii is not the same organism that infects cannabis. G. orontii infects Arabidopsis thaliana. However, we have designed a test that can detect the unique DNA signature of the powdery mildew species that infects cannabis from a 4mm leaf punch. In fact, our youPCR Powdery Mildew detection assay can detect powdery mildew DNA from cannabis leaves that show no visual signs. This means that we are detecting the DNA from the mycelium network that the pathogen creates prior to sporulation. This early detection can be useful to growers who can remove and quarantine infected plants so they do not further infect the grow room.

If powdery mildew penetrates the plant, the next question is whether it gets into the plant’s vasculature and infects other parts of the plant. There is no published literature that supports that theory, and it is likely the mycelium network is made externally.

Example of sample preparation.

However, we collected four samples from visually clean leaves off of a plant that had visual powdery mildew elsewhere on the plant. Of those four samples, three tested positive for powdery mildew. This does not prove the theory that PM can travel through the plant’s vascular system. It is possible that the other parts of the plant were infected by separate infection events. It may also be that some powdery mildew DNA gets into the plant’s vasculature when the haustorium penetrates the plant. In any case, it is a topic that warrants further study.

Powdery mildew does not affect all cannabis strains and varieties equally. Some varieties of cannabis are resistant, while others are very susceptible to the pathogen. We also believe that powdery mildew resistance is not binary, but additive. In hops, for example, we have seen that there are lineages that are partially resistant.

In any case, powdery mildew resistance is a trait that can be selected for. Using early detection technology, such as the youPCR Powdery Mildew assay, growers can screen plants for infection before visual signs are present. By continuing to screen for powdery mildew infections, and selecting against plants that test positive, breeders can enrich their resistance.

As you can see, there is still much to be learned about powdery mildew on cannabis. As with most plant pathogens, the best defense is prevention. This can be done with strict environmental controls, periodic screenings of grow rooms, and screening of incoming clones. The latter two can be done using the youPCR Point-of-Grow Powdery Mildew assay.

Resources:

1. Want to get in touch with Medicinal Genomics? They can be reached via the following methods:
   1. Website: https://www.medicinalgenomics.com/
   2. Phone: 866-574-3582
   3. Email: info@medicinalgenomics.com

The best growers I’ve met engage in various forms of plant training, such as super-cropping, low-stress training, and pruning/topping to increase their yields and grow bigger buds. In this article, I’ll be going over one of the tried-and-true plant training methods and combinations that can produce yields well over 3 lbs. per 1kw light.

It starts with your rooted clones. Make sure to top them when you begin to see some lateral growth on the lower nodes. Topping at this time promotes lateral growth and branching. This will also cause your cannabis plants to diverge at the top and create two dominant heads instead of one. Creating more tops creates more colas in your canopy, thereby increasing your harvest.

Topping at least one more time, at the right time, will create four dominant heads. This will also give the lower branches time to catch up with the canopy. It takes roughly five to seven weeks for your plants to reach a stage where they are large, round, and bushy with 6-8 tops. Be sure to halt the topping phase 2 or 3 weeks prior to flower. This will ensure your tops have plenty of time to get established as dominant branches. Then, using a low-stress form of training called Screen of Green (ScrOG), we lower a steel grid trellis horizontally onto the plants. You can see a video of us using our Canopy Crawler to achieve a ScrOG here!

The material used for the ScrOG method is important. The steel grid trellis of the Canopy Crawler provides the plants with resistance that they can push off of to promote quick lateral growth. Each node on each branch will begin to receive signals from the plant telling it to grow vertically. Fabric or plastic trellis netting can flex when branches are pushing through, which slows down lateral growth and counteracts the ScrOG method all together. Full plant resistance is the key to generating more bud sites.

Combination of a steel trellis with a net trellis.

Lowering the ScrOG trellis spreads the plants out horizontally. After the screen is lowered, leaves and branches turn towards the light the next day and begin to grow vertically again. While still in the vegetative cycle, over the next 7-14 days the goal is to spread or “stretch” the plants’ branches all the way out to the edges of the screen. Simultaneously, the lateral branching starts to dominate in-between. Branches that reach the edge of the crawler can simply be turned back towards to center without any stress or harm to the plant.

A branch that has been "stretched" to the next square.

No need to worry if any breakage happens during the stretch process. You can simply tie up any broken lateral branches for support and it will continue to grow stress-free. We have heard reports of broken branches healing and coming back strong, so don't write off any broken branches for at least 24 hours. Most of the time you'll see it come back stronger then before.

24 hours after "stretching."

During this stretching process, I recommend defoliating everything beneath the canopy. This includes fan leaves, bud sites, small lateral branches, and anything else that will receive insufficient light for growth. Defoliating prior to the flower cycle is necessary to reduce the prevalence of underdeveloped buds. By defoliating beforehand, you focus nutrients towards the top of the canopy, and improve airflow around the plants.

Once a branch top is in every square of the steel grid trellis at an even height, you’re ready to switch the light cycle to flowering mode. The full and even canopy is key to maximizing your light utility and yield. This is because during the first 2 weeks of the flower cycle, cannabis plants will continue to stretch and grow vertically before transitioning into full bud development. An even distribution of colas improves light penetration throughout the canopy and allows for utilization of every inch of optimal light. What you get is a sea of dense colas and minimization of underdeveloped buds.

Defoliation under the canopy.

ScrOGging does take a little extra work to train the plants, but with the reduced plant count, we find ourselves saving on cloning time, transplantation efforts, water, pots, and grow medium. This makes it worth the effort. You will absolutely notice increased yields per plant with this training technique. Growers we work with see an average 25% increase in their yield when they move to using the steel trellis ScrOG technique. The magic to our method is full plant resistance using a steel trellis to create an even canopy; by utilizing the optimal light in an even canopy, with steel trellises, all of your product contains consistent, high-quality, dense, sellable buds.

Have you ever used the Screen of Green method? Please share your experiences in the comments section below!

Resources:

1. Want to get in touch with Canopy Crawler? They can be reached via the following methods:
   1. Website: http://canopycrawler.com/
   2. Phone: 530-285-0577
   3. Email: contact@canopycrawler.com

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Want to get in touch with Green Mountain Harvest?

You can reach them via the following methods:

1. Phone: 303-981-4273
2. Email: greenmountainharvest@gmail.com
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For the next few months, we intend to publish articles about the effects of humidity on the growing process during the different seasons. We’ll emphasize different options for different growing facilities and how climate is the primary obstacle to the highest quality cannabis. We encourage you to send us any question you may have, and our team will be happy to answer them.

Editor’s Note: If you’d like to reach out to DryGair, feel free to reach them with the contact information listed below.

It is well-known that climate control is essential when growing plants, especially cannabis. Since plants are living, breathing organisms, they adapt to the environment in order to regulate their internal conditions, a process known as homeostasis. The energy and nutrients each plant expends to maintain its homeostasis alters its development and yield

One of the biggest questions that new growers must answer is how to control their environment to maximize plant growth and yield. Nowadays, growers can monitor fine details, such as the humidity and temperatures on plants' leaves and soil. These factors influence growing decisions and yield. A lot of thought and money is invested into the climate controls of a new grow facility.

In our experience, each greenhouse and grow room is different. They can vary by a wide variety of factors, including size, crops, and exterior climate. Additionally, there are many ways to control climate -- from small, localized machines to complicated infrastructure. When choosing appropriate climate controls, growers should carefully consider multiple factors, including:

1. Covered area in greenhouses
2. Grow room size
3. Energy sources
4. Energy efficiency
5. Integrated equipment
6. And more!

DryGair is an Israeli-based company that specializes in humidity control in greenhouses and growing facilities. We provide solutions to the growing requirements of different crops in different countries and climates. Our team has extensive experience in all fields of agriculture, especially the cannabis industry.

Resources:

1. Want to get in touch with DryGair? They can be reached via the following methods:
   1. Website: https://www.drygair.com
   2. Phone: +972-9-7730980
   3. Email: info@drygair.com

In the final installment of our series on measuring horticultural lighting performance, we examine other factors that play a role in horticultural lighting.

If you'd like to read the previous articles, you can find them here:

1. Part One
2. Part Two

The input wattage of horticultural lighting systems is one of the most incorrectly-used metrics to describe the lighting capabilities of a fixture. The input wattage of a fixture doesn’t tell you anything about the fixture’s performance, as two fixtures with same input wattages can, and likely, will have drastically different light outputs (PPF). If a fixture tech spec sheet only shows you the input wattage, then you are still left wondering how many photons that fixture can produce. For example, if a 1000-watt fixture and 500-watt fixture have the same PPF, what would you buy?

Horticultural lighting systems convert electrical energy into light energy (measured in PPF) just like a fan converts electrical energy into kinetic energy. Photon Efficacy (PE) indicates how efficiently a certain fixture can convert input electrical energy into light energy in the form of PAR (µmol/Joule or µmol/s/watt). In addition to driving the luminaires, input electrical energy may also be used to power drivers, fans, and any other moving parts in a fixture. These extra components decrease Photon Efficacy.

LED performance and lifetime is highly dependent on efficient heat management. As a consequence, efficient passive heat-dissipation techniques are superior to active cooling techniques, because active cooling requires electrical energy input, thereby reducing photon efficacy. Additionally, active cooling lowers the reliability of the lighting fixtures due to the nature of extra components and complex designs that increase the likelihood of component failure.

Fixtures with higher photon efficiencies reduce the total energy required for a grow-operation, reducing electrical operating expenses. They also help grow-operations qualify for energy efficiency rebates, depending on the local power company.

The Coefficient of Utilization (CU) is the fraction of photons reaching the canopy from a fixture. CU depends on the distance from the fixture to the canopy, fixture light distribution pattern (beam angle), and fixture form factor. A proper lighting simulation will provide a quantitative measure about the CU. Higher CUs reduce light loss and thus are considered more economical. However, there is no magical beam angle that fits all horticultural lighting applications. Depending on the cultivation setup and structural arrangement, optimum lighting layouts must be designed uniquely to achieve maximum CU.

LED fixtures that have proper thermal management techniques differ from HID fixtures in that HID fixtures radiate significantly more heat. This makes HID fixtures unfavorable for close-proximity lighting, as the plant canopy will experience excessive heating and growers will need to compensate with additional cooling units and fans to accommodate for the extra heat on the canopy. This is why most HID fixtures require significant distance between the fixtures and the plant canopy and are not typically suitable for high-density, vertical farming applications.

A vertical farm of leafy greens and herbs in Virginia.

The ability to modulate the PPF of a fixture is a useful tool for precision agriculture. Properly designed LED lighting fixtures provide the ability to control PPF. Light levels can be altered based on crop growth stage and dimming an LED fixture increases its lifespan.

Safety is another important consideration when it comes horticultural lighting fixtures. Fixtures that are properly protected from power surges, excessive dust, and high humidity can save lives and money. Similarly, avoiding the use of harmful chemicals, such as mercury, protect workers and consumers alike.

A lighting fixture is just one item of the total grow-operation equation. The lighting recommendations should tie in with the environmental conditions, growing techniques and financial goals. The right horticultural lighting system is a partnership between lighting manufacturer and the grower that ensures mutual success.

Before selecting your lighting system, do some homework on your crop and business goals. Thoroughly understand lighting requirements in conjunction with other environmental factors. Review all of the aforementioned lighting metrics to select the proper lighting system that meets your goals. Remember, great lighting also comes with great customer support. Reach out and call as many lighting solution providers as you can to get a feel for their knowledge, support, and willingness to help you succeed.

Resources:

Want to get in touch with Tharindu? He can be reached via the following methods:

1. Email: tharindu@fluence.science
2. Phone: (512) 387-8453

Society is currently experiencing a revolution. Like the industrial revolution and the computer revolution, we are now experiencing the DNA revolution. The secrets to the vast and amazing abilities of living things, and human health, lie in the DNA code. So what is DNA?

DNA is a molecule, made up of 4 building blocks. These building blocks, called monomers, are referred to as nucleotides. DNA is typically found as long chains called chromosomes within the nucleus of eukaryotic cells. Despite the fact that cells cannot be observed without a microscope, each individual cell contains several feet of DNA, depending on the species. If you laid out all of the DNA in your body end-to-end, it would extend to the moon and back, multiple times.

DNA serves as the basic form of data storage required to encode life. DNA has the capacity to store an enormous amount of data, similar to a computer hardrive, but orders of magnitude more efficient. To illustrate: Computer code is binary and has two choices (0 or 1) per position, while DNA has 4 choices per position (A, C, G, or T). For example, if there are 5 positions, each with 4 choices, you end up with 4 x4 x4 x4 x4 = 4⁵ = 1024 potential combinations. A binary code, on the other hand, would only have 2 x2 x2 x2 x2 = 2⁵ = 32 potential combinations. As you add positions, the differences become exponentially greater. In the Cannabis genome there are 820 million positions or base-pairs of DNA (humans have 3.2 billion). The number of possible combinations is truly incredible!

DNA sequences encode for the 20 amino acids that are used as the building blocks for proteins found within cells. Proteins can be thought of as the basic working units of an organism, similar to the workers in a construction company. The specific biochemical properties and combinations of the individual amino acids determine the function of a protein, while the proteins themselves can be shaped differently to make different structures. THCAS (THCA Synthase) and CBDAS (CBDA Synthase) have very similar amino acid sequences (roughly 90% identical), but the difference in amino acids between the two proteins result in different end products.

Editor's Note: Synthases are proteins that create certain molecules. THCA synthase makes THCA, and CBDA synthase makes CBDA.

Pictured Above: DNA codes for different amino acids which can then be combined into proteins.

A gene is most commonly defined as a DNA sequence that codes for a specific protein. E. coli has about 5000 genes, while more complex organisms have 25,000-30,000 genes. For any given gene, there exists some natural variation, which can result in a protein with a different function (or even no function). Thus, genetic differences (genotype) can result in physical differences (phenotype). The DNA sequence also determines the timing of gene expression to coordinate developmental processes, such as flowering time. In layman’s terms, the DNA sequence is like a building blueprint.

Editor’s Note: Geneticists have a variety of terms they use in genetic parlance. A “P” generation stands for the initial parents of a cross. The “F1” generation means “Filial 1”, or the children of the parents. The “F2” generation means “Filial 2”, or the children of the F1 generation. This can be continued indefinitely.

In any given cannabis strain, certain genes have been brought to the forefront through inbreeding. Strains become increasingly stable by inbreeding, decreasing heterozygosity (two different copies of a specific gene). Crossing two strain often results in hybrid vigor, where the child displays the strengths from each strain and an overall increase in vigor for many traits. When F1 hybrids are allowed to progress to the next generation (F2), there is often a loss in the hybrid vigor, thus clonally-propagated F1 plants are desirable for production. However, the loss of vigor in the F2 also comes with increased variation – which can be the starting material for a breeding program and in fact may produce even better production strains. Proper selection in the F2 and subsequent generations, results in gene combinations that can phenotypically exceed the F1 plants.

Legend: Crossing a pest resistant (Normal) parent (DD) x a sensitive (Lesion) parent (dd) produces identical F1 plants (Dd). In the F2 we can identify stable lines (DD), using DNA sequencing, for further breeding. Thus, pest resistance would be FIXED in this strain.

In the F2 plants, the genes from the parents have been shuffled, due to naturally occurring chromosomal rearrangements, resulting in novel genotypes and phenotypes. Stabilizing a genotype/phenotype is accomplished by sibling crosses (inbreeding) in subsequent generations (F3, F4, etc.), with heterozygosity decreasing with each generation.

With next-generation sequencing technologies, billions of DNA sequences are generated every day, and technologies to understand and make use of these sequences are improving rapidly. This information offers endless possibilities for improving human well-being as well as for breeding better plants with desired traits; it’s simply a matter of finding the right genes.

Editor’s Note: Finding the right combination of genes is not creating GMOs, merely classical breeding! Knowing which plants have which genes can lead to a more efficient and effective breeding program.

Resources:

1. Want to get in touch with Marigene Consulting? You can reach them via the following methods:
   1. Website: http://www.marigene.com/
   2. Phone: 970.372.5363
   3. Email: info@marigene.com
2. Want to learn more about genetics? Please let us know in the survey below.