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Pathogens & Pests

Proper Identification & Solutions
By Zac Ricciardi & Ben Owens

Pathogens will be the next blindsided infestation for the cannabis industry. Years ago, people were finding Root Aphids and Russet Mites and had no approach to mitigate the situation, they thought that they just had lots of fungus gnats or a nutrient deficiency and weren’t correctly identifying the problem. Furthermore, because of that first misstep, the rest of what they did was a waste of money, which is why it is crucial to educate ourselves as growers on the symptoms of common issues and to be willing to ask for help identifying unknown problems before then become complete infestations.

If you’re chasing a nutrient deficiency but the problem is russet mites, you’ll never make it better and the nitrogen you’re feeding will end up feeding the mite population thus making it worse because they now have a regular food source and no impingement of their existence. Proper identification is huge for growers; being able to properly identify the pathogen/pest that they are dealing with will give them the best approach for handling issues.

Any time anyone is doing a large scale planting, anything bigger than a personal garden, I really recommend soil analysis and a water test. This can help identify any of the pathogens, viruses or bacteria that could be present in that media/environment . Unfortunately, there are significant costs associated with these tests that may make them inaccessible to all growers, which is why we wanted to break down some common pathogens and pests, and provide some advice on how to deal with issues as they arise. Don’t forget: in order for a pathogen or pest to take over a plant or grow, all three conditions of the Pathogen Triangle (Pressure, Environment, Host) must be met.

COMMON CANNABIS PATHOGENS

Fusarium

Fusarium is a very persistent wilt disease that can decimate entire crops furiously. Fusarium is commonly introduced as a water-borne pathogen and has been found in Colorado city water sources. It isn’t systemic but once a host catches it, all of the clones from that plant will be prone to symptoms, especially if feeding with untreated water. The only options are to start from seed or tissue culture.

Fusarium is a pathogen whose telltale signs really don’t present themselves until later in flower, often indicated by buds not developing as they should. You’ll notice that the lower buds are bigger and mature faster than the top buds and you’ll start to see leaves crisp up from the outside in; The part that is connected to the stem will look somewhat healthy and remain turgid but the outer leaf will begin to die. Because fusarium symptoms take a while to exhibit themselves it makes it hard to identify in younger plants which may look healthy in the beginning of the cycle.

Treatment:

1.0-1.2ml/gallon Zerotol 2.0 treatment for all feeds. This will PREVENT the pathogen from taking hold via the water column.

Note: Reverse Osmosis (RO) systems alone will not filter out this pathogen.

Alternaria

Similar to Fusarium and popping up in Colorado water sources, alternaria is a pathogen that causes chlorosis and necrosis, inducing black spotting on the leaves of your plants. The telltale sign is a healthy leaf at the base of the stalk where the petiole comes out, but the further you move away, the more unhealthy the leaf gets. Unlike fusarium, alternaria is systemic and while you can treat and mitigate its propensity in genetics, “it’s a bandaid on a bullet wound.” Alternaria can be introduced through water, effective plant material, and insect pressure (aphids, thrips, or piercing/sucking insects). Making sure you have a proper IPM rotation in place will prevent the insect vector.

Treatment:

1.0-1.2ml/gallon Zerotol 2.0 treatment for all feeds. This is will PREVENT the pathogen from taking hold. 

Note: Reverse Osmosis (RO) systems alone will not filter out this pathogen.

Botrytis

Botrytis, commonly known as “Bud Rot”, relies heavily on a conducive environment and a susceptible host. Chunkier, dense genetics whose structure is stockier are the most susceptible to this pathogen as moisture builds up inside the buds and becomes localized to that environment. Anaerobic environments with high moisture pockets are the most conducive to botrytis.

Treatment:

Increase airflow utilizing old school techniques like a bent pop-sicle stick placed inside the cola to prop open the bud structure. This is what we used to do in Santa Cruz to save multiple crops as a non-chemical approach to warding this pathogen off. Alternatively, you can utilize Zerotol 2.0 at a 1% solution (37ml/gallon with non-ionic surfectant like Natural Wet) as a sanitizer; Botrytis grows from the inside out, so if you can use this sanitizer on the stem, any spores in there are addressed. Then, follow up with pop-sicle stick.

Powdery Mildew (PM)

Powdery mildew is something everyone fears “introducing” into their grow but it is something that is all around us right now; we live in a conducive environment to things like mildews, which is why a cold snap in the fall or winter can cause rampant PM on your crops. Similarly, warm days and cold nights can cause PM to pop up overnight in your cannabis grows. Making sure that you maintain consistent temperatures and relative humidity that doesn’t drastically fluctuate will prevent moisture from forming and mildew from growing. 

PM is like a dandelion in structure: it has a root system and flower on top. When you see mildew on top of leaf, we are seeing the symptom not the cause. The cause is that the root system (hyphae) are already present.

Treatment:

There are two ways to remove PM: physical removal of tissue OR utilizing OxiPhos in conjunction with Zerotol 2.0 to induce a systemic response (ISR) that targets the hyphae of PM. Oxiphos allows a plant to be on its own defensive, mitigating the root network of PM, changing the environment on the surface of the leaf so that it is inhospitable and unviable for PM’s root system. 

You can also use chemigation and/or foliar treatments. If you are seeing it in Veg or right before flip, spray the plants as the peroxide will sanitize what you are seeing. If you are seeing it in flower, earlier than Week 6, I’d still spray it as Zerotol breaks down into oxygen and water. If you see PM on a leaf or cola you are removing, spray first and then remove to avoid spreading spores during removal. 

Pythium

This pathogen is most commonly referred to as “root rot” and often comes from a conducive environment that is high in moisture and temperatures. 

Plants will exhibit chlorosis and lack of vigor. Almost appear sluggish or droopy. Roots will be brown and slimy. 

Treatment: 

Utilizing Zerotol 2.0 as a drench at 1:100-1:250 depending on pressure levels will eradicate this pathogen. Proper dry downs and no standing water will also help prevent pythium from becoming an issue.

COMMON CANNABIS PESTS

Thrips

Thrips are a pain. If you see damage to the leaf tissue where it looks shiny in appearance or looks like portions of the leaf have been “scraped” off, you likely have thrips. Thrips themselves hang out on the tops and veins of leaves. They range in color from yellowish to whitish to brownish and look very similar to a splinter or a small speck of coco fiber. They are very tiny and very voracious. 

Thrips have two parts to their life-cycle. The first takes place as they develop and live in the soil. The second takes place once they have migrated up the plant to the tops and leaves. 

Treatment:

When treating for thrips, it is important that you treat both the soil and the canopy using “sprenching” (spray the canopy, drench the substrate). If you only spray the canopy, your problem will never go away completely. Utilizing Azaguard in conjunction with Bioceres is a great rotation to get rid of the pest.

Fungus Gnats + Root Aphids

Fungus gnats are most commonly seen right after you water your plants. They primarily live in the top layer of the soil, and feeds stir them up until the water level normalizes in your substrate. Fungus gnats are often confused with Root Aphids. Fungus Gnats have a very defined abdomen and thorax, and their wingspans are proportional to their bodies.

Fungus Gnat

Whereas fungus gnats are more of an annoyance rather than a detriment to the plant itself, root aphids cause damage to your plants’ feeder roots, impacting their ability to uptake vital nutrients. The telltale sign of root aphids is if you see the edges of your leaves starting to curl (in addition to noticing flying bugs). Root Aphids are very bulbous; you can see where their chest ends and their wings are longer than their bodies by ~30%.

Root Aphid

Root aphids and Fungus Gnats have two adult life stages: one as crawlers and one as flyers. In their first adult stage, root aphids don’t fly (fungus gnats do). If you see flyers, they have been there a long time; Root Aphids only fly when they have been exposed to a pesticide or continue to bump into so many other bugs. Flyers are indicative of a larger problem, and hopefully proactive measures prevent your issue from getting this out of hand.

Treatment:

Use a combination of Diatomaceous Earth (DE) sprinkled over the top of your substrate and foliar sprays, alternating organic pesticides like Green Cleaner, Bioceres WP, and Azaguard. The substrate treatment will kill and prevent additional larvae from being introduced and the foliar sprays will kill the existing adult population on contact. If you are already in flower, use DE to remove new generations from your equations and work to avoid overwatering. You can hang sticky traps to capture adults but be sure to regularly change these out as they are more for monitoring than curing your issue. Doing a “sprench” application will ensure that you cover any surface the pests may be dwelling. 

Spider Mites

These pests are easy to get rid of. They live on the underside of your leaves and tent to cause the top of the leaf tissue to look speckled, almost like it was shot with rock salt. By the time you see these dots, you have a moderate infestation, and are days away from webbing. Spider Mites vary in color from brown to reddish brown to yellow and their gestation period is three days.

Treatment:

A 3-day rotation of Azaguard, insecticidal soap, and Green Cleaner will keep up with gestation period and attack them at every lifecycle. Mycoinsecticides like Bioceres and Venerate have shown promise in rotation with these other chemistries against spider mites.

Russet Mites

These bugs aren’t fast moving but proliferate themselves rapidly and are a huge pain in the ass. They are invisible to the naked eye which makes them hard to scout for. I recommend taking a plastic baggie with you around in the garden and placing problem leaves that need to be scoped in the protective sleeve as to not spread issues.

Symptoms are lack of vigor, chlorosis, and “tacoing” of your leaves. New growth appears yellowish/brown. 

Treatment:

Utilizing azaguard in rotation with insecticidal soaps and sulfur is a great way to kill russets. Make sure you’re getting full coverage to ensure a complete contact kill. 

Aphids

Aphids are a huge problem for outdoor and indoor growers alike. They produce rapidly (some are born pregnant) and can destroy a crop quickly! They range in color from light green to yellow to white. They are easily scouted for and hang out on every part of the plant. Sticky monitor cards are a must for aphid control.

Treatment:

A rotation of Azaguard, Bioceres WP, and insecticidal soap or pyrethrin product of your choosing works very well. Consistent sprays until control is achieved is recommended. Changing out the yellow cards bi-weekly is also recommended.

FIXING ISSUES IN THE GROW: NOTHING IS UNFIXABLE

Nothing is unfixable. There is a certain amount of effort that goes into IPM for it to be done correctly, but diligence is often not what is lacking, rather, understanding of what to look for. With the above information, hopefully you’ll be better suited to correctly identify an issue and treat it accordingly. 

A Few Tips:

Read through both Cleanliness 101 and Cleanliness 102: IPM pieces on this site. They contain a wealth of good practices that can prevent issues before they arise.

Use Zerotol or Sanidate to sterilize your tools.

Sanidate is completely food safe, a standard that many facilities are already moving towards, and the entire industry will need to implement once national oversight and legalization rolls out. In commercial applications, Sanidate is recommended for cleaning all hard surfaces (anything that isn’t a sensor or soft metal like brass) as it can kill everything from COVID to E. coli. 

Zerotol is the Corvette version of H2O2, a completely stable combination of Hydrogen Peroxide and Vinegar that breaks down into inert ingredients. (oxygen and water). ZeroTol can be used in almost all applications: sterilization, root drenching, foliar sprays, and sprenching. Diligence portion is what may be lacking in this case.

Wash anything that is dirty or has visible organic matter, nutrients, spills, or other residuals. I recommend using a combination of a soap/Green Cleaner alkaline cleaner and Zerotol/sanidate  for a follow up sterilization.

ZeroTol can be used to kill water-borne pathogens before they become a problem. ZeroTol is safe to use in this way at a 1.0-1.2ml/gallon preventative rate.

Using products (oxiphos) that create ISR in your plants will lessen their susceptibility to issues. 

Communication is key if multiple people are involved in your grow.

I recommend using Irrigation Flags for pest management to coordinate what treatments have been undergone to avoid repeating treatments and over-treating, and ensuring that team members know which plant had what. If you are in there doing plant work, and you notice an infected plant, you can color code what you’re seeing; if blue means spider mite, stick it in there, and then your homie who comes in after knows that this plant is a problem plant and knows to focus on it. 

Sticky Traps and Pest Monitors

These tools work great when used correct. By definition, these are monitor cards. They are gridded to allow you to easily get a sense of the changes in your pest population at a glance. Traps can help you evaluate progress as you target soft body insects like thrips, aphids, and gnats. But don’t get comfortable using these tools as immobile pesticides, as this can create a conducive environment for other problems, as plenty of contaminants can adhere to the dead bodies on your traps. 

Sticky Trap Monitors work to your benefit when used correctly and changed regularly. Typically, we would change them every 21 days even if clean, or, after a week, if there were too many to count, then we’d change it out. When it is no longer an effective tool for quickly counting, swap it out. If everything else is going well, changing traps weekly or biweekly will mitigate additional risks.

While this is a general guide to common pathogens and pests, there are plenty of others that can cause problems if preventative measures aren’t taken. As issues present themselves, refer back to this guide and the other relevant information on cleanliness and IPM to try to tackle your problem correctly from the start. If you notice multiple symptoms that could all indicate different deficiencies, consider doing a general root treatment to tackle any number of potential problems in your substrate; If your roots are unhealthy, the plant is trying to be susceptible to outside threats. 

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Don’t shy away from academic sources as far as symptoms and diagnosis. A significant amount of the stuff I have learned has been from legitimate universities and studies. There are quite a few places around the world where hemp is legal; look at what their universities are doing, look at studies on hops (one of the closest related plants to cannabis). Hops are legal everywhere and are susceptible to the same pests that hemp and outdoor cannabis would be as well. See what is effective for treating the issues you are experiencing hops and mirror what they are doing.