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Grow Diagnosing plant disorders and pests


Former member
There are few topics discussed more thoroughly on the internet than cannabis cultivation. Sadly, every grow room sometimes becomes an infirmary.


This is one of the first pair of full fan leaves at the second internode at two weeks. Based on web searches, suspect light burn is beginning to bleach leaves.

Turned off COB half of lamp, leaving 150 W conventional LEDs at 35 inches. (Hoped this would be high enough to use COBs early.) Also put lamp on 18 hour timer and installed circulation fan above canopy to dissipate heat.

Important to monitor environment following changes, as the COBs raise heat and lower humidity. Wouldn't want mold spores to germinate.

When the plan recovers, will try turning COBs back on, or wait until flowering, depending on speed of growth under conventional LEDs only.
What makes you think it's a light issue rather than a nutrient deficiency?

Good question! In this case, mainly experience. Happened previously when first adding COB LEDs. (Warned against using them this early by manufacturer, but wanted to try them at a greater distance.) Also, matches images on the web. At the first sign of problems, just do an image search.

Unlikely to be a nutrient deficiency. It's early in the plant's life. Looks most like a Cal-Mg deficiency, but already corrected this previously by supplementing bottled nutes. Could also be pH. If the pH is out of range, the plant won't be able to absorb nutes. Go a little lower on next feeding, just to be careful.


Whether you’re looking into expanding your crop or you’re entering the growing industry for the first time, it’s essential to know how to get the best yield. One of the most devastating things that can happen to your marijuana fields is an infestation from common cannabis pests. These insects and other creatures can quickly decimate your plants and harm your yield if you don’t take steps for cannabis pest control.

Originally posted on 2 February 2016, Updated on 2 May 2020

Many of these insects will eat the plant leaves or roots, taking the nutrients away from the plant so that it dies. Plants that do not die can have significant reduction of their yield or quality of cannabis buds produced. However, it isn't difficult to identify the symptoms and prevent the common cannabis pests before they destroy your grow. In this section, we will look at the most common cannabis pests and and show you exactly what they are, how to spot them, and how to prevent and control the pests.


Spider mites aren’t spiders, but they are related to spiders as part of the mite family. These mites can be difficult to eradicate. Their tiny mouths have sharp teeth that pierce plant cells and suck out the matter inside. They are very tiny and look like tiny black dots on the underside of the leaves. You’ll also know you have spider mites when yellow, white, or orange spots begin to show up on the top of the leaves. If the infestation isn’t caught in the beginning stages, a webbing will begin to appear between leaves.

Spider mites are less common when using hydroponics but can affect your plants no matter where they are grown. Spider mites must be killed, and Azamax is a common cannabis pest control killer.



Aphids often resemble tiny flies with wings and pierce through leaf matter to suck out the contents inside. Aphids also live in colonies and cluster together on the leaves causing significant damage quickly. They also leave a sticky residue behind, which attracts sooty mold and turn leaves black and unsafe to consume.They can be dark or pale in multiple colors, and are found on the underside of the leaves. Most aphids that attack marijuana plants are green and leaf-shaped.

Aphids can be sprayed off of the plants using a power sprayer. Heavily infested plants may need to have leaves and stems pruned. Growers can also use insecticidal soap to kill aphid colonies. Another option is to use Neem oil extract to keep the insects away.


Fungus gnats are a nuisance to the soil cannabis plants grow in, and the larva will damage plant roots as they eat through the biological materials in the soil. They can be common indoors and out.

Fungus gnats are dark brown or black and are usually found buzzing about the soil of marijuana plants as their larva grows in the dirt. They often begin to appear when the soil has been too saturated for too long. Fungus gnats are only 2mm long but are easy to spot as they fly.

To prevent fungus gnats, you must focus on water management and not overwater the soil. Sticky cards can be used to attract and kill many of the gnats or use Neem oil, which prevents many types of cannabis pests. Diatomaceous earth is a powder that can be applied to the soil to improve soil health as well.


Whiteflies eat the cannabis leaves and suck the nutrients from the veins of the leaves. They look like small moths that fly around cannabis plants and hang out on the underside of leaves. They also lay white eggs that stick to the leaves and are hard to remove.

Insecticidal soaps can be sprayed onto the plants for cannabis pest control. Neem oil can also be used for prevention, but care must be taken not to spray the Neem oil on the buds as the taste will become bitter. There are also other organic pesticides such as Spinosad that can be used to eradicate cannabis pests.


Thrips bite the marijuana leaves and suck out the nutrients leaving behind shiny metallic-looking spots. The leaves will eventually lose enough nutrients that they begin to die.

They are tiny insects that move fast and look like pale worms as larvae but can have dark or gold wings as adults. The insects are often seen on the leaves, but growers will also see the shiny spots left behind on the front side of the leaves.

Like many cannabis pests, insecticidal soaps, and organic cannabis pest killers can be effective at treating and getting rid of thrips. Neem oil can be sprayed on the plants to prevent thrips from returning as the oil is bitter and aversive to the insects.


Caterpillars crawl across cannabis leaves and munch their way right through, leaving large holes in the leaves. Caterpillar poop can be seen on leaves, and the caterpillars themselves are easy to spot and often significant. Some of them are brightly colored, hairy, and can be fast-moving. They are most commonly seen in outdoor operations.

Caterpillars can be eradicated with a biological insecticide that kills the larva and starves out the adult caterpillars without harming other insects such as pollinators. Caterpillar sprays can also help with other cannabis pest control.


Leaf miners live inside of the leaves and mine tunnels as they eat away the inner plant material. They are not a specific insect but refer to the larva of any of the insects that can make their way inside of the leaves, including flies, beetles, and moths. They leave behind long, discolored trails that appear on the leaves.

To be rid of leaf miners, growers must remove the affected leaves since the bugs are living inside. Spinosad products can also be used to kill the insects as they eat the leaves. Neem oil and caterpillar spray are natural ways of preventing and treating leaf miners.


Snails and slugs eat the entire leaf of the cannabis plants, killing the leaves. They leave behind a trail of slime and eat big chunks from the leaves of marijuana plants. Holes in the leaves will also have scalloped edges and can be confused with caterpillar damage. They are most likely to come out at night when temperatures are cooler and are most common in crops grown outdoors.
Getting rid of slugs and snails is hard, but one thing growers can do is allow frogs, beetles, and toads to remain in their outdoor fields as they eat slugs and snails. Physical barriers using eggshells, lime, or diatomaceous earth can also prevent slugs and snails from reaching the plants.
The best way to control your cannabis pests is to prevent them from happening in the first place. If you are growing your plants indoors, make sure your grow room is completely sealed as most of these pests come from outside. You should also focus on quality of your grow operations. Grow from seeds, not clones, whenever possible and if you do use clone seeds, you should thoroughly check for pests before planting. Growers should also buy high-quality soil and check the soil for signs of pests before using it. Growers can also sterilize the soil before use.
One of the best ways to prevent pests organically, and also helps with soil quality is to plant more than just marijuana. Companion plants such as basil and garlic can be planted around your marijuana crops to help with cannabis pest control. Many cannabis pests do not like the smell or taste of basil or garlic and these plants make useful boundaries to keep insects away. You can also introduce natural predators in your grow spaces such as toads, frogs, ladybugs, and birds. These animals eat insects and can often rid your crops of more insects than sticky pads can if you are growing organically.
Nearly all cannabis pests are most common when marijuana is being grown outdoors. Most of the preventive methods used in cannabis pest control are very useful outdoors, including all of the pest control sprays, though they are effective indoors as well. Sprays such as insecticides are also very fast-acting and often the best way to completely rid pests from your fields.
The best way to prevent pests from decimating your cannabis plants, however, is to grow indoors and keep your buildings sealed against pests whenever possible.

Hop latent viroid continues to spread across Canada

A disease that severely damages cannabis plants is rapidly spreading across Canada.​

The hop latent viroid (HLVd) started emerging on cannabis crops in California as early as 2017 before first being noticed in British Columbia a few years later.

Once isolated to only a few whispers and rumours, it has now spread across Canada, infecting as many as 40% of licensed growers in the country, says Brian Coutts, a Strategy & Business Development Manager at A&L Labs. A&L is one of the few labs in Canada offering testing for the viroid.

“It’s out there more than anyone thinks,” says Coutts. “It was about two years ago now we had our first positive test, and it was in BC. Since then, it’s clear across Canada in every province.”

Coutts says by his estimate, he figures about 40% of cannabis growers in Canada currently have or have had to deal with HLVd, causing major crop losses for big and small scale growers alike.

The viroid both weakens the plant overall, creating smaller, more brittle plants and it affects cannabinoid and terpene production.

“Everyone is chasing that THC number, as they think that’s what the consumers want,” says Coutts, “and the number one problem with the viroid is that it can bring down those THC levels.”

“It’s out there more than anyone thinks. It was about two years ago now we had our first positive test, and it was in BC. Since then, it’s clear across Canada, in every province.”
One cannabis grower in Canada who has been vocal about his experience with the viroid is Benjamin Padovani, the owner of Treez Botanicals, a micro cultivator in Ontario. Padovani had to recently destroy all his plants, including clones in veg and flower rooms, as well as his mother plants and his entire genetic library, after discovering hop latent viroid.

Although he’s not 100% sure how he picked it up, Padovani says he suspects it was from a batch of clones he acquired prior to licensing. Because Health Canada allows growers to bring in an unlimited amount of starting materials in the form of clones, mother plants, seeds, and tissue culture (basically anything but flowering plants) prior to licensing as part of a one-time transfer, many applicants try to gather as many genetics as possible before they get their licence.

The value of this approach is having a robust and unique genetics library that can help a new grower stand out in the market. But with few controls in place and growers not always testing everything they bring into their facility, the risk of diseases like hop latent viroid or powdery mildew, as well as pests like thrips or aphids, can be a risk.


Issues with powdery mildew are well known in the industry, with—some growers hiding it and others being open about their experience in managing and eliminating it, a process that entails, like with the hop latent viroid and other similar issues, destroying all infected plant matter and thoroughly cleaning the facility.

“I had my genetics for years and never had this issue,” explains Padovani. “But because Health Canada only lets you bring in genetics the one time, I thought that I would do one last push to bring in some more before I was licensed, and I think it came in that way”

“I never bothered testing them and wasn’t careful because you never think it will happen to you. But it happened.”

“I noticed maybe two or three of my plants in one room looked like they had totally different characteristics than all the other plants of the same variety. Then in my second room, I noticed about 30% were completely showing signs of the virus. Then I noticed it was in my mothers, too.”

Padovani then looked at the plants under a microscope and saw a much lower level of trichomes than the uninfected plants, as well as much less of a smell.

Once he realized what it was, he then took samples and sent them to a lab for testing that confirmed his fears. When the results came back positive, he then quickly made the decision to just destroy all his plants and began working to clean his facility as thoroughly as possible, while also calling around to find a new supply of genetics so he can begin growing again.

“Once I did more research, I realized I was never going to get rid of it until I just torched everything. Now I’m cleaning everything in the facility and working on getting genetics from someone to start over.”

Coutts, at A&L, says the best approach to avoid bringing HLVd into their facility is making sure they buy genetics from a reputable supplier, testing everything that comes into the facility, and having working practices on site that will help prevent any infected plants that might still come through from passing anything on to other plants.

“You can pass it on with tools, by clothing. If you’re in one room and you’re brushing by the plant with your lab coat and then go into another room and touch other plants, you’ve spread it. It’s that easy. Clean your tools, use separate tools for each room, and leave enough room in your grow rooms so plants aren’t too crowded.”

He also says purchasing genetics that started as tissue culture is an important step.

“If you love the genetics that you’ve been growing, if they’ve been good to you and you love the THC content and the flower you’re producing, the only way to clean it up is tissue culture.”

One option for saving infected genetics is through Dr. Adel Zarei, the cannabis micropropagation lab manager at Safari Flower in Ontario. Zarei has spent the last few years developing a process using tissue culture to create new, HLVd-free plants from samples from infected plants.

Dr. Zarei says he has had success with several clients who had the viroid and wanted to save certain cultivars. The process can take around nine months and requires a small sample of the regenerative material from either the apical meristem or a tip of new root.

Following detection of any infection, he says the first step is to send it to a lab to confirm the presence of the viroid. Then, if the grower wants to save any specific varieties, they should take several samples and send them to a lab like Safari, before destroying the rest of their plant material and thoroughly cleaning their facility.

Then, once the facility is ready for new plants, always ensure you’re starting with something you can guarantee is disease-free.

“The last pass to eliminate or control this viroid is starting with a clean plant. Having a certified, virus-free plant would be a good choice to start with. “

Zarei says he started looking at propagation methods that could create clean plants from samples with viroid about two years ago when reports of it emerging in Canada were just becoming known.

Leaning on his experience working with the viroid in hop plants more than a decade ago, he says he’d like to see the industry focus more on research into what varieties of cannabis may be more resistant to the hop latent viroid. He points to similar research showing that some varieties of hops are more resistant to the viroid than others, speculating something similar is likely possible with cannabis.

Cannabis breeders need to take into account the strength of the plant in general, not just THC or other cannabinoids and terpenes, explains Zarei, but also specifically looking at resistance to the hop latent viroid and other known diseases that plague commercial cannabis production.

Back at Treez Botanicals, Padovani says he hopes more producers will be more willing to be open about this disease because it’s becoming so prevalent.

“I think it’s pretty much everywhere now, but it’s just not being talked about at all,” he continues. “That’s why I’ve been very open about it. I’ve posted on my social media because I think all the big guys have it, everyone has it, but they just don’t want to say it. I believe we’ll still persevere. I’m taking aggressive action now so that it hopefully won’t affect us in the near future. I believe we’ll bounce back, but it’s not easy.”

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