The 8 Biggest Pesticide & Fungicide Mistakes You’re Making

Features - Cultivation

November 1, 2017

Cannabis plant damage caused by insecticide overdose
Photo by Mel Frank

While many cannabis cultivators utilize biological insects throughout their integrated pest management (IPM) programs, they also turn to biological pesticides and fungicides to combat and prevent outbreaks in their cultivation facilities. (In Colorado alone, there are just under 300 approved pesticides and fungicides available to cultivators for legal use within their growing operations, documents on the state government’s website show.) A total of 29 states and the District of Columbia, as well as U.S. territories Guam and Puerto Rico, allow medical or recreational cannabis use. The number of states with approved lists of pesticides and fungicides is growing, and includes: Alaska, California, Nevada, Maine, Ohio, Oregon and Washington. Each have varying lists and guidelines.

But even as pesticides and fungicides become more widely accepted and available to cultivators, mistakes are being made—mistakes that cost cultivators a lot of wasted time and money. Cannabis Business Times spoke to experts, including those who make and supply the pesticides and fungicides you buy, and to cultivators who are learning from their mistakes, to help you avoid these eight common pesticide and fungicide pitfalls.

East Fork Cultivars cultivation in Oregon
Photo by Kim Nguyen,

1. Incorrectly identifying the pest.

You can’t fix a plant problem if you don’t correctly identify it, points out Arianna Taylor, an IPM specialist with Arbico Organics. Yet many growers mistake what they see, she says, then treat the wrong problem with the wrong product. “Consider fungicide,” Taylor says. “Misidentifying a fungus could lead to an extensive treatment [that] will do nothing helpful. Although the treatment itself may not have adverse effects, you have now sprayed the plant multiple times and added a heavy amount of moisture, water, time and money. Once you realize it is not working, you start the discovery process again, but the problem has persisted and is now uncontrollable, so plants must be pulled. Because of lack of proper identification, they have become a loss.”

Sometimes, you might need help to identify the pest or problem. Joseph Rosenberg, cultivation director of 7 Points Oregon, admits this had at one point been a problem for him. The company contacted an entomologist and later, a microbiologist, who have come in to help pinpoint the real issue and develop the right treatment plan. “We followed that up with regular lab tests to verify the results of the treatment that matched our visual monitoring of the plants,” Rosenberg says.

Photo by Kim Nguyen,

2. Playing mixologist with pesticides.

Applying multiple products to your plants won’t make those treatments more effective, warns Wally Daniel, M.S., director of production at HiFi Farms, located near Portland, Ore. Instead, mixing treatments could render each treatment useless—or worse, hurt your plants. “In the case of biological or microbiological pesticides, some labels explicitly state if they can be mixed with other pesticides or nutritional inputs,” he says, and yet a surprising number of growers still mix and match them. “If the label says don’t mix them with other products, don’t do it.”

3. Using pesticides curatively.

Medicine isn’t meant to cure a cold before you have it—you pop pills once the sneezing starts. But when it comes to sick plants, pesticides are meant to work the opposite way, says Matthew Krause, Ph.D., product development manager for BioWorks. “Almost all biological pesticides are preventive,” he says, meaning they should be applied before pathogens or insect pests reach levels that can cause disease or injury. “When the first biopesticides were launched over 40 years ago, growers applied them as they had curative chemical pesticides, only to find that the biologicals were not very effective. However, as information about preventive use of biopesticides continued to expand, growers increased their successes with biopesticides in organic and conventional growing systems.”

4. Spraying too much (or too little).

When you spot a problem or pest, you want it gone—now. So, “a lot of times when people have bad problems, they use the highest rate [of spray],” says Matt Andrus, technical specialist with Arborjet. “Sometimes that kind of heavy application can work, but it also damages the plants. And if you’re spraying the plants and damaging them, then you yourself have become the pest. [Instead,] let’s pay attention to the pest we’re after and what strength is needed.”

Even if you don’t damage your plants by over-applying pesticides or fungicides, your plants (and pests) could become immune to the treatment, warns Arbico’s Taylor. “Overuse of some products could increase pest insects with stronger resistance to certain insecticides,” she says. “More is not always better. Right product, right time and proper application are going to increase the effectiveness of your controls.”

Editor’s Note: For more on resistance to biological pesticides, click here to read October 2017's Hort How-To column on biological control by clicking here.

Photo by Kim Nguyen,

5. Not doing a patch test before spraying everything.

“It’s vital to … test what you’re applying to make certain it actually works,” says Aaron Howard, co-founder and COO of East Fork Cultivars in Oregon’s Illinois Valley. “This seems like a no-brainer, but you’d be surprised how many growers broadly apply pesticides without testing their efficacy.” Trial tests are something Howard does at East Fork Cultivars to “test [a pesticide’s] effectiveness against the targeted pest, and whether it’s phototoxic and phytotoxic, then monitoring for efficacy so we’re not applying things that don’t work,” he says.

6. Treating symptoms, not the cause.

You can’t treat what you can’t see, right? Wrong, says Taylor. “We make decisions on treatment based on what we see in the immediate [present], and usually that treatment involves what is visible,” says Taylor. “Unfortunately, this is usually treating symptoms rather than finding the cause of the issue.” In fact, some healthy-appearing plants can still have issues. “And when those issues arise, we need to get to the root of the problem,” she says. “Take molds: When we see them, they have usually begun to grow and spread throughout foliage, so we grab a fungicide labeled for the specific species of fungus, and we spray infected areas. But where is that mold coming from? Where are those spores living so that they are able to continuously infect our plants? We choose products without considering a holistic approach to treatment,” and that’s not always best, she says.

Photo by Kim Nguyen,

7. Not storing products correctly.

You can’t store many bioinsecticides or biofungicides the same way you might store a chemical product, warns Krause. “Growers are used to storing chemicals in the greenhouse or a room or a shed—they have no real reason to be concerned with temperature swings,” he says. “But when you’re dealing with any living biopesticide product, you’re working with a living organism that can be sensitive to heat and unstable temperatures, much like your plants, pets, and you. And it’s the same for the biopesticide products. There are some microbial biopesticides that are stable at very wide temperature ranges, but there are others that have much narrower temperature ranges in which they can be stored. Hence, it is very important for the grower to check the label of the product for proper storage conditions.”

8. Not applying the product correctly.

How tough can it be to spray a plant with a pesticide or fungicide? It’s actually not as easy as you may think. “When you’re spraying on a plant, coverage is crucial,” says Andrus. “You can’t miss a spot.” Why? “A lot of times, organic pesticides work on contact, [which means] you have to spray the creatures that you’re trying to disrupt,” he explains. And if you don’t hit all the pests, they will continue to thrive. “Take mites as an example: They’re known for being tiny, and they can hide a lot of different places,” says Andrus. “Specifically, you could have 50 adult spotted spider mites and 50 of their eggs within a 1-centimeter by 1-centimeter area. So, say you miss a few square inches. Two weeks later, you swear there are more mites than before you sprayed. You can literally spray 98 percent of your plant and still have a problem because of that 2 percent.”

Jillian Kramer is a New York City-based freelance journalist.