Just like people, CBD hemp can get sick from bacteria, fungi (mold), viruses, and parasites. Unlike people, when disease strikes, there are a limited number of treatment options available. As a result, becoming infected with one of these pathogens quite often means the death of the plant. Worst yet and in many cases, the infection rapidly spreads through out the entire crop wiping most of it out in as little as two weeks. Yikes!
We thought checking the soil pH and watching for all the signs and symptoms of nutrient deficiency was challenging enough – see Nutrient Problems & Symptoms and Leaf Symptoms and Nutrient Deficiencies. Nope. Being vigilant about preventing insects and microbes from getting a foothold on the plants is even more important. Nutrient deficiencies can often be corrected whereas infection by a pathogen is often lethal – see Common Hemp Diseases and Identify Plant Problems.
We are in no way experts when it comes to diagnosing and treating microbial and pest infestations. Still, our general takeaway after reading about the various types of pests and diseases is that when the temperature isn’t too cold, 70-85 degrees F is ideal, the soil isn’t too wet, and there is good air movement, risks are significantly lessened. Certainly when the seedlings are in the greenhouse, controlling these variables is fairly easy to do. When they’re in the field, our drip irrigation system along with moderate Wisconsin summertime temperatures will help.
In addition, having transitioned to organic many years ago, what we’ve learned is that our crops are healthier; they’re less prone to disease and bug influxes. Contrary to what proponents of conventional farming would have you believe, spraying with chemical herbicides and pesticides along with applying chemically produced nutrients does not produce plants that are either as robust or full of nutrients as those grown using deeply organic practices. Now this may be considered a controversial statement to some, as currently the studies to prove out this fact simply haven’t been done, but the antidotal evidence is overwhelming.
Who doesn’t know someone that is wheat intolerant? Who hasn’t heard of someone that has been cured of various maladies by eliminating processed foods and eating organic? In terms of conventionally grown foods and the various chemicals that are used, one chemical that is now ubiquitous to modern farming is RoundUp. The EPA recently released usage statistics showing hundreds of millions of pounds are used yearly.
Could all the RoundUp (glyphosate) that is sprayed on the fields to prevent an inundation of weeds (weed flourish in depleted soils) harm health? Yes, yes, we know Monsanto/Bayer would have us all believe their products are not only safe but produce equally healthy plants. Our experience and the experience of others say otherwise.
Specifically, our crops are eaten with prejudice by the indigenous fauna; critters instinctively prefer our organic fields over the chemically treated crops next door. Furthermore, our crops are more disease and drought resistant than conventional fields nearby. The sum of the parts does indeed constitute the whole (entourage) and food derived from chemicals falls short of food grown organically in healthy soils. Some day the science will catch up to this reality. And this may be soon given the recent 2 billion dollar award to a couple and a 289 million dollar award to a man in court cases that linked cancer to RoundUp.
Types of Diseases & Pests
As noted above in the links to Common Hemp Diseases and Identify Plant Problems, the list of illnesses and bugs that can harm hemp is fairly long and many can cause a lot of damage. Given this, we quickly concluded that we needed to come up with a list of proactive measures to keep insects and disease away; we needed an Integrative Pest Management (IPM) plan – see Organic Integrated Pest Management and GrowersNetwork Integrated Pest Management.
The first item in our IPM plan was selecting hemp plants with genetics suited to our climate. We are growing CBD hemp strains from Oregon CBD. The strains were selected in part because they do well in our Wisconsin climate.
The second item in our IPM plan is using deeply organic practices wherein every input, into the fields and onto the plants, is not only free of chemicals but done in a way to increase microbial communities that support plant and soil vitality. Unlike indoor grow operations that fear the introduction of microbes, to the extent that workers change clothes before entering grow rooms, we’re using natural ways to promote good microbial populations. We don’t want to try to keep microbes out; we just want to encourage the good ones to hang around using truly organic compost and compost teas. We want a “system” that comes closest to mimicking nature at its best with the belief that this produces product with the widest and most potent benefit – a product with the fullest entourage effect.
Besides promoting beneficial microbes, a third part of our IPM plan is getting out into the fields daily and looking at the plants using a handheld microscope. Bugs and fungi are incredibly small so we will be using a 60x microscope. As part of this process, we need to come to understand these plants, to develop a feel for them. We need to know them well enough to be able to walk a field and notice subtle difference in the overall presentation of a plant that indicates disease or infestation invisible to the naked eye.
On top of these IPM strategies, we’re also adding the judicious and regular application of organic deterrents using a spray rig along with having a handful of other biological remedies on hand. After a bit of research, our sense is that the two biggest threats are powdery mildew (mold/fungi) and mites (insect). Given this, we’ve developed an IPM plan that includes foliar sprays (on the leaves) to keep them away. As an added benefit, our plan to keep these two threats at bay also prevents a host of other harmful microbes and insects from destroying the plants. Below is our IPM strategy for powdery mildew (PM) and mites.
Powdery Mildew
The microscopic spores that grow into mold are everywhere. Scattered by the wind, they lie in wait until the conditions are right to grow into mold that looks like white fuzz. This is a natural process. Mold is nature’s way of breaking down dead and weakened plant matter into compost for the next generation of plants to feed on. We don’t mind mold except when it decides to devour our plants before harvest.
In terms of powdery mildew (PM), the super tiny spores blown in by the wind land on hemp plants and begin to grow when the temperature drops between 50-60°F after a rain followed by warmer temperatures between 70-85°F for the next few days. The UC Davis Gubler-Thomas Powdery Mildew Model developed for California grapes gives us a sense for the conditions that lead to a PM epidemic in CBD hemp and is summarized below. Once the PM begins to grow, within a week the fungi (mold) is producing millions of new spores that increase the risk of infection and spread of PM. While still in the greenhouse, carefully controlled temperature spikes can be used to prevent PM but once in the field, it’s up to sprays on the plant (foliar) to prevent and control PM.
Note: We’re assuming the model for PM on grapes applies to hemp but aren’t sure of this. At a minimum, the grape model gives us a sense for the conditions that lead to powdery mildew. Powdery Mildew Risk Assessment Model for Grapes
The plants must first be infected before several days of warmer temperatures create the right conditions for PM to flourish. To be infected, approximately 1/10th of an inch of rainfall is followed by at least 8-12 hours of leaf wetness at temperatures between 50-60°F. Once infected, a running total using a scoring system is kept that indicates the likelihood of an outbreak of PM. The running total is only started once the conditions for an infection have been met (wet and cool). Powdery Mildew Scoring System
- Add 20 Points: for each consecutive day where there are six consecutive hours of temperatures between 70-85°F
- Subtract 10 Points: when the temperature does not meet the requirement
- Subtract 10 Points: when the temperature exceeds 95°F for at least 15 minutes
- 0-100 Limit: the running total can never be less than zero or greater than 100
Powdery Mildew Risk Assessment
- 0-30 Points: PM is not spreading and preventative measures may be curtailed
- 40-50 Points: risk for a PM epidemic is moderate; PM is reproducing every 15 days
- 60-100 Points: risk for a PM epidemic is high; PM is reproducing every 5 days
Preventing Greenhouse Powdery Mildew
From our reading of the UC Davis Gubler-Thomas Powdery Mildew Model, one way to prevent powdery mildew in the greenhouse is prevent the temperature from ever dropping into the 50-60°F range. If we keep our greenhouse warmer (upper 70’s and above), then even though we’re watering frequently and the humidity is higher, there is little risk of a PM outbreak.
Maintaining warmer temperatures also prevent the relative humidity (RH) from dropping below the dew point resulting in droplets of water condensing on surfaces including the plants and increasing the risk of other molds from spreading – see Dew Point Calculator. For a point of reference, in Bud Rot: How to Identify and Prevent this and Powdery Mildew or Cannabis Mould, the recommend a relative humidity is 40% – 70% during the vegetative stage and 40% – 50% in the flowering stage. When the RH is below 40%, the risk of a PM outbreak, or another mold, is negligible.
Obviously, keeping the greenhouse temperatures warmer is the best approach to PM. However, should the temperature dip in the greenhouse resulting in the plants becoming infected, by allowing the temperature to fluctuate such that there isn’t 6 consecutive hours of temperatures between 70-85°F would prevent the infection from getting a foothold. Likewise, carefully allowing the temperature to spike to 100°F would knock out some of the PM population. As we see it, preventing an outbreak after infection by controlling temperature wouldn’t be easy to do without advanced temperature controls. And this brings us to other methods.
Preventing & Treating Powdery Mildew
In general, preventing mold growth starts with cultivating healthy plants through good nutrient and pH management. Avoid northern slopes and shady areas. Align rows with the prevailing wind for good air circulation. Use plastic mulch along with removing interior leaves and weak branches to minimize high humidity and moisture conditions. Remove organic mulch, straw, or random vegetable matter from around the bottom of the plants. Moldy bud, or moldy portions of plants, should be cut out immediately.
Wait to plant in fields until temperatures warm and humidity is moderate. Ideal relative humidity during vegetation is 40% – 70%, drops to 40% – 50% during early and mid flowering, and less than 40% during late flowering. Sensi Seeds writes that temperatures above 80F or below 65F without fluctuation help to create “ideal breeding grounds for bacteria”. Bacteria and PM lead the way to gray mold (bud rot).
Begin spraying at the start of a wet spell as fungicides are better at preventing rather than killing mold. Avoid spraying in the evening. Furthermore, the greater the number of applications of any given fungicide, the greater the risk of the mold developing a resistance to that fungicide. Fungicides at low risk of resistance development include sulfur, copper, phosphorous acid products, horticultural oil, potassium salts, and hydrogen dioxide.
Powdery Mildew Treatments
Compost Tea
- nutrients in tea feed plant making it stronger
- use a balanced tea with beneficial fungi and bacteria
- beneficial microbes in the tea fend off with harmful pathogens
Potassium Bicarbonate
- kills and prevents downy and powdery mildew, scab, late blight, and other fungi
- increases the pH plant surfaces (alkaline) limiting mold spore germination
- always use a “spreader-sticker” agent like dish soap to ensure even and lasting distribution – also see OroBoost or use Kinetic when plants are stressed
- agitate vigorously and often
- Ed Rosenthal recommends an ounce of potassium bicarbonate in a gallon of water and 1.5 cups of milk. Note: Since milk contains proteins and sugars, the risk for grey mold/bud rot (Botrytis) is increased. Only use milk in vegetative stage.
- home grower recommend 1 tbls potassium bicarbonate, 2 tbls vegetable oil, and 1 tsp organic dish soap (Dr. Bronner) per gallon water
- baking soda (sodium bicarbonate) works similarly but is only a preventative
- eHow recommends 4 teaspoons of baking soda and 1 tablespoon of horticulture oil per gallon of water
- apply every other day during infestation then at lower rate every 7-10 days
- $8/pound
Horticultural Oil
- controls mites, leafhopper, whiteflies, mealybugs, powdery mildew, scale, alternaria, rust, various viruses, etc.
- eradicates powdery mildew — most fungicides only prevent infection
- virtually no risk of developing resistance
- OK to use on flowers
- do not apply in damp conditions or in temperatures above 80F
- never apply oil and sulfur within 2 weeks of each other
- highly refined petroleum oil prevents phytotoxicity – dead spots, stunted growth, etc
- Note: Neem oils like Trilogy are plant based oils and can be useful as a preventative
- JMS Stylet recommends 1:100 to 1:200 (one gallon in 100 or 200 gallons of water)
- Bonide (not OMRI) recommends 2.5-5 tablespoons/gallon (one or two gallons in 100 gallons of water) every 7 days as needed
- $21/gallon JMS Stylet or $27/gallon Bonide All Seasons
Sulfur
- controls powdery mildew, brown rot, scab, blossom blight, leaf spot, red spider, and various mites
- more effective at preventing powdery mildew than treating an infection
- virtually no risk of developing resistance
- sulfur can be hard on the epidural layer – strong concentrations damage plants
- do not apply during flowering – affects taste
- to prevent leaf burn, do not apply when the temperature is above 90F, or will be soon
- never apply sulfur within 2 weeks of horticultural oil
- use a “micronized” spray on plants under 12”
- keep tank agitated to avoid bleaching
- apply sulfur in the morning before beneficial bugs are out
- Yellow Jacket recommends between 2-10 pounds per 100 gallons of water applied every 7-14 days (1-5 tablespoonfuls per gallon)
- Southern Ag recommends 4 pounds per 100 gallons applied every 4 to 7 days when powdery mildew appears and continued as long as disease threatens (2 tablespoonfuls per gallon)
- $1.60/pound
Actinovate AG
- consists of Streptomyces lydicus bacteria
- controls powdery mildew, downy mildew, greasy spot, Botrytis, Sclerotinia, Monilinia, Alternaria, Erwinia and bacterial spot
- mentioned by Oregon Sungrown Growers Guild
- Novozymes recommends 6–12 ounces dissolved in 50–100 gallons of water applied to foliage and blossoms every 7 to 14 days
- $102/pound ($6.40/ounce)
Serenade
- consists of QST 713 strain of Bacillus subtilis bacteria
- controls foliar diseases such as powdery mildew, downy mildew, greasy spot, Botrytis, Sclerotinia, Monilinia, Alternaria, Erwinia and bacterial spot
- do not let mixed solution stand for long periods or sit overnight
- keep tank agitated
- mentioned by Oregon Sungrown Growers Guild
- AgraQuest recommends 4 ounces (1/2 cup) of concentrate per gallon of water – 4 fluid ounces of concentrate per 1,000 sqft.
- $2.90/4 fluid ounces of concentrate
ZeroTol 2.0
- consists of hydrogen dioxide H2O2 (aka. hydrogen peroxide)
- controls alternaia, black spot, powdery mildew, fusarium, leaf spot, blights, brown spot, etc.
- algaecide/bactericide/fungicide used as a foliar spray and to clean greenhouse materials and tools
- mix 1:100 for curative and 1:300 as a preventative every 5-14 days
- $68/gallon
Preventing & Treating Mites
The new growth on infected plants will be paler, stunted, and malformed with stems that are half the normal thickness. Buds will be shriveled with brown pistils. Leaves may be yellow and might fold into a taco shape. Damage starts at the bottom and moves upward. An infestation can occur within 1-2 weeks and can wipe out 1/3rd of a crop. When infestation is rapid, use “adult” predators – see Hemp Russet Mite. (Image: Mites)
At a minimum, you’ll need a 14X “jeweler’s lens” (60X handheld microscope is better) to inspect the underside of leaves, the center of curled leaves, stem junctures, or the middle of flowers. They look like a small worm with 4 legs on the front. Besides seeing adults, look for translucent egg sacs on the bottom of leaves.
An outbreak may be thwarted if all infected plant matter is carefully removed in plastic bags within the first week before egg laying starts. Once infestation spreads up the plant, it’s almost impossible to save the plant. Don’t hesitate to discard entire plants. Simply touching a healthy plant after coming into contact with an infected plant can spread mites. Use a 1:10 bleach-to-water solution to clean tools and wash clothing.
Preventatively, clear away debris and vegetation from around plants. Mites like hot, dry, and dusty conditions so using a foliar spray helps. Avoid over fertilizing as mites love the excessive green growth. Once infected, use several treatments in succession without waiting to see intermediate results. To minimize the risk of mites becoming immune to any given treatment, use a variety of products.
In the USA, Russet Mites have been reported in California, Colorado, Oregon, and Washington. Mites do not survive the winter in freezing climates.
Mite Treatments
Horticultural Oil
- controls mites, leafhopper, whiteflies, mealybugs, powdery mildew, scale, alternaria, rust, various viruses, etc.
- suffocates insects so virtually no risk of developing resistance
- OK to use on flowers
- do not apply in damp conditions or in temperatures above 80F
- never apply oil and sulfur within 2 weeks of each other
- highly refined oil prevents phytotoxicity – dead spots, stunted growth, etc
- JMS Stylet recommends 1:100 to 1:200 (one gallon in 100 or 200 gallons of water)
- Bonide (not OMRI) recommends 2.5-5 tablespoons/gallon (one or two gallons in 100 gallons of water) every 7 days as needed
- $21/gallon JMS Stylet or $27/gallon Bonide All Seasons
Sulfur
- controls powdery mildew, brown rot, scab, blossom blight, leaf spot, red spider, and various mites
- more effective at preventing mites than treating an infestation
- some mites may develop a resistance to sulfur
- sulfur can be hard on the epidural layer – strong concentrations damage plants
- do not apply during flowering – affects taste
- to prevent leaf burn, do not apply when the temperature is above 90F, or will be soon
- never apply sulfur within 2 weeks of horticultural oil
- use a “micronized” spray on plants under 12”
- keep tank agitated to avoid bleaching
- apply sulfur in the morning before beneficial bugs are out
- Yellow Jacket recommends between 2-10 pounds per 100 gallons of water applied every 7-14 days (1-5 tablespoonfuls per gallon)
- Southern Ag recommends 4 pounds per 100 gallons applied every 4 to 7 days when powdery mildew appears and continued as long as disease threatens (2 tablespoonfuls per gallon)
- $1.60/pound
Insecticidal Soap
- controls aphids, mites, plant bugs, leaf-hoppers, earwigs, mealybugs, psyllids, scale insects, tent caterpillars, etc.
- fatty acid in soap suffocates insects – soap must coat insect
- spray small area to test for phytotoxicity – dead spots, stunted growth, etc
- OK to use on flowers
- not absorbed by plant
- more for prevention than infestations
- never apply sulfur within 3 days of soap
- do not apply when the temperature is above 90F
- oil acts as sticker-spreader
- apply when conditions keep soap from evaporating quickly – ineffective once dry
- hard water in excess of 300 ppm should be conditioned
- Certis USA recommends 2:100 (two gallons in 100 gallons of water) at 75 to 200 gal/acre
- 410 Magazine DIY recipe: 2 cups of oil mixed with 4 cups of dish soap in 100 gallons water
- Garden Betty DIY recipe: 6 cups of oil mixed with 15 cups of dish soap (Bronners) in 100 gallons water every 7 days (every 4 days for infestation)
- add 2 cups vinegar to 100 gallon DIY recipe for powdery mildew
- $32/gallon (DES-X)
PFR-97 20% WDG
- consists of the naturally occurring fungus Isaria Fumosorosea Apopka Strain 97
- controls whitefly, aphid, thrips, and mites
- relative humidity must be 40-50%
- apply in early morning
- frequent applications at a lower concentration are better than fewer strong doses
- mix in 2 gallons of water for 20-30 minutes before diluting in spray tank
- apply in rotation with Naturalis L and Grandevo – Kill Hemp Russet Mites & Broad Mites on Cannabis
- Certis recommends 14-28 ounces per 100 gallons applied every 7 days (3-10 days depending on infestation)
- $29/pound ($25/14 ounces)
Naturalis L
- consists of the fungus Beauveria Bassiana
- controls aphids, whitefly, mites, thrips and some flies
- works differently than PFR-97 so to prevent resistance, apply 4-5 days after PRF-97
- control starts after 3-5 days – does not “knock down” pests immediately
- does not harm bumblebees and other pollinators
- apply in rotation with PRF-97 and Grandevo – Kill Hemp Russet Mites & Broad Mites on Cannabis
- not readily available in USA – see BotaniGard 22 WP
- BioWorks recommends 2 pounds/100 gallons for mites every 5-10 days
- $74/pound
Grandevo
- consists of the heat-killed bacteria Chromobacterium subtsugae and its proteins
- insects get a lethal “tummy ache” when they eat the protein
- can be applied in any humidity or sun conditions
- 2 tablespoons per gallon – mix instructions
- make sure to use a sticker/spreader like OroBoost or Kinetic
- very gentle on beneficial insects as they don’t eat the plants
- apply in rotation with PRF-97 and Naturalis L – Kill Hemp Russet Mites & Broad Mites on Cannabis
- Marrone Bio recommends 3 pounds/100 gallons/acre for mites every 5-7 days – takes 2-3 applications before decline starts
- $53/pound
Venerate XC
- consists of the heat-killed bacteria Burkholderia spp. A396 and its proteins
- controls mites, aphids, leafhopper, lygus, stink bug, thrips, etc.
- targets the larval life stage of insects
- does not hurt predators
- Marrone Bio Instructions recommend 4 tablespoons per gallon (1.5 gallons/100 gallons water)
- $82/gallon
PyGanic
- consists of pyrethrins – an insecticide derived from chrysanthemums
- rapid knockdown and killing of insects
- toxic to bees
- use once or twice first to knock down 80% followed by other controls
- pyrethrins are neurotoxic but are “relatively safe” for humans and animals
- higher doses in people can cause headaches, tremors, convulsion…
- not recommended for flowering plants
- safest product when you need immediate results
- use sparingly as insects can quickly develop resistance
- do not apply more than 10 times per season or within 3 days except under extreme pest pressure
- breaks down within 48 hours from UV light
- MGK recommends 4.5 to 17 fluid ounces in sufficient water for thorough coverage per acre
- $193/gallon ($25/17 ounces)
Alternatives
- Frequent Watering – mites breath through there skin so can be controlled by spraying the plant 3-10 times a day
- Protective Plants – marigolds, sunflowers, petunias, basil, rosemary, and zinnias keep insects away
- Insect Frass – mix 1 tablespoons in 1 gallon of water then let settle for 30 minutes; spray every two days to kill eggs (especially spider mites)
- Diatomaceious Earth – will not stop an infestation but will slow the spread of the mites
- Isopropyl Alcohol – kills adult mites on contact. As it evaporates quickly, plants tolerate it well. Mix equal part of alcohol with water and use as a spot spray
- Compost Tea – in Get Rid of Broad Mites, Jorge Cervantes discusses the use of aerated activated compost tea (AACT) to prevent infestation and recommends an Beauveria bassiana strain GHA insecticide like BotaniGard.
Predators
The use of “beneficials” (predator bugs) that eat harmful insects can be used as part of an IPM plan. This is especially true during the flowering phase as the use of some sprays is not advised as the spray affect flavor. Typically, “juvenile” predators are used as a preventative by hanging satchels of them about 8”-10” off the ground. They disperse slowly over a month and work there way up the plant. For a full-blown infestation, “adult” predators are used that are very hungry and capable of consuming pests right away. Note: Around us, Asian Ladybugs that are used on soybean fields are everywhere and should provide some protection for our hemp as they eat aphids, spider mites, scale insects, whiteflies, leaf beetle larvae, some insect eggs, and small caterpillars. (Image: Ladybug)
Predators often eat not only the harmful adult insects but their larvae too thereby thwarting further outbreaks. And unlike many sprays, pests are unable to develop a resistance to beneficial predators. It’s a good idea to find an insectary you can work with prior to the start of the season as we’ve heard predators often don’t survive shipping.