and why should you prevent it?
Powdery mildew is a fungal disease affecting a wide range of plants. It typically appears first on plant leaves but can spread to all parts if left unchallenged. It is one of the most common diseases for nursery plants such as flowers, vegetables, woody plants and cannabis. Species of powdery mildew are specialized to infect only plants in one genus or one family. This means infection will not spread to other species of plants in other plant families. For example, one species of powdery mildew infects Calibrachoa, Verbena, petunia and cucurbits. This means growing squash and cucumber in the same greenhouse as the aforementioned flower species since cross contamination can occur in an outbreak.
Powdery mildew can infect plants under a wide range of conditions. This is one of the aspects that makes it so challenging to combat. Temperature, relative humidity, light and air circulation are all prominent environmental factors influencing the possibility and prominence of powdery mildew outbreaks. It tends to thrive in shaded, warm and slightly drier climates than other mildews and will spread rapidly when there is high humidity.
Powdery mildew is easily recognizable as circular, powdery white spots. It is most often noticed on the top of leaves, but it can grow on the underside as well. Early symptoms can greatly vary and include irregular patches of chlorosis (lacking chlorophyll), purplish red or brown areas, and/or lesions that will then develop the trademark powdery appearance when it enters the pore production stage. These pre-pore production patches are often confused with nutritional deficiencies, leaf spot diseases or spray injury. Younger foliage is more susceptible to powdery mildew, but it can easily infect mature leaves as well. As it progresses, the white spots will spread to cover most of the leaves or other affected areas. If left unaddressed, it will turn leaves yellow and dry them out.
Once powdery mildew becomes active, it will leach away a plant’s nutrients, making the plant weaker and the blooms less, reducing overall yields. In extreme cases, it can kill plants. There is still debate as to whether or not powdery mildew is systemic. What is known is that it has the ability to reproduce both sexually and asexually. Under the correct conditions (slightly different for each powdery mildew species), spores mature and are released to spread the infection. Powdery chains develop to distribute the spores, which become airborne by the gentlest of air flow. The spores are unique from other mildews in that they do not need moisture on the plant surface in order to penetrate and infect a plant. Once powdery mildew becomes active it spreads quickly via air drafts or currents, easily contaminating large swathes of a crop or entire rooms when growing indoors or in greenhouses. Spores can also be transferred via people, animals and insects.
Powdery mildew is most often controlled and remediated using chemicals and bio organic methods. Fungicides are employed at the first sign of powdery mildew and generally used until plants are harvested. Non-chemical approaches include applications of mixtures containing milk, bicarbonates, heavy metals or minerals (such as copper and sulfur) and oils (such as neem and tea tree). The eradication of powdery mildew is most likely if caught very early. It is difficult if not impossible to eradicate existing, established colonies. Again, the biggest challenge with powdery mildew lies in the fact that once it is easily noticed, it has already established itself and started to spread spores.
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Powdery mildew has garnered a lot of attention in the industrial hemp and cannabis industry. For the purposes of this discussion, we will simply label both industrial hemp and cannabis as ‘cannabis’. Industrial hemp and cannabis are essentially the same plant in the Cannabis genus with hemp being defined as less than 0.3% THC and cannabis defined as greater than 0.3% THC. The species of powdery mildew that infect the cannabis family of plants are different organisms that those that affect other common indoor crops, such as tomatoes, cucumbers or lettuce. What isn’t known is how many different species there are that affect cannabis. A 2017 study genetically sequenced infected cannabis plants from the U.S. and Canada. It found that the DNA did not match any known powdery mildew sequences to date. Needless to say, there is still much to learn about powdery mildew, especially in the cannabis industry.
Powdery mildew is problematic not only because it can weaken the productivity of a cannabis plant, but if it spreads to the buds, they cannot be sold. With the unique consumption methods of cannabis, consumer safety is a huge concern. Some people believe contaminated buds can be salvaged for use in concentrates, such as live resin or CO2 oil. But there is not a clear consensus regarding the safety of these products when derived from infected buds. Ethanol extraction and CO2 are currently thought to be the best way to rid extraction products of powdery mildew. More testing is needed to confirm this and consumers are advised to look for dispensaries that list product testing results.
Powdery mildew can become active at any time given the right environment, including harvest or the drying process. Cannabis is generally dried and cured slowly to bring out the best qualities of the bud. This means the plants will spend a good deal of time open to potential contamination. Cannabis is generally best dried slowly at lower temperatures. During the first 72 hours, water loss from the plants is rapid, increasing humidity in the air. Keeping the area clean is imperative, as many spores germinate in moist environments with temperature ranges between 50-70 degrees Fahrenheit. Controlling temperature, humidity and airflow during drying and curing is just as important as it is during the different stages of growing cannabis.
Industrial hemp crops are treated differently than the previously mentioned cannabis depending on the end use. Most hemp crops destined for CBD extraction are rapidly dried in hours rather than days or weeks. In this drying process, heat and lots of airflow are introduced. Contamination and detectable levels of powdery mildew spores aren’t usually an immediate problem at this stage due to low humidity and dilution of spores. However, problems usually arise during the long term storage of the dried hemp while waiting for processing. At this stage moisture intrusion and absorption can contribute to the growth of the previously spread spores. Most harvests occur around October and depending on processor availability, could sit through the damp winters and subsequent warming spring seasons which create ripe conditions for crop loss.
Strategic solutions for short or long term storage of industrial hemp are to ensure storage rooms are properly sealed from outdoor environments, provide chemical- or refrigerant-based dehumidification and provide active air filtration and circulation.
With an eye towards compliance of both domestic and international Good Manufacturing Practice (GMP), the integrity of the harvest and subsequent storage are cornerstones that ensure a safe transfer of product to the processor or downstream integration into the food chain. It is the responsibility of growers, processors and integrators to ensure the quality of both raw and finished product and to follow ISO 9000/9001 quality manufacturing procedures for acceptance into both domestic and international markets.
Many plants are more susceptible to powdery mildew in greenhouse and indoor settings. Greenhouses often have ideal conditions for powdery mildew: dense canopies, restricted/uneven airflow, conducive temperatures and ideal relative humidity for spore germination. Greenhouses are also more susceptible to outdoor contaminants and environmental factors, especially in humid locations. Measures such as opening vents (a contaminant highway when not filtered), running fans, and using dehumidifiers or A.C. units are all tactics that can be used to help maintain an optimal environment, not only for growing your plants, but also for preventing powdery mildew.
Indoor grow rooms, while less susceptible to outside climate and contamination factors, still face distinct challenges in managing powdery mildew, especially if certain aspects are not initially set up correctly. The bigger the indoor operation, the more important awareness becomes regarding the challenges of acquiring and maintaining uniform temperature and humidity in the entire space.
Vapor Pressure Deficit (VPD) is a concept every greenhouse and indoor grower is typically aware of. It is a combination of relative humidity and temperature in a single value and a more accurate way to predict plant transpiration and water loss than just relative humidity alone. VPD is the difference between how much moisture is in the air and how much moisture the air could potentially hold when saturated. A high VPD (usually measured in pounds per square inch or kilopascal) means the air is still capable of holding a large amount of water. A low VPD means the air is closer to saturation. When it comes to growing, the VPD within plants versus the VPD in the air matters. There is a large gradient between plants that are nearly saturated with water and the air, which is what enables plants to transpire and dry out. For starts and younger plants, a lower VPD is generally better as it will reduce the drying of young plants (as well as cut down on the misting and watering required). For mature plants and finishing, a higher VPD is recommended as denser plants will be able to transpire and cool themselves. Just like different plant species have a range of different needs to optimize their growth, different powdery mildew species thrive in differing conditions. This means the VPD will need to be maintained differently depending on what you are growing. (Of note are tomatoes. When grown with too low of a VPD, the plants become stressed and causes the fruit to crack).
In outdoor grows and field crops, plants respond naturally to the variable VPD as long as the plant or strain is appropriate for climate zone. It is thought that this makes the plant healthier and more resistant to powdery mildew and other pests. Indoor environments will better serve their crop if they are designed to synthesize these outdoor variable VPD conditions.
The two primary reasons plants need air is to make food (photosynthesize) and breathe. Just like people, plants will begin to suffer in contaminant filled air. Dust, dirt and debris can accumulate on a plants leaves, reducing gas exchange and blocking light. Other contaminants are not visible to the naked eye but just as harmful and more challenging to combat. Powdery mildew, with spores 1-3 microns in size (unseen by the naked eye), exists as one of the most formidable airborne foes for indoor growing environments. Maintaining clean air is a strong preventative step against powdery mildew and other contaminants.
Contact us for more information about taking preventative steps against powdery mildew and maintaining clean air environments for your plants!