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, and woody plants. 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 is not advised 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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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.
Fully enclosed indoor grow spaces, 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 should be 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 foundational preventative step against powdery mildew and other contaminants.
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