Powdery and downy mildew can be a big problem for growers of ornamental, edible or medicinal plants. In each case, these pathogens cause economic damage by rendering the plant or produce unsalable. Each pathogen relies heavily on spores released by the fungus to spread to other plants. How your HAF system works may be a cause of spread and climate conditions that lead to infection.
As both pathogens spread primarily through airborne spores, air movement has an impact on transmission. These spores are released from an active infection and land on plant tissue, causing infection in moderate temperatures. Powdery mildew does not require moisture on the leaf while downy mildew spreads best when leaves are damp and humidity is high. Timing of spore release is an issue as well. Powdery mildews produce and release spores throughout the day, while downy mildews produce spores at night. All of these factors should be considered when designing an air movement system and strategy.
Momentum, Not Speed, As a Tool for Moving Air.
Many greenhouse and indoor air movement systems rely on single speed fans running at higher RPMs. Typically, these create air speeds varying from very fast to moderate to minimal. This unevenness is caused by the high speeds of the fan. Near the fan, air speeds are high. However, as the air movement encounters resistance in the form of contrary air patterns, plant density and room structure, these speeds rapidly decline. As most fans rely on a basket or cage to protect the blades, contrary patterns are created by each fan as only the blade angle is responsible for the air direction. As a result, air is thrown in different directions from each fan. This causes turbulence and unevenness as the patterns collide.
With slower-speed fans using directional shrouds, the air moves in one direction and at lower speeds. These lower speeds are subject to less degradation due to resistance factors and the singledirection creates less turbulence. The key with these systems is continual, stable movement that creates directional momentum. This stability ensures that the air movement is even and that the environment is as well. Humidity, temperature and CO2 are all distributed homogeneously throughout the space when air movement is even.
In most cases, slower speed in HAF is achieved by the addition of a speed controller that reduces the RPMs of the fan. Some systems can connect to climate control computers to allow for timing and environmental factors to adjust the speed of the fans throughout the day. Variable speed allows for air speeds to be adjusted based on the crop and the space, ensuring the best result. Fans should be designed for this use and the system should be designed for the space, considering speed regulation. Some slower fans are wound to run at lower RPMs. These fans produce slower air speeds as well, but cannot be adjusted throughout the day or to fit the space and crop.
Reducing Infection Through More Gentle Air Movement
Another advantage of slower moving air is reduced spread of infection. Consider Fall and the seasonal chore of raking leaves. Any grower living in a climate with seasons and deciduous trees knows the fun of collecting leaves. On calm days, leaves fall from the tree, landing on the ground below. On windy days, these leaves are scattered throughout the neighborhood. While this may reduce the amount of work for the Fall raker because their leaves are now in their neighbor’s yard, this illustrates how mildews spread. With wind and turbulence, the leaves are released from the tree more quickly as there is a vigorous force acting on them. The leaves are spread for greater distances, carried on the wind. These leaves are like both powdery and downy mildew spores. They release under more vigorous air movement and are carried by this turbulent, fast air throughout the growing area. Slower air movement causes less spore release and carries spores shorter distances.
In addition to creating an optimal environment, slower fans focusing on momentum help plants transpire evenly by removing the excess humidity and microclimate surrounding the leaves equally throughout the crop. More turbulent fans do this less evenly, causing varying climates and humidity levels throughout the crop. In many instances, areas where air movement is uneven cause pockets of higher humidity levels that can lead to the growth of mildews. More even air movement leads to fewer pockets where these diseases flourish.
A properly designed system with lower-speed, momentum-based fans can help by evening the climate to help reduce mildew growth while not causing the spread that can be seen with high-speed, turbulent fans. Dramm can help create the right air movement for your growing operation. Find your Dramm Technical Representative online to discuss how we can help optimize your air movement or request a fan layout at quote using our online tool.
