Spray Coverage Key To Uniform Pest And Disease Control In The Greenhouse

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Greenhouse growers often are heard complaining that they aren’t getting uniform control of an insect or disease, or that a plant growth regulator has been erratic. Michigan State University Extension says in many of these cases, the problem isn’t the pesticide or plant growth regulator; it turns out that the material was not applied uniformly or didn’t get to where the problem was located.

Apply From Several Directions

Penetration into the leaf canopy of greenhouse crops can be a major headache. Unless there is very good air movement, when plants are widely spaced or the canopy hasn’t yet filled in, foggers, air blast sprayers, smokes or spray applications that don’t flip over leaves will deposit material mostly on the top surface of the upper leaves; relatively little penetrates deep into the canopy or to the underside of the lowest leaves. Changing the spray droplet does increase canopy penetration, but the leaves must still be flipped over by the force of the spray for optimal coverage of the lower surfaces. Applications must be made from several directions to hit leaves shielded by other leaves or other plants. This type of application method will greatly improve insect and disease control in crops with dense canopies.

Check Nozzles For Correct Pattern And Droplet Size

Droplet size is also key to coverage of the plant surface and penetration into unfolding leaves and other hiding places. The smaller the average droplet size, the greater the coverage and penetration. Droplet size is directly dependent upon the type of nozzle, the nozzle size and the pump pressure. Generally speaking, the higher the pump pressure, the greater the proportion of small droplets in the spray pattern. Growers need to check that the nozzles they’re using are of the correct pattern and size, aren’t worn and that the pump is providing the necessary pressure. Many growers change nozzles every year to ensure good coverage. For good coverage of plant leaves, hollow cone nozzles are much better than flat fan nozzles. Also, watch for dripping nozzles because that means they are plugged.

Drench With The Right Volume Of Solution Per Container

Uniform application is also dependent upon the amount of the spray or drench used. Drenching with too small a volume of solution per container means that part of the media might not be saturated and any roots in this area won’t take up the material. On the other hand, applying too much solution can cause overdosing and phytotoxicity. Applying ultra-concentrated materials is convenient because the application equipment is light and can be easily moved, but applying very small amounts of materials uniformly over large areas is tricky.

The bottom line is that no material will work as well as it could unless it is applied uniformly and to the proper area. Don’t let the pressure of everything else that must be done this season cause you to take shortcuts in pesticide and growth regulator applications; you are really wasting valuable time and may put the crop at risk.

Read the full article, “Spray coverage is key for effective treatment results in greenhouse.”

This article was published by Michigan State University Extension. Visit the Michigan State University Extension website for more information on related topics.

Thomas Dudek (dudek@msu.edu) is the district Extension horticulture and marketing educator for Michigan State University Extension.

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