MSU Extension

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 – some plants affected, while others not at all or very little. Michigan State University Extension states that 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.

While many products will move from the top of a leaf to the bottom or from a leaf into the stem, it's unrealistic to expect these materials to move from one side of the plant to the other, from one leaf to another or from one plant to another. However, that's what must happen if foliar sprays or drenches aren't uniformly applied. Plant growth regulators like paclobutrazol (Bonzi, Piccolo, Paczol) and uniconazole (Sumagic) don't move out of leaves, but the site of action is the stem and meristem of the plant. Avid, an insecticide, moves from one surface of the leaf to the other, but not from leaf to leaf. Imidicloprid (Marathon, Mallet), another insecticide, moves up into the plant from roots, but not down from treated leaves. The control material you are applying must come into contact with the pest, or in the case of growth regulators, the growing sites, before anything good can occur.

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. Charging 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 in order 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.

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 you'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 insure 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.

Very small droplets tend to float in the air rather than fall out on to the leaf surface and don't move long distances. Larger droplets fall out readily and can be sprayed long distances, but don't cover the surface as well. While using a "hand gun" and spraying only from the center isle may be convenient for you, the plants on the outer edges of the houses are only being reached by very large droplets, so a great deal of the surface is wide open to pest attack.

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.