MSU Extension

The following information should be useful to greenhouse growers as we wind down the spring season .I have noticed some disease and insect problems that were attributed to poor coverage by the non-uniform application of pesticides.

Greenhouse growers often complain that they aren’t getting uniform control of an insect or disease or that a growth regulator has been erratic; some plants affected, others not at all or very little. In many cases, the problem isn’t the pesticide or plant growth regulator; the material was not applied uniformly or didn’t get to where the problem was located.

While many pesticides will move from the top of a leaf to the bottom, or from a leaf into the stem, it’s unrealistic to expect materials to move from one side of the plant to the other, from one leaf to another or from one plant to another. But that’s what must happen if foliar sprays or drenches aren’t uniformly applied. PGR’s that contain paclobutrazol and uniconazole-P don’t move out of leaves but the site of action is the stem and meristem. Avid an insecticide-miticide moves from one surface of the leaf to the other but not from leaf to leaf. Marathon II another insecticide moves up in the plant, not down. The control material must come into contact with the pest, or in the case of a plant growth regulators, the growing sites, before anything good can happen.

Penetration into the leaf canopy 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 deposit material mostly on the top surface of the upper leaves; relatively little chemical penetrates deep into the canopy or to the underside of the lowest leaves. Charging the spray droplet ( i.e., electrostatic sprayers) 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.

Droplet size is the 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 dependent upon the type of nozzle, the nozzle size and the pump pressure. Generally, the higher the pump pressure, the greater the proportion of small droplets in the spray. 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.

Very small droplets tend to float in the air rather than fall out onto 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 an “orchard 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 disease or insect attack.

Uniform application is also dependent upon the amount of the spray or drench used. Drenching with too small a volume of solution 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 pesticide or PGR 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 spring cause you to take shortcuts in pesticide and growth regulator applications; you are really wasting valuable time and may put the crop at risk. Consider checking your sprayer nozzles this summer and replace them if they are worn in the least bit.