Ensure plants receive thorough chemical coverage

Complaints are often made by greenhouse growers about the lack of uniform control of an insect or disease or that the effect of a growth regulator has been erratic. Tom Dudek at Mich. St. Univ. Extension said in many cases, it’s not the product that’s causing the problem. The material was not applied uniformly or didn’t reach the area on the plants where the insect or disease was located.
Penetration into the leaf canopy can be difficult at times. Unless there is 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 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.
The smaller the average droplet size, the greater the coverage and penetration. Generally, the higher the pump pressure, the greater the proportion of small spray droplets. Check that the nozzles being used 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.
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 growing medium might not be saturated and some roots may not take up the material. Applying too much solution can cause overdosing and phytotoxicity.