Proper nozzle setup and sprayer calibration are keys for a good product distribution in the canopy. Many diseases such as soybean rust have their initial development in the lower portions of the plant, which is proven to be very difficult for the spray to access. A limited access to the spray may translate in poor disease control. In general during a spray application, a decrease in spray deposition is verified from the top to the bottom of the plant. Droplets exiting the nozzles find several obstacles in their way that sometimes make their mission somewhat impossible: to deposit the fungicide equally in all parts of the plant. In this article we will examine ways to guarantee good coverage during fungicide applications.

Let’s begin by examining the hydraulic nozzle. The nozzle is responsible for the breakup of the liquid into droplets (a process called atomization) and for their dispersion into the environment. This breakup is achieved by forcing the liquid through an orifice. This metered orifice will provide a constant flow of the liquid according to the pressure being exerted. The size of the droplets and spray pattern will be a function of the type of nozzle (flat fan, air induced, hollow cone) and angle of spray (80°, 110°). No single nozzle can create droplets of a unique size. The spray will always be formed by droplets of several sizes. Since droplets can be very small, the units used in their measurement are microns (μm). To exemplify how small these units are, is enough to say that one inch is equal to 25,400 μm. 

But if the spray exiting the nozzle is formed by droplets of different sizes, how do we express the droplet size that a particular nozzle/pressure combination is producing? We calculate the median volume diameter (VMD), a number expressing that 50% of the spray volume is contained in droplets higher than that and the other 50% of volume in droplets smaller than that. Often in nozzle manufacturers' catalogs we find information such as this “nozzle A has a VMD of 335 μm at 40 PSI of pressure.”  This information tells us that 50% of the volume is contained in droplets which are larger than 335 μm and vice versa. Why is this information important? Because research has shown that droplets that are in the 250-350 μm range have a more consistent delivery of spray throughout the plant profile. Bigger droplets carry more volume (a 500 μm droplet carries 8 times the volume of a 250 μm droplet) but may produce an uneven coverage. Droplets in the 250-350 μm range are referred to as medium-size droplets.

Therefore, selecting a nozzle/pressure combination that will create medium-size droplets is the best way to guarantee good spray distribution. Pressure selection is also very important. If the spray exiting the nozzle is formed by droplets of the right size, then the right pressure will give them enough momentum to get past the top of the canopy. If the sprayer fails in this energy transfer because there is not enough pressure, droplets will not be able to get past the top of canopy and the deposition in lower portions will be compromised.

A careful reader by now is formulating this question: “If the spray is formed by droplets of several sizes, what will happen to those small droplets when pressure is applied to the spray? Will it drift? And the answer to this question is drift-control agents. By adding a drift-control agent to the spray mixture, the production of fine droplets (those smaller than 150 μm) is reduced, and spray drift is kept to a minimum.  Of course, all pesticide applications have to follow the proper rules of observing weather conditions and suspend application in high wind situations. There's just not a way around that.

So far we have discussed 3 methods to optimize fungicide spray distribution: the production of medium-size droplets, the selection of proper nozzle pressure and the use of a good drift-control adjuvant. Let’s now discuss a more controversial point: volume of application. Generally speaking, spray deposition increases with the increase in volume of application. The choice of an application rate is bound by many factors, including, but not limited to: sprayer capacity, operation efficiency, speed and pressure limitations. An increase in application rate can substantially decrease operation efficiency by increasing refilling time. Application rate has to be chosen based on experience, field conditions, crop growth stages and label recommendations.

Proper sprayer maintenance such as frequent nozzle cleaning and replacement of worn tips will aid in the spray deposition. Sprayer speed has not being considered an issue in spray deposition. Keep these issues in mind during spraying to guarantee a quality distribution of products.