Spray program guidelines

Scientific application of a spray program ensures its effectiveness. All chemicals must be applied at the right place (target) and the right time and in the right way. Pesticides and fungicides are dangerous chemicals, they are designed to kill pests and diseases, but they can also kill humans if not used correctly. The application method is the most critical factor in chemical control. The equipment or adverse weather conditions cause 80% of poor control.

Understanding the target and target area

The target is where the chemicals must reach to have effective control. The target area can be different parts of the plant or insect.

The target is determined by:

• Fungicides applied to pathogens on leaves.
• Insecticides that target the stomach of insects must be applied where the insects feed.
• Contact insecticides must make good contact with its targets. Targets can be a disease or an insect.
• Systemic chemicals affect plant growth and metabolism. So the whole plant must be targeted. The best place is usually fast-growing soft leaves.

Chemical drift

Chemical drift is unavoidable due to air movement. Drift means that some of the chemicals did not reach the target, and that part is ineffective or wasted. There are two types of drift; exodrift where part of the chemical drifts outside the target area because of air movement and endodrift where the chemical falls onto the ground but still close to the target.

The volume of chemical application in a spray program

Fungicides, pesticides and herbicides are in concentrated water-soluble form, and further diluted with water. Some have to be applied with high volume, and some are applied with a spray mist. So the total mixing volume of water with the chemical is essential. There are 5 volume classifications:

1. High volume: >600L
2. Medium volume: 200-599L
3. Low volume: 60-199L
4. Very low volume: 5-50L
5. Ultra-low volume: <4.99 L

High volume is not necessarily more effective than ultra-low volume spray program applications. High volume is mostly used in large fields with large tractors, whereas ultra-low volumes are applied by backpack. What is important is that the active ingredient of the chemical must reach the target at a lethal concentration. With modern technology lower volumes can be used and are beneficial for the following reasons;

• Lighter equipment
• Cheaper transport
• Less compaction of the soil
• More modern and effective equipment.
• Low volume is more cost-effective than high volume sprays.
• Less of the active ingredient is wasted by drift and other factors

One disadvantage is that with low volumes, the concentration of the active ingredient becomes more crucial and must be measured more accurately.

The droplet size of applied chemicals

Droplets form by forcing the liquid chemical through a special nozzle. The design of the nozzle and the pressure of the system determines the size of the droplet. In general, the finer the droplet, the better cover is obtained, but also the more drift there is. So its a delicate balance between droplet size and pressure.

Smaller droplet size and lower volume of water is recommended. It’s more effective in controlling the target, and fewer chemicals are lost. Smaller droplets also evaporate more quickly and become ineffective.

Once the droplet leaves the nozzle, various things can happen. It can ricochet, it can combine with other droplets and roll off, it can stay on the target and dry out, leaving the active ingredient behind. If the active ingredient is at the right concentration, it sticks to the target even under rain, irrigation or dew. The residue washes off over time. Some of the active ingredients biodegrade by coming in contact with UV light or even soil colloids or organic matter.

The pH of the mixing water

The pH of the water is critical and does influence the effectiveness of the active ingredient. Each pesticide, herbicide and fungicide has its ideal pH in which it is mixed. In general, the following applies:

• Fungicides: pH 7
• Herbicides: pH 4.5 – 5.5
• Insecticides: pH 4.5 – 5.5
• Foliar feeds: pH 45 – 5.5

Conclusion

It is vital to follow a good spray program that prevents the development of resistance in the target over time. The active ingredient of chemicals belongs to different groups which determine their mode of action. Exposing the same target to a specific chemical removes all the susceptible targets and leaving the ones that have natural resistance. This is called selection pressure. Those targets that have natural selection keep on breeding and their offspring have resistance to the active ingredient. So it means that you cannot control those targets with that specific active ingredient any more. The only option is to select another if it exists at all.

You can read more about basic resistance strategies in another article here.