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Spray program guidelines

spray guidelines vegetable farming

A spray program must be applied on a scientific basis otherwise it will not be effective. 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 important factor in chemical control. 80% of poor control is caused by the equipment or weather conditions were not right.

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 for leaf pathogens and must be applied on the leaves
  • Insecticides that target the stomach of insects must be applied where the insects feed
  • Contact insecticides must make good contact with the target such as a disease or insect.
  • Systemic chemicals affect the whole plant so the whole plant must be targeted. The best place is usually fast growing soft leaves.

Chemical drift

There will always be some type of chemical drift due to air movement. Drift means that some of the chemicals did not reach the target and that part is ineffective or wasted. There is 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.

Volume of chemical application

All chemicals must be mixed with water. Some have to be applied with high volume and some are applied with a mist. So the volume of water that is mixed with the chemical is very important. 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 necessary more effective than ultra low volume 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 a a lethal concentration. With modern technology lower volumes can be used and is beneficial for the following reasons;

  • Lighter equipment
  • Cheaper transport
  • Less compaction of the soil
  • More modern and effective equipment can be used
  • Basically it is more cost effective
  • 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.

Droplet size of applied chemicals

Droplets are formed by forcing the liquid chemical through a special nozzle. The design of the nozzle and the pressure of the system will determine 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 fine balance between droplet size and pressure that is used with the equipement.

Smaller droplet size and lower volume of water is better. It’s more effective in controlling the target and less 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 will stick to the target even under rain, irrigation or dew. The residue can also wash off over time. Some of the activie ingredients will also biodegrade by coming in contact with UV light or even soil colloids or organic matter.

pH of the mixing water

pH of the water is very important and does influence the effectiveness of the active ingredient. Each pesticide, herbicide and fungicide has its ideal pH in which it must be 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


It is very important to follow a good spray program that will prevent the development of resistance in the target over time. The active ingredient of chemicals belong 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. You will have to select another if it exists at all.

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



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