About phytotoxicity

Sometimes, your crops will be ‘allergic’ to certain chemicals, warns Paul Donovan.

About phytotoxicity
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Many people are allergic to certain chemicals and substances. In the same way, some plants can have an allergic reaction to a specific pesticide, fungicide or chemical fertiliser. This is called phytotoxicity. Even ‘green’ products, such as insecticidal soap, can be phytotoxic. Phytotoxicity can occur if a chemical is applied in the incorrect mix or the plant is stressed at the time of application. It can also be caused by insecticidal drift, where insecticide sprayed on one land is blown onto a neighbouring crop that may be sensitive to it. Pesticides can, over time, also accumulate in runoff which then spreads to a crop.

Symptoms
Signs vary from plant to plant as well as from one pesticide to another. Usually, the leaves develop brown or yellow patches. They might even dry out and fall off. In some cases, the plant or fruit will be stunted, and in rare cases, it will go through a stage of excessive growth.

Prevention

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  • Always read the label before spraying a pesticide onto a plant. The manufacturer will list the plants on which the pesticide should not be used.
  • Limit the number of times the plant is sprayed. A series of treatments, especially over a short period, greatly increases the risk of phytotoxicity, even in resistant strains. Stick to the dosage rates on the label.
  • Avoid using the chemical on plants under stress, such as those that have just been replanted.
  • Always use a clean container when applying pesticides.
  • Avoid spraying under cool, humid conditions or when the leaves are wet, as the pesticide will sit on the leaves, increasing the risk of phytotoxicity. This is especially true when using pesticides containing emulsifiable concentrates, spreaders, stickers or solvents.
  • Using carbonates, organophosphates or oil-based pesticides at a low temperature can cause phytotoxicity. Equally, using sulphur or hydrocarbon-based pesticides at high temperatures can have the same result. At all times, therefore, it is better to spray when the temperature is moderate.
  • Spray the pesticide from a distance of 40cm to 50cm and apply evenly and thoroughly. If you spray any closer than this, you risk overdosing.
  • Don’t mix pesticides. Incompatible chemicals increase the risk of damaging the plants. 

Paul Donovan, based in Botswana, promotes the use of biological control agents and advises farmers on pest control.