Chemical Control Methods

Chemical application is, at present, the primary method used for the protection of crops (Fig. 1.244). However, the use of chemicals has a significant impact on the environment. It raises concerns about public health and the quality of food and drinkable water, and is an increasing expense on farm production costs in both developed and developing countries.

Thus, it is important to understand the principles of chemical application, and in particular how spraying equipment works, and the adjustments that it needs.

Formulations. Plant-protection products can have two different mechanisms of action:

By Contact: the effect is restricted to those areas on which a sufficient amount of active ingredient has been deposited; the overall efficacy of treatment depends on the degree of coverage of the target, and hence on the density and homogeneity of distribution.

Systemic: the active ingredient is absorbed by the leaves or roots and conveyed via the lymphatic system to the points of action; the treatment is effective if a sufficient amount of active ingredient has been absorbed overall.

The active ingredients (AI) used for crop protection are classified according to their function as:

Herbicides: Substances that kill or inhibit weed development. Weeds damage crops by: competing for environmental resources (light, nutrients, water, space, CO₂, O₂), host­ing parasites and diseases, and by directly releasing toxic substances. Contact herbicides destroy the shoot systems of the weeds on which they are deposited and retained. Systemic herbicides are absorbed by leaves or roots and transported inside the weed, where they interfere with its vital processes. Soil-applied herbicides act on weed seeds in the early stages of development; they can be used before the crop has emerged (pre-emergence) or in subsequent stages (post-emergence).

Fungicides: Substances that kill or inhibit fungi development. Fungi damage plants by causing: stunted growth, malformations, rotting or drying. Contact fungicides are active only on those surface areas on which they have been deposited; they are used as preventive treatments. Systemic fungicides are absorbed by leaves and distributed inside the plant; they can also cure internal suffering layers.

Insecticides: Substances that kill insect pests. These pests damage crops by: directly destroying the epigeal or hypogeal parts; restricting the flow of nutrients to certain parts of the plant; inoculating viral or fungal infections. Insecticides are active both directly on the insect, and also after they have been absorbed into the plant’s organism. In both cases, the pesticide’s action on the parasite is by passage through the scarf-skin, by swallowing, and by inhalation.

For commercial use, the AI is prepared in a variety of formulations designed to be applied “as is” or to be diluted in water. The most commonly used formulations are listed in Table 1.52.

The new formulations–water-soluble granules, capsules, gels, tablets–and the new types of containers–i.e., water-soluble bags that dissolve directly in the tank–are char­acterized by: lower risks to the operator, ease of dosage, ease of storage, virtually no problems with disposal of the containers.

To facilitate the water-dilution operation, a number of adjuvants are available. These include diluents to facilitate mixing with water, droplet spreaders, drift retarders, adhe­sion agents, foam inhibitors and more.

Dry formulations are used widely in areas where water is scarce and for some particu­lar treatments (i.e., sulfur on vines). Granule formulations allow very precise application, avoiding the dangerous drift phenomena typical of powder applications. The character­istics of “dry” materials are discussed in Section 9, Granular Applicators.

Fogs and aerosols are generally used for the treatments in greenhouses. The small droplets of these formulations are characterized by a high level of drift, which is ad­vantageous in enclosed environments where it facilitates penetration into all parts of the crop canopy. Soil fumigation is a treatment normally used for disinfecting the soil prior to planting a new crop. It is carried out by introducing the active ingredient in gaseous form beneath plastic sheeting spread over the ground, or by injecting into the soil, liquids or granules that subsequently emit vapors.

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