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  Section: Principles of Horticulture » Plant protection
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Application of herbicides and pesticides

Plant protection
  Physical control
  Cultural control
  Biological control
  Chemical control
  Insecticides and acaricides
  Application of herbicides and pesticides
  Toxicity aspects of pesticides
  Selection for plant resistance
  Integrated control
  Supervised control
  Legislative control

This subject is described in detail in machinery texts. However, certain basic principles related to the covering of the leaf and soil by sprays will be mentioned. The application of liquids and wettable powders by means of sprayers may be adjusted in terms of pressure and nozzle type to provide the required spray rate. Cone nozzles produce a turbulent spray pattern suitable for fungicide and insecticide use, while fan nozzles produce a flat spray pattern for herbicide application. In periods of active plant growth fortnightly sprays may be necessary to control pests and diseases on newly expanding foliage.
Figure 16.10 Knapsack sprayer being used in nursery stock
Figure 16.10 Knapsack sprayer being used in
nursery stock
  • High volume sprayers apply the diluted chemical at rates of 600–1000 l/ha in order to cover the whole leaf surface with droplets of 0.04–0.10 mm diameter. Cover of the under-leaf surface with pesticides may be poor if nozzles are not directed horizontally or upwards. Soil applied chemicals, such as herbicides or drenches, may be sprayed at a larger droplet size, 0.25–0.5 mm in diameter, through a selected fan nozzle. The correct height of the sprayer boom above the plant is essential for downward-directed nozzles if the spray pattern is to be evenly distributed. Savings can be achieved by band spraying herbicides in narrow strips over the crop to leave the inter-row for mechanical cultivation.

  • Medium volume (200–600 l/ha) and low volume (50–200 l/ha) equipment, such as knapsack sprayers, apply herbicides and pesticides onto the leaf at a lower droplet density, and in tree crops, mist blower equipment creates turbulence, and therefore increased spray travel, by means of a power driven fan. Ultra-low volume sprays (up to 50 l/ha) are dispersed on leaving the sprayer by a rapidly rotating disc which then throws regular-sized droplets into the air. Larger droplets (about 0.2 mm) are created by herbicide sprayers to prevent spraydrift problems, while smaller droplets (about 0.1 mm) allow good penetration and leaf cover for insecticide and fungicide use.
Fogging machines used in greenhouses and stores produce very fine droplets (about 0.015 mm diameter) by thermal and mechanical methods, and use small volumes of concentrated formulation (less than 1 l in 400 m 3 ) which act as fumigants in the air, and as contact pesticides when deposited on the leaf surface. Dust and granule applicators spread the formulations evenly over the foliage or ground surface. When mounted on seed drills and/or fertilizer applicators, granules may be incorporated into the soil. Care must be taken to ensure good distribution to prevent pesticide damage to germinating seeds or planting material.

Safe practice
In chemical control, the hazards are:
  • possible acute poisoning of humans, pets, farm animals, bees, and wild animals;
  • possible accumulation of pesticides that lead to toxic levels in humans, pets, farm animals, bees and wild animals;
  • possible cancer inducing effects in humans;
  • possible damage to cultivated and wild plants especially by herbicides;
  • possible contamination of streams and dams;
  • possible development of strains of rodents, insects, mites, and fungi, resistant to pesticides.
When using chemical control, risks can be minimized by:
  • restricting chemical applications to only those situations that justify such a control measure. In many instances, other controls measures may be preferable and less hazardous;
  • carefully choosing the least hazardous chemical to effectively control the problem organism;
  • carefully reading the instructions on the product label;
  • carefully choosing the correct clothing, where necessary;
  • carefully measuring the correct amount of concentrate water (where relevant);
  • calculating (where appropriate) the amount of pesticide and water necessary for application to the crop area in question;
  • carefully mixing the two, avoiding spillage on to skin, clothing and the surrounding area;
  • carefully applying the product so that the same area is not covered more than once, at any one time;
  • carefully applying the product under suitable dry, wind-free weather conditions;
  • carefully applying the product so that other humans, beneficial animals, waterways and adjacent plantings are avoided;
  • carefully avoiding spray drift, especially with herbicides;
  • carefully storing pesticides in a secure, safe, dry place away from children and pets.

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