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Liquid applications of insecticides and fungicides are generally considered to be more effective than dusts since they can be applied more efficiently, particularly to the undersides of leaves. Dusts require to be mixed with a carrier base such as talc powder and are sometimes difficult to store, particularly in humid conditions when they' tend to absorb moisture and become solid.
Dusters
Hand operated dusters are usually of the bellows type and are used mainly on vegetable crops grown on a limited scale. Tractor-mounted dusters are used on a commercial scale, particularly in low rainfall areas where supplies of water for crop spraying may be limited.
Sprayers
sprayer is a machine /device that used to spray a liquid, and sprayers are commonly used for projection of water, crop performance materials, weed killers, maintenance of pest chemicals, as well as manufacturing and production line ingredients.
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Most sprayers fall within the following categories.
Knapsack types: These are generally carried on the shoulders, although some newer versions are carried slung from the shoulder. There are three main types: syringe, pneumatic and hydraulic.
Syringe type: This is suitable only for small areas and consists of a double-acting syringe mechanism which produces a continuous flow of liquid dispersed into fine droplets.
Hydraulic sprayers
These are commonly used for mosquito control and in cocoa farms for spraying insecticides and fungicides.
The lever-operated or pumped type of sprayer consists of a plastic or galvanised metal container to one side of which is fitted a handle for pumping the liquid through a plastic tube to the nozzle. In some areas, a boom with up to four nozzles is operated by one man but a wide boom, up to 3 m in length and with 6-8 nozzles spaced along it, can be carried between two men, each operating a knapsack sprayer. Other types of hydraulic sprayer can be pressurised before spraying begins and
are less tiring in operation than the lever-operated sprayers.
Motorised pneumatic sprayers: These are widely used for pest and disease control in plantation crops and are relatively expensive since they are powered by a small engine although they are shoulder-mounted. The droplet size can be varied by using different types of nozzle and the effective range of the spray is sufficient to make this type of sprayer suitable for use on tree crops.
Tractor-mounted sprayers: These basically consist of: tank, pump, pressure control valve, flow control valve, boom and nozzles.
Gauze filters are incorporated into the system at various points to prevent blocking of the pump and nozzle assemblies.