Protected Cultivation 195
17.5.4 Overhead Systems
In overhead misting systems, sprinklers are installed under the roof framework of
polyhouse, and this ‘rain’ water down pours onto plants. This type of irrigation sys-
tem is easy to automate and produces high humidity. This high humidity allows pro-
tecting crops against frost damage. For the best coverage, space overhead sprinklers
to around 50%–60% of the wetting diameter of the sprinkler.
17.5.5 Bench Misting
Bench misting uses a central line of sprinklers or hoops placed at or just above the
level of plants. Bench misting requires plants to be placed on raised benches and
these must be made of materials that are impervious to water, such as a metal. This
allows having just a single misting bench in the polyhouse and a different watering
system in the rest of the polyhouse.
17.5.6 Drip Irrigation
Drip irrigation is one of the most efficient methods of watering, typically operat-
ing at 90% efficiency. Runoff and evaporation are at a reduced rate compared to
other irrigation systems such as sprinklers. In drip irrigation, tubes that have emit-
ters run alongside the plants receiving irrigation. The water leaves from the tubes
through the emitters by slowly dripping into the soil at the root zone. This method
of irrigation minimises leaf, fruit and stem contact with water resulting in reduced
plant disease. It reduces weed growth by keeping the area between plants dry. Drip
watering is excellent way to water in polyhouse as it keeps the humidity low leading
to less pest and disease problems. Water is directed to exactly where it is needed
either with an individual dripper, especially for pot grown plants, or inserted into
a pipe for beds (Dubey and Singh 2012). Protected cultivation improves the water
productivity due to the evapotranspiration (ET) reduction and larger outputs of
protected growing (Stanghellini 1993). Drip irrigation also increases the water
productivity, relative to conventional irrigation in greenhouses (Castilla 1994), far
from the 65 kg/m^3 obtained in sophisticated greenhouses, with soilless culture and
very long cycles (Stanghellini 1993).
17.5.7 Hand Watering
Hand watering is the most basic method of irrigation for a greenhouse or polyhouse.
Watering cans or hoses with nozzles allow tailoring watering to the needs of indi-
vidual plants. This is also the most labour-intensive method for plant irrigation and
may not be effective for large scale cultivation or nursery raising under polyhouses.
17.6 Disease Management in Polyhouses
Papaya is very sensitive to viral diseases under protected environment. It often
spreads in the plantation by insect vectors such as whitefly, thrips and aphids.