Copy these 5 pbo files inside the folder res/addons and replace if necessary.
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There are two versions of Cropduster Planes:

The "(Po-2) Cropduster" and "(Simple) Cropduster".

The Cropduster planes are found in Civilian/Air/
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There are also two versions of Cropduster Pilots:

The Cropduster Pilots are found in Civilian/Man
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To Cropdust just click on menu "Spray On" that a smoke will activate for 10 seconds after repeat again. sorry I couldn't make a better script for Start and Stop the smoke. If someone knows how to do you can modify the file and upload just don't forget the credits.
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Credits:
The (Po-2) Cropduster was modified from PO-2 Plane from Liberation Mod.
The (Simple) Cropduster was modified from Biplane by Vosku.
The Smoke Script was modified from Cessna Bird dog by Scud.
The Cropduster Pilots where taken from BISCamel Bonus Addon BETA from Bohemia Interactive Studio and The Codemasters Software Company Limited ("Codemasters").
Mod by Felizpe.