Introduction

Adjuvants are added to spray mixtures to increase the effectiveness of the main active ingredient.  There are many classes of adjuvants, each with its own spectrum of activity.  Any specific individual adjuvant may have more than one mode of action.
When the cost of an adjuvant is less than the cost of the chemical there may be clear economic advantages to using an adjuvant rather than using a high rate of a chemical.  It has long been recommended to reduce the rate of NAA (50%) when a suitable surfactant was added to the spray mixture. 
The use of surfactants with plant growth regulators may significantly enhance their activity.  Obviously, plant growth regulators are applied to modify plant growth and to be effective they must enter the plant.  This is in sharp contrast to most other pesticide applications where a uniform deposit is desired on the outside of the plant. Because the exterior of leaves have waxes, cutin, pectin, and cellulose between the chemical and the cell contents anything that we can do to aid a uniform deposit and penetration through these barriers to uptake must be seriously considered. 
The wholesale use of adjuvants with orchard sprays may not be warranted.  Many pesticides contain adjuvants to stabilize the product and to make it effective in the spray tank.  Thus, growers should be cautious in using adjuvants and should rely on reputable sources of information for data regarding the selection of a suitable adjuvant. 

Adjuvant Descriptions
Acidifier: Acidifiers are adjuvants that have the ability to reduce the pH of solutions.  They are useful when the spray water one uses has a pH higher than the desirable pH for the chemicals to be sprayed.  However, acidifiers can only lower the pH so their indiscriminate use is not recommended.  In some cases if spray water pH is correct or lower than the recommended pH for the chemical in question then the use of an acidifier could reduce the effectiveness of the pesticide. 
Anti-foaming agent: This is another name for defoamers so see that classification. 
Anti-transpirant: Anti-transpirants are chemicals that can reduce transpiration of plants.  These can be applied as main chemicals themselves or they may have activity in enhancing the activity of other chemicals.  Anti-transpirants can be effective when transplanting nursery stock especially when leaves are present and low humidity and/or high winds are expected. 
Buffering agent: Buffering agents have the ability to raise or lower pH to the designed pH of that buffering agent.  Thus, buffering agents can be useful when the spray water pH is above or below that to be utilized. 
The quantity of buffering agent needed to be used in a spray tank may be difficult to determine.  Various buffering agents have different powers to buffer.  In addition, merely measuring the pH of a spray solution may not give an indication of the quantify of buffering agent needed.  For example, sulfuric acid and acetic acid (the acid in vinegar) may both have low pH values.  However, sulfuric acid is a much stronger acid than is acetic acid and much more buffering agent would be needed to change the pH of a sulfuric acid solution verses an acetic acid solution.
Compatibility agent: Compatibility agents are adjuvants that allow easier mixing of two or more components in a solution.  The use of compatibility agents may allow the use of two or more chemicals in a tank that would otherwise be incompatible. 
Crop oil: Crop oils normally contain 95 to 98% petroleum oil that have 1 to 2% surfactant added to them.  Crop oils can be effective as penetrants and as surfactants.
Crop oil concentrate: Crop oil concentrates normally contain between 80 and 85% petroleum oil with 15 to 20% surfactant.  These products can be effective as penetrants and as surfactants.
Defoamer: Defoamers are adjuvants that reduce foaming when there is excessive foam in a spray tank.  Excessive foaming can be a significant problem with some agitation systems in sprayers and especially when the water level in a tank gets down so that a mechanical agitation system would be causing excessive foam.
Drift retardant: Drift retardants are used to reduce off target drift of pesticides and are often used with aerial applications.  Since aerial applications normally are made with highly concentrated spray mixtures spray nozzles that create small droplets are often used. Since small droplets can travel farther than larger droplets some drift retartants work by increasing mean droplet size. 
Extender: Extenders are adjuvants that can extend the useful life of a spray chemical.  This can be accomplished by increasing the adhesion of the chemical to the leaf or by reducing any factor that can diminish chemical effectiveness or by enhancing the weatherability of the chemical. Since many pesticides are broken down by ultraviolet light some extenders have the ability to intercept UV light. 
Penetrant: Penetrants are adjuvants that have the ability to aid the penetration of chemicals into plants. 
Spreader: Spreaders are surfactants.  See surfactants.
Spreader-stickers: Spreader-stickers are compounds that perform two function at the same time.  They stick and spread chemicals to plant surfaces.  See stickers for that description and see surfactants for the spreader part of these compounds. 
Sticker: Stickers are adjuvants that aid the attachment of a chemical to a surface and can lengthen the time that a chemical will be attached to a plant surface.  Stickers generally make pesticide deposits less easily removed from leaves by forces such as rain or wind. 
Sticker-spreader: Sticker-spreaders are compounds that perform two function at the same time.  They stick and spread chemicals to plant surfaces.  See stickers for that description and see surfactants for the spreader part of these compounds.
Surface active agent: Surface active agents are surfactants.  See surfactants.
Surfactant: Surfactants are adjuvants that reduce surface tensions of solutions so that the solution can spread and cover surfaces more effectively.  This is probably the best known class of adjuvants.  Most adjuvants are a double ended molecule, one end being water soluble and one end being oil soluble.  Therefore, these molecules can line up between water-like compounds and oil-like compounds and make them more compatible.  Surfactants can be uncharged (nonionic), positively charged (cationic), or negatively charged (anionic).  When using surfactants follow label recommendations for the major product in the spray tank since certain pesticides specify one of these three types of surfactants. 
Suspension agents: Suspension agents aid the suspension of one material in another.  These types of adjuvants are often used in liquid fertilizer mixes and also in liquid pesticide formulations to help maintain a uniform product mix.
Thickener: Thickeners are adjuvants that increase the viscosity of solutions.  Calcium dips were proven to be more effective in increasing fruit calcium levels when a thickener was added to the calcium dipping solution.  Thickeners are often added to low calorie foods such as pancake syrup and salad dressings to mimic the physical characteristics of higher sugar foods. 
Vegetable oil concentrates: These adjuvants are similar to crop oil and crop oil concentrates except that vegetable oils are used instead of petroleum oils.
Wetting agent: This is another name for surfactants.  See surfactants.
For references, contacts and table of adjuvants available for use on tree fruits (table 43 at this site):
http://www.cas.psu.edu/docs/casdept/hort/tfpg/part3/part34b.htm

More information on adjuvants is available from Montana State University.

http://scarab.msu.montana.edu/extension/Agadjtoc.htm