100 MASS. EXPERIMENT STATION BULLETIN 252 



3. To increase wetting power by reducing the interfacial tension at the 

 leaf-spray interface and thereby facilitate spreading and distribution. 



A highly dispersed copper fungicide assures better suspension and more 

 uniform distribution than one of larger aggregates, especiallj' when ap- 

 plied as a mist or film of invisible drops. Greater adhesiveness of the re- 

 sulting residue is also obtained. Drench applications on the other hand are 

 more likely to collect in drops, particularly on the edge of the leaves, or 

 run off carrying most of the fungicide. The force with which the spray 

 is applied is also a factor in wetting and spreading. The resistance of 

 leaves to wetting varies with the character of the leaf surface (epidermis) 

 and the nature of the spray solution. The epidermis of leaves may be a 

 relatively thin membrane of cellulose or heavily cutinized (cuticularized), 

 waxy, hairy, etc., and differs greatly in respect to wetting. The same 

 applies in a measure to fruits, buds, spurs and older growth. The leaves 

 of apples, peaches, pears and grapes are cutinized, and fruits like apples 

 and plums and the leaves of the cabbage are waxy and not easily wet as 

 they interpose a film of air between the leaf and spray. In most in- 

 stances, the addition to the spray of organic substances of similar constitu- 

 tion to those in the plant, i. e., albuminous substances and plant infusions, 

 and their adsorption or surface concentration at the leaf-spray interface, 

 reduces the interfacial tension and permits the spray to touch, wet and 

 form a continuous film over the surface. In the case of waxy surfaces, 

 the substances added generall\' have a fairly large molecule with a small, 

 active group soluble in water and with the inactive portion adsorbed in 

 the surface layer of the spray solution. When the inactive portion is 

 similar to or soluble in the wax, its adsorption at the leaf-spray interface 

 reduces the interfacial tension and permits the spray to spread. Soap and 

 soap emulsions, when compatible with the fungicide, are usually effective 

 on such surfaces and other substances will be noted. 



It is evident, therefore, that wetting is essentially the same on all plant 

 surfaces and that an effective solute must have a low surface tension 

 (soaps), or the property of surface concentration (saponin, proteids), and, 

 in some instances, a solvent action on the epidermal coating in addition. 

 Since there is no method for measuring interfacial tension between plant 

 surfaces and spray and as the nature of the surface interposed by the 

 plant is problematical, it is impossible to foretell the action of added sub- 

 stances known to reduce surface tension of the spray solution on the plant 

 surface-spray interface. The fact remains, however, that an increase in 

 wetting and spreading is necessary in many instances, is desirable in all 

 cases, and is secured mainly by reducing the interfacial tension. There- 

 fore, the substances to be added for the various leaves, fruits and other 

 surfaces must be determined largely by trial and error. 



Theory of Wetting and Spreading. 



Numerous investigators have contributed extensively on the subject and 

 a few will be noted. Mausier (62), Gastine (32) and Ruth and Kelly 

 (82) found a low surface tension of the spray favored spreading. Brun- 

 nich and Smith (6) assert that wetting power depends primarily on sur- 

 face tension. Lovett (58) claims that neither suspension nor viscosity 

 of the spray solution is a true index of spreading but considers suspen- 



