Prevention or Cure of Plant Diseases 15 



stronger than could safely be used when the plant is in 

 foliage. For this purpose use : — 



Copper sulfate with lime sufficient to color. 

 Lime-sulfur mixtures. 



4. The use of protective sprays or dusts. — Examples: 

 grape })lack-rot, apple scab. A protective spray or dust is one 

 applied to the plant upon its bark, foliage, or fruit, to kill 

 any offensive germs that may fall upon its surface, or to pre- 

 vent their growth upon or into the plant. Such sprays are 

 distinguished from the cleansing sprays, which are not 

 primarily for protection against future invasion, but rather 

 for the purpose of cleaning off or killing all spores actually 

 upon the plant. Protective applications are usuall}^ em- 

 ployed when the plant is in foliage, and therefore must be 

 used with due regard to the susceptibility of the plant to 

 the poison employed. They should be applied: — 



1. Often enough to replace any previous protective ap- 

 plications that may have been removed by rain or other 

 means. 



2. To give protection to any new surface of twig, leaf, 

 or fruit which has developed since the last protective treat- 

 ment was made. 



The first protective application should be given as soon as 

 possible after susceptible surfaces have developed, and 

 before the attacking organisms are expected to make their 

 invasion. The time varies of course with each special crop, 

 with each disease, with the climatic relations and section 

 of the country. Nor can any absolute rule be laid down 

 for the use of subsequent protective applications. Fre- 

 quency of rain is an important controlling factor. A fungi- 

 cide, if rained upon just after it is applied and before it has 

 time to dry, is easily washed off. 



New tissue and new surfaces develop every day; there- 

 fore, theoretically to secure complete protection, sprayings 

 should be very frequent, much more frequent than is prac- 

 ticable. To meet the needs occasioned by new growth, pro- 



