PREVENTION OR CURE OF PLANT DISEASES 21 



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

 or fruit which has developed since the last protective spray 

 was applied. 



The first protective spray should be given as soon as 

 possible after susceptible surfaces have developed, and 

 before the attacking organisms are ejcpected to make their 

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

 with each disease, with the climatic relations and section 

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

 for the appHcation of subsequent protective sprays. Fre- 

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

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

 time to dry, is much more easily washed off than is the 

 case if it dries well before the rain falls. 



New tissue and new surfaces develop every day ; there- 

 fore theoretically to secure complete protection, sprayings 

 need to be very frequent, much more frequent than is prac- 

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

 tective sprays are usually given at intervals of from ten to 

 fourteen days, though special rules must govern special cases. 



The chief protective sprays are : — 



A. Bordeaux mixture, full strength or weak. 



B. Ammoniacal copper carbonate. 



This solution possesses the advantage that it does not 

 spot the fruit and may therefore be used near the period 

 of maturity, when the Bordeaux mixture, owing to its 

 spotting effect, would be less desirable. 



C. Lime-sulphur mixtures. 



5. Excision of the affected parts. — Examples : Ash 

 white rot. Excision is resorted to chiefly in the case of trees 

 which from their location or quality possess value suffi- 



