SECTION VIII. — FACTORS INFLUENCING BLK^HT I'REVALENCE. 115 



In 1912 a number of experiments were made vvitli nitioliui which is 

 an artificially prepared manure containing both lime and nitrogen. 

 The experiments were chiefly designed to test its destructive effect on 

 the froghopper and not its general manurial effect on the condition of 

 the soil. They will be referred to again later under Control 

 Methods (p. 142). 



AGE OF CANE. 



The age of the canes in a field can be considered fi-om the number of 

 years that the stool has been in the ground, or the number of months 

 that the shoots have been above ground. 



In the first case we are concerned with the difference in resistance 

 to blight of "plant canes" in their first year, and first and second ratoons. 

 There is no doubt of the greater immunity of the first year canes. 



In 1912, a year of severe blight, a number of estates gave the 

 returns of acreage damaged. From these returns the following 

 particulars are obtained. 



For ten estates in 1912. 



Area under plant canes 2,980 acres. 



Plant canes damaged 1,093 „ =37 percent. 



Area under first ratoons 8,405 acres. 



First Ratoons damaged 1,789 ,, ^^ 52 per cent. 



Area under older ratoons .3,144 acres. 



Older Ratoons damaged 1,482 „ ^=47 per cent. 



In 1917 a similar return was given by many estates, but the 1st and 

 2nd ratoons were not given separately. 



For thirteen estates in 1917. 



Area under plant canes 3,861 acres. 



Plant canes damaged 1,712 ,, =44 per cent. 



Area under ratoon canes 6,677 acres. 



Ratoon canes damaged 4,311 ,, = 64 per cent. 



It will be seen that in these two bad years the plant canes are about 

 50 per cent, less liable to damage than the ratoon canes. 



It is however a distinct feature of the blight that it is less selective 

 when it is severe than when it is slight, so that in the years when the 

 outbreaks are only slight there is a very much greater relative freedom 

 from attack in plant canes. In 1919, when the area under plant canes 

 was probably greater than ever before, only a few small spots were 

 attacked, much less than one-tenth of the area damaged under ratoon 

 canes. 



In the figures for 1912 it will be noticed that the older ratoons are 

 sHghtly less attacked than the first ratoons. The difference in the 

 numbers is small and from other experience I do not think significant, 

 as fields of second ratoons which are doing poorly are frequently 

 abandoned altogether and not included in the returns, while third 

 ratoons and older are only kept in cultivation on the better soils. 



