80 AGRICULTURAL RESEARCH AND THE FARMER : 



can only be taken as indications, and as a basis for the work 

 which is still in progress, but they may be briefly referred to 

 here. It was found that apple scab on fruit and leaves has 

 been reduced by every fungicide tested; that scab infection of 

 the wood is also greatly reduced by summer spraying, thus 

 reducing the sources of re-infection in the following summer; 

 and that the effectiveness of the fungicides tested in the control 

 of scab varies considerably. Bordeaux mixture was found to 

 be the most effective, the two formulae used giving practically 

 identical results; next in effectiveness came lime-sulphur, which 

 appeared however to reduce the crop; ammonium polysulphide 

 with soap was the least effective against scab, but the trees of 

 the two varieties sprayed with it bore more than double the 

 crop of similar trees sprayed with lime-sulphur. Ammonium 

 polysulphide with soap was found to be the most effective against 

 apple mildew, and Bordeaux the least. Summer spraying with 

 the fungicides reduced attacks of brown-rot, thus improving 

 the keeping qualities of the fruit; it also considerably reduced 

 bud infections of canker. Finally, it was found that the size 

 of the fruit was usually increased by the spraying, and further 

 that the effects of spraying are cumulative, spraying done in 

 1919 having affected the trees and fruit in 1920, regardless of 

 the treatment in 1920. 



University of Manchester. 

 Mosaic and Leaf Curl or Leaf-roll Diseases of Potatoes. — These 

 two diseases are undoubtedly present in serious proportions 

 throughout Great Britain. It is not uncommon for some potato 

 varieties to be infected with Mosaic to the extent of from 50 per 

 cent, to 100 per cent., while in the case of Leaf-roll, although 

 the percentage of infection may not be so high, the effect upon 

 the plants is more serious, since the yield and size of the tubers 

 may suffer a reduction of as much as 75 per cent. In both 

 cases the tubers carry infection. Although it is now known 

 that both diseases are infectious and that the active principle 

 or virus is carried by insects from plant to plant, the actual 

 cause of the trouble is still obscure, and all the efforts so far 

 made to isolate an organism from the affected tissues have 

 failed. Research was therefore commenced in the spring of 1920 

 to ascertain, if possible, the exact cause of both diseases, the 

 method and course of infection, the relative susceptibility of 

 various varieties of potato, and the part played by insects, 

 especially aphids, in spreading the diseases. The experimental 

 work, both in the laboratory and field, has been of a particularly 



