351 



WINTER COVER WASHES. 



By a. H. lees, M.A., 



Plant Pathologist, Universitij of Bristol, Agricultural and 

 Horticultural Research Station. 



Winter cover washes or more properly late spring cover washes 

 were first tried on the large scale by Mr Howard Chapman of Kent. 

 He found by experience that a lime wash applied as late as possible 

 before the buds burst in spring produced a very decided lessening of 

 Psylla attack and consequent increase of crop. Cover washes have 

 also certain other subsidiary but very real advantages: 



(1) They can be applied when labour is easily obtainable. A 

 summer wash has to be put on at the end of April, nearly two months 

 later, when labour is urgently needed for other operations. 



(2) They tend to keep the bark clean. 



(3) They add a small amount of lime or chalk to the soil. 

 Since its introduction Hme wash has been tried by many growers, 



some of whom have become enthusiastic advocates of it, while in the 

 hands of others it has produced no apparent good. It seemed there- 

 fore desirable to make some investigation into its action. Two 

 hypotheses were current to account for its good action, since undoubtedly 

 it has had good action in some places and under some conditions. 

 The first suggested that its action was due to the causticity of the hot 

 lime, which had some effect on the protoplasm inside the egg or at 

 any rate so influenced the egg shell as to render the ovum fatally sus- 

 ceptible to outside conditions. The second supposed that the lime 

 covering had no direct action but only an indirect mechanical one, 

 effectually sealing in the eggs and preventing the hatching of the 

 enclosed larva. It was obvious from the outset that the first explana- 

 tion was unlikely. When one considers that the chitin of insect eggs 

 can resist being boiled in strong caustic soda, it is not likely that the 

 feebly caustic lime would affect it. The second thus appeared a more 



