PLANT DISEASES. 75 



that is, the fungus was supposed to enter the tree only through 

 wounds. Work on the pest was started some years ago, 

 to find out exactly how the fungus infects the tree. In the 

 first place the fungus was isolated and artificial infections were 

 carried out. Infection under natural conditions was then 

 studied, in the hope of being able to devise means of preventing 

 such infection. It has been found that the most important 

 means of entrance for the parasite is not through mechanical 

 injuries, but through the " leaf scars " which are left on the 

 stem after the leaves fall. Immediately after the fall of the 

 leaves a period of very active infection may take place in the 

 autumn, especially as regards certain varieties of apple, and 

 another period of active infection may occur in the spring just 

 before the buds burst. The microscopic details of such infection 

 have been worked out, and it appears that the fungus enters 

 the stem through small cracks that appear in the leaf scar tissue. 

 The results therefore point to spraying as a method of preventing 

 canker, and spraying experiments are in progress to ascertain 

 whether this method is effective. Copper sprays have been 

 used, since the canker fungus is not sensitive to sulphur. The 

 first year's experiments carried out in 1920 were somewhat 

 promising, but in 1921 the dry season prevented much canker 

 from appearing, even on the control branches, so that significant 

 results have unfortunately not been obtained. Another method 

 by which the fungus enters the tree is through scars formed by 

 the " scab " fungus, Venturia incBqualis. 



It has also been shown that a further means of infection is 

 through the galls caused by Woolly Aphis, thus confirming au 

 opinion generally held by growers that the two diseases have ar. 

 association. The importance of combating attacks of Aphis is 

 therefore two-fold. In this case it has been found that infection 

 takes place in the autumn of the year in which the Aphis appears, 

 and not — as supposed — in the succeeding year. 



Bark Canker of Apple Trees. — A disease of apple trees 

 discovered at Long Ashton during 1920 and not hitherto reported 

 in this country is under investigation. It has also recently 

 been found doing considerable damage in a nursery in Somerset. 

 It was noticed first in America in 1918, where it was given the 

 name of Bark Canker. The causal organism appears to be a 

 fungus called Myxosporium corticolum. It is possible that this 

 disease is fairly widespread, as owing to the somewhat similar 

 appearance of the scars to those of the canker fungus it might 

 easily be mistaken for the latter. 



