Proceedings 53 
and form the source of infection the next year. As a rule the 
grower is entirely unaware that these bunches of ‘dried-up’ 
plums have anything to do with the cause of the rotting of his 
crop, and of course takes no steps to remove them. When he 
does so, and at the same time cuts out the dead wood—on 
which also the fungus lives through the winter—-the trees become 
healthy again. 
Apple ‘canker’ is the worst disease of the apple known. It 
is caused by the fungus /Vectria ditissima, but we find that 
the fungus is, so to speak, the secondary cause. This species of 
Nectria is an example of the class of Fungi known as ‘wound 
parasites.” If the spores of this fungus are placed on the unin- 
jured wood of an apple tree, they are unable to infect it. If, 
however, the surface of a branch or if the stem is wounded,— 
that is to say if the bark or skin is eracked or torn so as to ex- 
pose the inner tissue—the spores are able to grow into the 
tree, and ultimately form a ‘canker,’ and commonly kill the 
branch or stem. Such wounds arise from a number of causes; 
hail-storms, attacks of hares, careless pruning, frost-blisters, &c., 
but in 90% of cases it is found that an insect is associated with 
‘canker.’ This insect is the ‘green fly’ known as the ‘American 
Blight’ or ‘Woolly Aphis,’ which punctures the bark of the 
branch in order to feed on the ‘sap,’ and by means of these 
minute punctures or wounds the ‘canker’ fungus finds its way 
into the tree. The knowledge of its particular life-history, #.¢. 
the fact that this fungus is a ‘wound-parasite,’ enables thé grow- 
er at once to understand why it is that ‘American Blight,’ and 
‘canker’ increase side by side; and also make him realize that 
the first step in the cure of ‘canker’ is to rid his trees of the 
insect, and thus remove the chief factor which is rendering his 
trees susceptible. It frequently happens that with the removal 
of this insect through spraying, the fungus (against which no 
spraying is of any use) disappears also. 
The value of a knowledge of the full life-history of the fun- 
gus is nowhere better seen than in the disease known as ‘Pear- 
leaf Cluster-cups,’ caused by the fungus Gymnosporangium 
Sabinae, This disease, which affects more particularly pear- 
trees trained against walls, is characterised by the appearance 
