336 



Cherry Leaf Scorch. 



[SEPT., 



of the previous season are still hanging on the branches of the 

 trees. In severe attacks scarcely a leaf falls from the diseased 

 trees in the autumn, so that at the beginning of winter the 

 affected orchard at a distance might be taken to be composed 

 of trees which had been suddenly killed in full vigour of growth, 

 the leaves having remained attached just as we find that the 

 withered leaves remain on a severed branch. During winter, 

 driving rains and snow and gales strip off a few of the 

 dead leaves or cause them to rot, but so resistant are the 

 diseased leaves and so firmly attached to the branch that in 

 the spring, at the time when the new leaves are beginning to 

 appear, a very considerable number of the leaves of the previous 

 season are still hanging on the tree. Fig. I is a photograph 

 of a diseased tree taken towards the end of winter, and shows 

 the large- number of dead leaves still hanging on the boughs. 

 Although partly obscured from sight by the new leaves, 

 clusters of dead leaves can be found on such a diseased tree 

 through its period of blossoming right on to the time when the 

 cherries are ripening. 



Now these dead leaves are those which have been attacked 

 when living by the fungus Gnomonia erythrostoma. They 

 still, when dead, bear the fungus, and it is by these leaves alone 

 that the disease is carried on from one season to the next. 



A diseased leaf gathered from a tree in the spring months 

 shows a number of minute black points, just visible to the naked 

 eye, scattered over its lower surface.* Each black point is 

 the beak of one of the fruit-conceptacles of the fungus (see 

 d in Fig. IV (i)). In the conceptacle we find a large number 

 of minute sacs (asci), each containing eight winter-spores 

 (ascospores). In Fig. IV (i) a conceptacle of the fungus is 

 cut open down the middle to show its contents ; at 2 an ascus 

 and four ascospores are shown ; at 3, four ascospores germinating 

 after being kept for 48 hours in a decoction of plum juice. 



The conceptacles are formed in the autumn, but are then 

 immature ; they ripen gradually during the winter months, 

 until by spring, at the time when the new leaves are unfolding, 



* Accompanying the Cherry Leaf Scorch fungus may be found, almost without 

 exception, a totally distinct fungus, which can be easily distinguished superficially by 

 its much smaller size, and by being densely clustered. This fungus is, I believe, a 

 species of Mycosphaerella. 



