l8o FUNGOUS DISKASKS OF 1'LANTS 



A striking correlation seems to obtain between the serration 

 of leaves and susceptibility to curl, the serrate varieties being 

 very slightly susceptible as compared with those which have the 

 glands of the leaves reniform or globose. Other interesting rela- 

 tionships have been suggested. 



The fungus. While the leaf curl has evidently been known 

 in England as a peach disease since 1821 or earlier, the fungus 

 was first described by Berkeley in 1857. It has therefore been 

 known to botanists for half a century. Infection takes place at 

 the time of the opening of the buds (most frequently), but it may 

 also result (occasionally) by the growth of a perennial mycelium 

 from the old wood, in which it has rested over winter, into the 

 expanding peach buds. According to Sadebeck, the mycelium 

 winters over in the primary cortex and medullary tissues of the 

 one-year-old branches. 



In order to examine the mycelium to the best advantage a 

 section should be made of a leaf or twig before the fungus has 

 appeared upon the surface. The distribution of the mycelium 

 within the host tissues may then be more easily followed, owing 

 to the greater protoplasmic content. A microscopical study of 

 hand or microtome sections indicates that the intercellular my- 

 celium is quite generally distributed in the parenchyma of the 

 leaf and in the cortex of the young stems. Three types of my- 

 celium have been recognized (Pierce), and these may be detected 

 in leaf or in shoot : 



1. The most common type is designated vegetative hyphce. 

 These are very diverse, both in the diameter of the tubes and 

 in the character of the branching, as shown in Fig. 66, b. Ad- 

 jacent cells are separated by peculiar plate septa. 



2. The second class of hyphae are known as distributive hyphce, 

 and these are in the main composed of long cells of more or less 

 uniform diameter, coursing, for the most part, parallel to the stem 

 axis, and they are found abundantly in the pith or beneath the 

 epidermal cells (Fig. 66, c). 



3. Fruiting hyphce. The vegetative hyphae which have devel- 

 oped beneath the epidermis push up between the epidermal cells, 

 and there is formed between the upper edges of the epidermal 

 cells, and also between the epidermis and the cuticle, an extensive 



