PEACH SCAB AND ITS CONTKOL. 7 
On twigs. — On tender young green twigs, the first macroscopic 
evidences of infection, typically, are barely visible, more or less im- 
perfectly defined, very slightly raised, irregularly circular to oval 
areas, almost indistinguishable in color from the surrounding normal 
surfaces. As the lesions develop, their central areas become light 
brown, and later darker brown, and each is bordered by a slightly 
raised peripheral zone, usually about half a millimeter wide and of 
approximately the normal green color. The coloration and zonation, 
however, may vary considerably with such factors as the age and 
character of the twig, seasonal conditions, and the stage of develop- 
ment of the disease. At the end of the growing season, well-developed 
lesions appear as smooth, irregularly oval, light-brown to dark-brown 
areas of normal elevation, with slightly raised purplish to dark-brown 
borders, which usually vary from one-half to 1 millimeter in width 
and shade off peripherally into the color of the surrounding normal 
surface (PI. I, figs. 3 to 5, and PI. IV, fig. 2). Such lesions usually 
measure 3 to 5 by 5 to 8 millimeters, the greater development being 
parallel to the axes of the twigs. The injury is superficial, involving 
only a few layers of the cells, the diseased areas becoming separated 
from the adjacent normal tissues by the formation of cork layers. 
Under favorable conditions, conidiophores, which bear typical conidia, 
may be produced upon the surfaces of the infected areas at any time 
after the lesions become macroscopic. During the summer and fall 
following the infection, the conidiophores are ordinarily produced 
singly, and seldom in such quantity as to be macroscopically evident. 
In the following spring, however, they are produced abundantly in 
olivaceous tufts. These sporiferous tufts may develop over the entire 
surfaces of the lesions, or they may be confined to peripheral zones, 
usually 1 to 2 millimeters in width, leaving the grayish brown central 
areas bare (PI. I, fig. 6, and PI. IV, fig. 3). Such lesions may enlarge 
slightly during the spring and early summer of the second season. As 
the summer progresses and the bark is differentiated, however, they 
gradually begin to lose their identity. By the third summer, they 
usually become indistinguishable, while the fungus rarely survives 
the second winter. Lesions on water sprouts or very rapidly growing 
twigs may lose their identity with the rapid cork formation of the 
first summer. Twig infections are miscellaneously distributed upon 
the wood of the current year's growth. They may be sufficiently 
numerous to become confluent and form irregularly shaped lesions, 
which may attain a length of several centimeters. On account of 
their superficial nature, however, they rarely cause appreciable injury 
to the twigs. 
On leaves. — The first macroscopic evidences of infection appear upon 
the lower surfaces of the leaves as indistinct, imperfectly defined, some- 
what angular to circular discolored areas, usually one-half to 1 milli- 
