PEACH SCAB AND "[TS CONTBOL. 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 haK 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 surromidijig normal 

 surface (PL 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 macroscopicaUy 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 gTowth. 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 natiu-e, 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- 



