6 BULLETIlSr 395, U. S. DEPARTMENT OF AGRICULTURE. 



DESCRIPTION. 



On fruit. — The earliest macroscopic evidences of infection upon the 

 green surfaces of the fruit appear as imperfectly defined, greenish to 

 oHvaceous, circular areas, usually less than half a millimeter in diam- 

 eter. This effect is due to the appearance of the olivaceous conid- 

 iophores and conidia of the fungus among the hairs of the fruit. 

 Upon blush surfaces, the early development of olivaceous color may 

 be accompanied or preceded by the disappearance of the normal pink 

 from the invaded areas, which remain much paler than normal until 

 masked by the development of the fungus. In such cases a narrow 

 peripheral zone of pale or yellowish green may mark the advance of 

 each diseased area. The developing lesions gradually become larger, 

 darker, and more clearly defined, owing to the death and disappear- 

 ance of many of the obscuring hairs of the fruit and to growth and 

 pigmentation of the fungus. When fully developed they are fairly 

 well defined, circular, olivaceous to black areas with an average diam- 

 eter of 2 to 3 millimeters, though under very favorable conditions 

 they may attain a diameter of 4 or 5 milhmeters (PI. I, figs. 1 and 2, 

 and PI. II). The direct injury is superficial, involving at most only 

 a few layers of cells, which typically become separated from the adja- 

 cent normal tissues by the formation of cork. The infections occur 

 practically exclusively upon the areas which are subject to thorough 

 wetting. The lesions are, thus, generally most abundant about the 

 peduncle and upon the upper portion of the wettable ^ surface (PI. 

 Ill, fig. 1, a). They may be more or less uniformly scattered over 

 the entire wettable surface, or they may be so abundant as to become 

 confiuent over large patches, frequently covering practically one-haK 

 of the surface of the fruit. The protected surfaces of even the most 

 severely attacked" fruits are usually free from infection (PI. Ill, fig. 

 1, &). As the peaches grow rapidly just prior to maturity, the corked 

 areas can not expand so readily as the normal tissues. This condi- 

 tion may result in mere inequahties of dei^elopment between badly 

 diseased and normal areas, but more commonly the growth stresses 

 occasion the cracking of the cork layers. These cracks may appear 

 upon individual lesions as small rifts extending shghtly into the flesh, 

 but upon very badly affected specimens they may extend the length 

 of the fruit and penetrate to the pit (PI. I, fig. 1). Badly scabbed 

 fruits do not ripen evenly and are frequently inferior in flavor. Those 

 which are severely scabbed about the attachment of the peduncle 

 may be shed prematurely. 



1 In the earlier stages of development of the peach, its hairy surface is very resistant to wetting. Witli 

 growth and weathering, however, that portion which is exposed to the action of falling meteoric water 

 becomes more easily wettable, while the area protected from such action, usually the opposite side of the 

 fruit, remains diflicultly wettable throughout the important period of scab infection. For the sake of 

 convenience, these areas subsequently will be referred to as "wettable" and "protected," though it should 

 be realized, of course, that these terms are relative, not absolute. 



