PEACH SCAB AND ITS CONTUOL. 11 



of the dead epidermal cells are compressed inward in extreme cases 

 until tbeir identity is almost lost. In such cases the fungus often 

 grows down between the epidermal cells, and occasionally hyphse 

 penetrate the intercellular areas of the first subepidermal layer. 



After the protective corky layers are fully formed, no further 

 pathological anatomical changes occur. As the bark forms and 

 roughens with the second year's growth of the twigs, the cuticle of 

 the diseased areas partially sloughs off; and, as the bark thickens 

 with subsequent development, the lesions gradually lose their 

 identity. 



Leaf lesions. — In the early stages of leaf infection, the slender 

 hyphae of the fungus are found between the cuticle and the cellulose 

 walls of the epidermal cells of the lower (dorsal) surface. As the 

 fungus develops, its ramifications become more abundant, and 

 under favorable conditions conidiophores and conidia are produced. 

 Its ultimate development, however, is much less vigorous than upon 

 twigs or fruit, no pseudoparenchyma having been observed on the 

 leaf lamina. The invasion appears to be merely subcuticular and 

 the fungus is very inconspicuous, even in well-stained preparations.' 

 The chlorophyll usually disappears from the mesophyll cells imme- 

 diately adjacent to the invaded areas, while in extreme cases this 

 abnormality extends into the leaf lamina to such an extent that the 

 areas above the lesions appear distinctly yellow or purphsh when 

 viewed from the upper (ventral) surface. 



When infection occurs upon the petioles or larger veins, corky 

 layers similar to those described for fruit and twig lesions are formed. 

 In such cases the development of the fungus is distinctly more vig- 

 orous than on the leaf lamina, but much less so than on fruits and 

 twigs. 



THE CAUSAL ORGANISM. 



TAXONOMY. 



Von Thiimen (1877), at Klosterneuberg, Austria, first described 

 the peach-scab fmigus, naming it CladosforiuTn carpopTiilum. Two 

 years later the same author (1879, p. 13) emended his original de- 

 scription. Saccardo (1886, p. 353) gives Von Thiimen's emended 

 description in slightly abbreviated form. Arthur (1889, p. 7) reviews 

 the references cited above and gives a translation of Von Thiimen's 

 emended description. Oudemans (1901, p. 388) describes as Fusi- 

 cladium carpopMlum Oud. a fungus growing on fallen young peaches. 

 He lists as a synonym Cladosporium carpopJiilum Thiim., but neither 

 gives Von Thiimen credit as authority for the specific name nor 

 states reasons fojp transferring the fungus to the genus Fusicladium. 

 Aderhold (1900, p. 541-549; 1901, p. 656-657), after extensive 

 investigations, concluded that Fusicladium cerasi (Rbh.) Sacc. 

 should be referred to the genus Cladosporium. On comparing this 



