8 BULLETIN 305, U. S. DEPARTMENT OF AGRICULTUEE. 



meter in diameter. The color of the affected surfaoe may be a pale 

 green, almost indistinguishable from the normal, or it may vary to 

 light brown or pinkish purple. The true color, however, is frequently 

 modified or masked by an abmidant development of olivaceous conidi- 

 ophores and conidia. These are usually present in greater or less num- 

 bers when the infection first becomes macroscopic. When conditions 

 especially favor their development, they may be borne in sufficient 

 quantities to give a dark-green color to the lesions and to constitute 

 their most conspicuous character. On the leaf lamina the fully 

 developed lesions are somewhat angular to irregularly circular. Their 

 average diameter is 1 to 2 millimeters, though in rare cases they may 

 attain a diameter of 5 or 6 millimeters (PI. I, fig. 7, and PI. IV, fig. 1). 

 On the petioles and midribs they are usually much longer and nar- 

 rower, generally measuring 3 to 8 by one-half to 1 millimeters. The 

 color of the blotch proper may vary from light brown to dark brown, 

 or it may be pinkish purple. The demarcation remams indistinct 

 and the elevation is usually unchanged, except in extreme cases, 

 when the lamina above the larger lesions may become convex below 

 and correspondingly concave above. When the lesions first appear, 

 no evidence of disease is visible upon the upper surfaces of the leaves. 

 In later stages, however, the tissues above and immediately around 

 the affected areas may lose some or aU of their green color and become 

 pale yellow or purplish. In extreme cases such areas may die, but 

 ordinarily no definite leaf spotting is induced. The lesions may occur 

 miscellaneously scattered over the lower surfaces of the leaves and 

 upon the petioles, or they may develop in such abundance as to 

 become confluent and form irregular patches (PL III, fig. 2). The 

 infection appears to occur with equal readiness on the petioles, 

 midribs, veins, and leaf lamina. It was demonstrated in the inocula- 

 tion experiments that badly diseased leaves may turn yeUow and fall, 

 but the writer has rarely observed this condition as the result of 

 natural scab infection. 



PATHOLOGICAL HISTOLOGY. 



Histological technique. — In kilfing material for histological work, 

 several standard fixing agents were employed, viz, Flemming's weak, 

 medium, and strong fluids, picro-formal, and chrom-acetic. The 

 stains used were Flemming's triple, Haidenhain's iron alum- 

 haematoxylin, Durand's haematoxyhn-eosin, gentian violet, and 

 safranin. 



For most purposes, Flemming's medium fluid and the triple stain 

 gave the best results. This combination not only stained the fungus 

 weU, but gave an excellent differentiation of fungus and host in twig 

 and fruit lesions. The cuticle of the host remained practically 

 colorless, while the dead epidermal and subepidermal ceUs took fittle 

 stain, being yellowish green in the finished preparation. In sharp 



