240 



THE FUNGI WHICH CAUSE PLANT DISEASE 



the leaf on such varieties as have leaves which are smooth beneath. 

 The pyncidia, however, have never been seen on the under side 

 of the leaf in our varieties. 



On stems, tendrils, peduncles, petioles and leaf veins the 

 spot in its first appearance is a small darkened depression which 

 soon becomes very black. On a cane the lesion rarely extends 

 more than a quarter of the way round, while on a tendril or leaf 

 petiole it may extend from half to all of the way round. On shoots, 



the lesions never extend so 

 deep as to cut off the sap 

 supply, but on petioles this 

 occasionally happens, rarely 

 so on peduncles, and quite 

 commonly so on pedicels and 

 tendrils. The first indication 

 of Black Rot on the berry 

 is the appearance at some 

 point of a small circular 

 blanched spot, scarcely 1 mm. 

 in diameter. The blanching 

 is so slight as to be detected 

 only by careful observation. It rapidly becomes more apparent 

 and has a whitish appearance, the contrast becomes more ap- 

 parent by the appearance of a brownish line at the mar- 

 gin. The whitish center increases in size and the brownish or 

 reddish-brown ring increases in diameter as well as in width 

 and is quite evident when the spot is 2 mm. in diameter. When 

 the spot is 3 mm. in diameter the ring is one-half mm. in width 

 and enough darker to give a bird's eye effect (a light circular 

 disc with an encircling darker band). The spot rapidly increases 

 in size so that in twelve hours more it may be 6 to 8 mm. in diam- 

 eter, and the encircling band nearly 2 mm. in width. After five 

 hours more, the spot is 8 or 9 mm. in diameter and there begins 

 to appear an outer darker band and an inner lighter brown one 

 which have in some cases a much lighter line between them. The 

 aureole is thus composed of two or three bands or rings. Eighteen 

 hours later, the spot is 1 cm. or more in diameter, is distinctly 

 flattened, and numerous minute brown specks appear on the 



Fig. 175. — Section of a pustule showing 

 microconidia. After Longyear. 



