6 BULLETIN 120, U. S. DEPARTMENT OF AGRICULTURE. 



When a virulent infection becomes established on a leaf, the mildew 

 frequently extends down the petiole to the twig, where it may con- 

 tinue to grow until it covers practically the entire surface of the bark. 

 In such cases the new leaves become badly infected as rapidly as they 

 come out. Plate IV, figure 1, illustrates this condition. Mildewed 

 shoots are stunted in their development, the internodes are very much 

 shortened, and a year's growth may be compressed into an inch or 

 two. Plate IV, figure 2, and Plate VI, figure 2, show a number of 

 examples, and it will be seen that in many cases the affected portion 

 of the twig is considerably thickened. The ends of many of these 

 diseased twigs die back during the winter, and in the following 

 spring a shoot is sent out from a lateral bud, as shown in Plate IV, 

 figure 2, a. In the case of the largest twig shown in Plate IV, 

 figure 2, this dying back of the terminals and their replacement by 

 laterals has occurred twice, as shown at a and a'. Some idea of the 

 abundance of these twig infections can be obtained from Plate I, 

 figure 1, an unsprayed tree, in which it will be seen that practically 

 all of the terminals have been attacked and the greater portion of 

 their foliage has died and dropped off, the few remaining leaves be- 

 ing stunted and distorted. Plate I, figure 2, shows a sprayed tree 

 in the same block as that shown in Plate I, figure 1. A close exami- 

 nation of the size and abundance of the leaves shown in Plate I, 

 figure 1, as compared with those shown in Plate I. figure 2, will 

 impress one with the necessity of controlling the mildew if the vigor 

 find future crop prospects of the tree are to be given proper consid- 

 eration. Similar comparisons may be made between Plates II and 

 III, though the illustrations do not bring out the strong contrast, 

 which exists between the sprayed and unsprayed trees. In winter, 

 when the foliage is off the trees, the grayish mildew covering remains 

 on the twigs, and they glisten in the sunlight when viewed from the 

 proper angle. One is astonished at their abundance, for unsprayed 

 trees fairly bristle with them, and it is probable that in a third of 

 the orchards of the valley more than 50 per cent of the terminals 

 are diseased. 



Under the microscope the fungus presents the appearance of a 

 much-branched and loosely interwoven tangle of very fine threads 

 lving on the surface of the leaf or shoot. Scattered all through this 

 tangle and forming a powdery layer on its surface are enormous 

 numbers of minute reproductive bodies called conidia, or summer 

 spores. Collectively the fine fungous threads are termed the my- 

 celium. The mycelial threads branch and rebranch as they grow 

 over the surface of a leaf or shoot (fig. 2), and at intervals short, 

 saclike processes, called haustoria. penetrate the outer, or epidermal, 

 layer of the leaf or twig and by means of these haustoria the fungus 

 absorbs its nourishment. Thus the mildew derives its food mate- 



