384 F. M. Rolfs 



injured early in the season usually carry a very light crop of fruit the 

 following year. Young trees are usually more affected than those six 

 or seven years old, and badly diseased young trees soon become stunted 

 and in some cases permanently injured. 



SYMPTOMS 



The disease affects the leaves, twigs, and fruit in a characteristic manner. 

 It develops on twigs and shoots of only the current year's growth, but the 

 wounds on the twigs often persist as open perennial cankers. 



The very common fungous disease of apricot and plum foliage due 

 to Cylindrosporium padi is similar in its symptoms to bacterial leaf spot, 

 but usually can be distinguished from the latter by the white downy 

 growth on the lower surface of the spot. Pierce (1894) reports a similar 

 disease on almond foliage in California due to the work of Cercosyora 

 circuniscissa. There is also a leaf spot of unknown cause which is con- 

 stantly associated with peach and plum rosette. Spray mixtures and other 

 toxic substances applied to the leaves often produce an injury which 

 is difficult, without the aid of a microscope, to distinguish from bacterial 

 leaf spot. Care must be taken not to confuse the lesions due to these 

 various agents. 



On the leaves 



The first evidence of the disease on the leaves is the appearance of 

 small gray specks (Fig. 59), which become somewhat angular in form 

 and soon take on a water-soaked appearance. They later become brown 

 or purplish brown, in some cases scarlet, and finally change to dark brown. 

 The spots measure from one to five millimeters in diameter and when 

 numerous they coalesce, so that large areas of the leaf may be involved. A 

 more or less circular diseased area is finally separated by a line of cleavage 

 from the healthy tissue, the injured tissue within this area promptly 

 contracting, drying, and falling cut. This gives the leaf the shot-hole 

 appearance. On the plum, in some cases, after the disease has apparently 

 run its course and the spot is well formed, a second and even a third 

 period of invasion may fellow and an area one-half inch or more in diameter 

 may develop from a single spot. Leaves that are badly diseased soon 

 fall to the ground. It is not unusual for an infected tree to lose from 

 fifteen to twenty-five per cent of its leaves by August, and in the more 

 extreme cases as high as seventy-five per cent, or even more, of the leaves 



