APPLE BLOTCH AND ITS CONTTtOL. 6 



dermis are not invaded directly and are affected only indirectly as 

 the diseased epidermis may permit the entrance of rot organisms or 

 as extensive cracking may affect the flesh. Not only may the fruit 

 be cracked open and otherwise made unsightly by the disease, but in 

 severe cases it may be so dwarfed and distorted as to be wholly worth- 

 less. Even on very young spots the tiny glistening pycnidia of the 

 causal fungus are always in evidence. 



On the leaves (PL II, figs. 1 and 2) the disease appears as small 

 nearly white spots scarcely more than a millimeter in diameter. Usu- 

 ally a single pycnidium of the fungus is to be found near the center of 

 the spot. Under ordinary conditions blotch is not to be considered a 

 serious foliage disease, but in cases where an orchard is severely in- 

 fected the spots on the leaves are numerous enough to affect seriously 

 the synthesis of carbohydrates and even cause defoliation. 



On the young twigs and water sprouts the disease is first noticed 

 as a dark raised area, soon dotted with numerous pycnidia. Later 

 the infected part becomes somewhat sunken. As the twig becomes 

 more mature the infected portion becomes lighter colored, and during 

 the next season the diseased area will appear as an older tan-colored 

 portion surrounded by a newer dark-colored area (PL II, figs. 3 

 and 4). In the older portion the fungus is dead, but in the newer 

 portion it will be found alive, extending the previous year's destruc- 

 tion. In ordinary cases not many twigs are killed and few are badly 

 injured (PL II, fig. 6) ; but in sections in which the disease is severe, 

 and especially on certain varieties, such as the Northwestern {North- 

 western Greening), the infection may be so severe as actually to kill 

 the tree. 



The fruit spurs may be attacked and in extreme cases even killed. 

 In uncared-for orchards or in orchards in which the disease has been 

 very prevalent and without proper control by means of spraying, 

 many of the fruit spurs may be injured seriously. 



CAUSE OF THE DISEASE. 



The cause of the disease is the fungus Phyllosticta solitaria, first 

 described by Ellis and Everhart (2) 1 in 1895. The type specimen 

 was found on the leaA^es of the wild crab apple (Pyrus coronaria L.). 

 The specific name is suggestive of the tendency of the fungus to form 

 a single pycnidium in each spot on the leaf. 



REVIEW OF THE LITERATURE. 



In 1902 Clinton (1) published a description of the disease as it 

 occurs on the fruit of the apple and considered it due to an unknown 

 species of Phyllosticta. He also noted that pycnidia on the fruit 

 produced fewer spores as the season advanced. 



1 The serial numbers in parentheses refer to " Literature cited," p. 11. 



