9 



outline, each producing its concentric rings of fruiting pustules. 

 Most of the diseased fruits fall to the ground by the time or before 

 they are half decayed, but some may become entirely rotten, dry up 

 in a mummified condition, and hang on the tree for a year or more. 

 This disease is illustrated in figure 1 , which shows an apple affected 

 with bitter-rot and a mummified fruit from the preceding year's crop. 



THE CAUSE. 



The familiar spots on the apple known as bitter-rot are caused by the 

 growth in the tissues of the fruit of the fungus Glomerella rufomaculans 

 (Berk.) Spauld. and v. Schrenk. The infection starts from very 

 minute spores, which, falling on the surface of the apple, germinate in 

 the presence of moisture, and the resulting tube-like growth penetrates 

 the skin and immediately begins to branch and grow rapidly, destroy- 

 ing the invaded tissue and soon producing the circular, brown, sunken 

 area already noted. The fungus penetrates toward the center of the 

 apple about in proportion to its rate of growth as seen on the surface, 

 and eventually the fruit, as it decays, becomes filled with the minute, 

 almost colorless threads of the fungus. 



After a time spore-bearing branches of the fungus are produced 

 near the surface of the diseased spot, and these soon rupture the skin, 

 giving forth pinkish masses of spores or conidia, by which the disease 

 is principally spread. From a single diseased spot are produced many 

 millions of spores, sufficient to furnish infection under certain con- 

 ditions for an entire orchard. On the decayed apples which have 

 fallen to the ground the perfect stage of the fungus is produced during 

 the autumn and perhaps also the following spring. This stage serves 

 to carry the fungus over winter and its spores are perhaps a source of 

 infection of the new crop of apples. With the warmth and moisture 

 of spring and early summer the fungus in mummified fruits on the 

 trees resumes growth, producing conidia or summer spores, which 

 serve to infect the new crop. 



Canker. — The bitter-rot fungus also occurs on limbs and twigs of 

 the apple tree, affecting the bark and cambium and producing what 

 are known as cankers. The diseased area is sooty black, sunken, and 

 from 1 inch to several inches in length, and the bark usually becomes 

 cracked. Both winter and summer spores are formed in these 

 cankers, which doubtless constitute an important source of infection 

 of the fruit. 



CLIMATIC INFLUENCES. 



Bitter-rot is essentially a hot-weather disease, and its period of 

 infection is mostly during the months of July, August, and September. 

 Injury from it is rarely noticed before the middle of June, but with 

 27050— No. 283—07 2 



