10 MISC. PUBLICATION 16 8, tJ.S. DEPT. OF AGEICULTURE 



BLACK ROT 



{Physalospora malorum (Pk.) Shear)* 



OCCURRENCE. SYMPTOMS, AND EFFECTS 



Black rot is a disease of apples, pears, and quinces that is found 

 in most of the producing sections of the United States east of the 

 Rocky Mountains. Practically all varieties of pome fruits are sus- 

 ceptible, although the greatest losses usually occur on those ripening 

 in the summer and fall. 



Black rot is characterized in its early stages by brown rotten spots 

 on any part of the apple. These spots vary greatly in size and are 

 usually irregular in outline. Under field conditions they may show 

 zones of different shades of brown, for which reason the disease is 

 sometimes called ring rot (pi. 1, B) . In late stages the spots enlarge, 

 become black, and show numerous small black pimples scattered over 

 the surface. In these pimples (pycnidia) are produced the spores 

 which are the chief means hy which the fungus is distributed and 

 propagated. Apple flesh affected by the rot is always quite firm. 



CAUSAL FACTORS 



Black rot is caused by the fungus Physalospora Tnalorum-. For- 

 merly it was known as Sphaeropsis malorymi^ but recent investiga- 

 tions have shown that this represents merely a particular stage in 

 the life history of the fungus. Consequently the fungus is properly 

 referred to under the name of its perfect stage, Physalospora 

 inalorum. The disease can be produced by spores of either stage. 

 The Sphaeropsis stage, by far the more common of the two, is found 

 on twigs, limbs, fruit, and leaves; the Physalospora stage has been 

 found so far only on twigs. 



The leaf spots appear first as minute purple specks, which slowly 

 enlarge, turn yellowish brown, and eventually become more or less 

 lobed because of secondary growth of the pathogene from one or more 

 points. It is this lobing and the final grayish-brown color of the cen- 

 ter of the spot that have given rise to the name " frog eye " some- 

 times used for these spots. There is occasional secondary infection 

 and enlargement of such spots by a species of Altemaria. 



The characteristics of cankers produced by the black-rot fungus 

 are as follows : The affected bark is at first slightly sunken and red- 

 dish brown in color; later, as the spot enlarges, the bark becomes 

 dark brown or almost black, and cracks appear over the affected area. 

 Very often there is a crevice, a clear line of separation around the 

 edge of the lesion ; in old cankers the successive stages in their devel- 

 opment are indicated by a series of concentric crevices, which give 

 the canker an appearance somewhat like that described for " frog 

 eye " of the leaves. Limbs are sometimes girdled and killed by the 

 canker during the second or even during the first year; or infection 

 may merely extend up and down the limb, eventually producing a 

 diseased area 2 or 3 feet long or even longer, but not killing the limb 

 until after 2 or 3 years. On old cankers, and especially around the 

 edges, the outermost part of the bark frequently becomes loosened 



^ Syn., Physalospora cydoniae Am. Auct. 



