42 MISC. PUBLICATION 14 0, U. S. DEPARTMENT OF AGRICULTURE 



on the roots and tops of bunched carrots. The decay is recognized 

 by the soft, mushy, and frequently slimy condition of affected 

 tissues. Stored carrots may be affected anywhere on the root but 

 frequently are attacked at the crown, with the decay extending deep 

 into the central part of the taproot. (See Celery, Bacterial Soft 

 Rot, p. 46.) 



BLACK ROT 



(Alternaria radicina F. Meier, Drechs., and E. D. Eddy) 



OCCURRENCE. SYMPTOMS, AND EFFECTS 



Black rot has been reported on the market in Xew York, Massa- 

 chusetts, Pennsylvania, and the District of Columbia. It is essentially 

 a market disease of topped carrots that have been held in storage. 

 Although reported on celery, parsley, and parsnips by certain Euro- 

 pean investigators it has been found in this country only on carrots. 



The decay is readily identified by the presence of black, slightly 

 depressed lesions at the crown or at any point over the surface of the 

 root (pi. 12, B). When it occurs at the crown the decay either may 

 be shallow or may extend deep into the central part of the root. 

 Lesions on the side of the root are more conspicuous than those at 

 the crown but are actually less damaging because the decay is con- 

 fined to the outer tissues. Such lesions are irregular in shape, vari- 

 able in size, and may occur either singly or otherwise. In all cases 

 the affected tissues are somewhat soft and are greenish black to black 

 in color. The characteristic black growth of the causal fungus may 

 or may not be present on the surface of the lesions. 



By artificial inoculation the disease can be produced on the foliage 

 of the carrot plant, where it resembles in great measure the macro- 

 sporium leaf blight. (See p. 44.) In this country it has not been 

 found occurring naturally as a leaf disease. 



Black rot is responsible for considerable loss in storage and on the 

 market, particularly in the Northeastern States. Because of its slower 

 rate of development it is not as important as bacterial soft rot, gray 

 mold rot, or watery soft rot. As a crown rot it renders the carrot 

 unfit for food. Where it is found chiefly as a side rot it disfigures 

 the root and furnishes favorable points of entry for the organisms 

 responsible for more destructive decays. 



CAUSAL FACTORS 



Black rot is caused by the fungus Alternaria radicina. This organ- 

 ism is undoubtedly capable of continued existence on dead plant 

 material in the soil. The carrot root is thus readily infected in the 

 field. It is entirely possible that infection may likewise occur in 

 storage cellars where diseased roots of the previous season have been 

 stored, or where contaminated soil or plant debris has been brought 

 in with the current season's roots. 



The pathogen can penetrate the apparently unbroken skin of the 

 root. Infection is facilitated, however, by the presence of wounds. 

 Fresh or old wounds, either at the crown or side, together with the 

 broken remains of small rootlets are the chief infection courts. In 

 Europe the pathogen has been reported to be carried on the seed, 



