210 Diseases of Economic Plants 



scale to scale within the bulb. In dry onions in storage its 

 progress is slow, but wet onions decay rapidly. The only 

 suggestion toward control is to keep onions dry in storage. 



Minor diseases 



Black-mold (Macrosporium parasiticum Thiim. and M. 

 porri Ell.). — Frequently onion leaves that have been in- 

 jured assume a velvety, black coating, which often appears in 

 spots, or covers them entirely. Treatment against other 

 diseases, and maintenance of the plants in full vigor is of 

 most service in this case. Another cause of black-mold -^^ is 

 Sterigitiatocystis nigra v. Tiegh. It most commonly appears 

 forming dark masses of spores, resembling a smut, under the 

 outer scales of the bulb. In this form, the injury is due only 

 to unsightliness. Frequently, however, the fungus grows 

 into the lower scales of the onion, causing a slimy, soft rot. 

 It is not essentially a field disease, but one of chief importance 

 during storage. Sanitary measures to prevent the develop- 

 ment of the mold on refuse in the field are advised. Infested 

 sacks, crates, bins, etc., should be disinfected. Pink-root -^^ 

 (Fusarium malli Taub.). — The affected roots turn yel- 

 low, later pink, then dry up. New roots that develop 

 also become diseased. The bulbs are dwarfed. The loss 

 sometimes is as great as 40 per cent of the crop. The 

 infection comes from the soil, and the general remarks 

 under soil diseases (p. 26) apply here. Two rusts occur, 

 Pucdnia porri (Sow.) Wint. on the chive, P. allii DC. on 

 onions. 



PARSLEY. See celery. 

 PARSNIP. See celery. 



PEA 



Spot '^^ {Mycosphcerella pinodes Berk & Blox., Asco- 

 chyta). — Spotting of pea leaves and pods has been known to 



