THE RELATION OF THE ENZYM PECTINASE TO INFECTION 
OF SWEET POTATOES BY RHIZOPUS 
L. L. Harter and J. L. Weimer 
(Received for publication April 28, 1922) 
It has been demonstrated that, if the proper conditions are maintained 
in the storage house, sweet potatoes can be kept for a considerable length 
of time with practically no loss from soft rot. If, on the other hand, the 
conditions are unfavorable, infection followed by decay may take place. 
The cause of the soft rot has long been suspected to be due principally 
to Rhizopus nigricans Ehrb. (14, 19). Before positive proof of its parasitism 
was obtained, some difficulty was experienced in isolating the organism 
from the rotting potato and still more in obtaining infection by it. If, as 
is customary in pathological technique, the plate plantings were made 
from tissue close to the healthy zone, a sterile culture was almost invariably 
obtained. If, on the other hand, the plantings were made from rotted 
tissue several millimeters back of the undecayed tissue, a pure culture of 
the organism was usually obtained. This suggested that there was some 
‘‘action in advance” of the growing hyphae. Harter and Weimer (17) 
showed that this action or dissolution of the middle lamellae in advance of 
the growing hyphae is due to a substance of the nature of an enzym, which, 
following the precedents established by Bourquelot and Heressey (1), Jones 
(22), Euler (11), Zeller (37), and others, they have designated as pectinase. 
Bruschi (6) found that the cells of plums on which Monilia cinerea 
(Bon.) Schroter had grown for two days were separated along the line of 
the middle lamella, and concluded from her results that the enzym pectinase 
was produced by the fungus. Munn (28) obtained similar results with 
the fungus causing the neck rot of onions. He demonstrated the production 
of oxalic acid, but from the nature of its action came to the conclusion 
that it has little or nothing to do with the maceration of the tissue. Munn 
clearly demonstrated that an enzym which he calls pectinase was responsible 
for the disintegration of the tissue noted. 
Although Rhizopus nigricans has been quite generally accepted as the 
cause of the soft rot of sweet potatoes, its causal relation has been somewhat 
difficult to prove. The usual method of placing spores or spores and 
mycelia upon the unbroken surface or upon a wound usually gave negative 
results, even when the potatoes were placed in a moist chamber lined with 
wet filter paper. No better results were obtained when the spores and 
mycelia were injected deeply into a wound made with a needle or forceps. 
If, however, the method described by Harter, Weimer, and Adams (19), in 
which the fungus was grown for one or two days in sweet-potato decoction 
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