628 Journal of Agricultural Research vol. vi, No. 17 



tuber, and, -if possible, to obtain some data as to methods for its control. 

 Part of the work was carried out at Stockton, Cal., and at various points 

 in the delta potato fields near that city. 



GENERAL APPEARANCE OF THE DISEASE 



In taking up the study of the disease in the field, potatoes were ex- 

 amined at the sorting benches in the warehouses at Stockton, and various 

 stages of the disease were observed. It was first apparent as a small 

 brown discoloration around some wound, such as the wound made by the 

 prong of a digging fork or by the breaking off of a "knob," which exposed 

 the tissue of the inner part of the potato. The rot apparently did not 

 affect tubers with unbroken skins. In the later stages of the disease 

 the potatoes were brown over the entire surface, soft, and easily crushed. 

 If sufficient pressure were applied to the tubers, a brownish watery liquid 

 was exuded through breaks in the skin. Sacks containing potatoes in 

 the advanced stages of this disease were frequently wet in patches where 

 the rotten tubers had been crushed against -the side. The interior of the 

 rotten potato when broken was usually a dirty white, soon changing to a 

 brown color around the edges. The center generally remained white for 

 sometime (PI. XC). 



ORGANISM CAUSING LEAK 



RHIZOPUS NIGRICANS 



Orton proved that the disease was caused by a fungus and concluded 

 that the causal organism was Rhizopus nigricans Ehrenb. (10). He based 

 his conclusions on the following premises: He observed a nonseptate 

 mycelium in the rotted tubers and obtained R. nigricans in cultures made 

 from these potatoes; he inoculated potatoes with this fungus and 

 produced a rot similar in all appearances to leak. 



That R. nigricans is able to rot Irish potatoes was also shown in 

 unpublished studies by Mrs. Ethel Field Tillotson. In her experiments 

 she used a strain of R. nigricans isolated from sweet potato (Ipomoea 

 batatas). Her method of inoculation was to germinate the spores of 

 the fungus in tubes of potato decoction and then pour the liquid, together 

 with the germinated spores, into cavities in the potatoes. The inoculated 

 tubers were placed in damp chambers, and in a few days the disease 

 was evident. 



With a strain of R. nigricans isolated from sweet potato by Mr. L. L. 

 Harter the present writer was able to inoculate Irish potatoes successfully. 

 The method developed by Mrs. Tillotson was followed in the earlier experi- 

 ments. It was found unnecessary, however, to germinate the spores before 

 inoculating the potatoes. Accordingly, in the later inoculations the 

 tubers were inoculated directly from a culture of the fungus by inserting 

 some of the spores and mycelium into rather deep wounds made in the 

 tubers with a sterile knife. The inoculated potatoes were then placed in 



