122 Journal of Agricultural Research voi. xvih.no. 3 



PERIOD OF INCUBATION 



The findings of the writer substantiated those of others — ^namely, that 

 with growing plants such as tomatoes, peanuts, com, cotton seedlings, 

 sweet potatoes, and others the period of incubation ranges from 2 to 4 

 days, depending on the tenderness of the growing tissue. Wilting usually 

 begins after the second day, and the plant usually dies in from 4 to 6 

 days after inoculation. In inoculations of tubers of Irish potatoes, 

 infection becomes apparent in about 6 days. After 15 days the potatoes 

 are about half rotted (PI. 5, B), the injury being a typical "melter," 

 the tubers becoming very soft and at the least pressure rupturing and 

 liberating a clear liquid with an agreeable odor of fermentation. With 

 sweet potatoes the period of incubation is about the same as that of 

 Irish potatoes, with the difference that the rot produced is not of the 

 "melter" type. The infected tissue becomes browned, water-soaked at 

 first but firm, then hard and stringy (PI. 5» C). With the cantaloupe 

 the period of incubation is usually from 3 to 6 days, after which the rot 

 progresses very rapidly. If the inoculated melon is so placed that the 

 point of infection touches the glass, the rot works so rapidly that it 

 practically melts away half of the fruit, leaving a ragged cavity (Pi. 3, E). 

 On the other hand, if after inoculation the fruit is so placed that the 

 point of infection is turned upward and not allowed to touch the glass, the 

 rot progresses slowly without producing a rapid soft rot. At the same 

 time the fungus hyphae permeate the fruit (PI. 3, F) and form a luxuriant 

 growth which spreads all over the surface of the cantaloupe (Pi. 3, D). 

 After the contents of the fruit are destroyed by the fungus, all that 

 remains is a compacted mass of mycelium, which later rounds itself up 

 into one mass of small sclerotia (Pi. 3, C). With the watermelon or the 

 squash the period of incubation varies from 8 to 10 days, the inoculated 

 fruit dry-rotting very slowly. The incubation period for the banana 

 varies from 4 to 8 days, and for the orange and the apple from 6 to 10 

 days. Once infection starts these fruits rot very rapidly. 



MODE OF INFECTION 



It has already been pointed out that abrasion of the host is not neces- 

 sary for infection. This is especially true with tender growing plants. 

 Of paramount importance to infection may be considered moisture and 

 especially air. F'or successful soil inoculations it is necessary to cover 

 the fungus not more than K to i inch deep. Numerous laboratory 

 experiments indicate that no infection is possible if either the mycelium 

 or the sclerotia are buried more than 5 inches deep. This suggests that 

 deep plowing would control the trouble. Furthermore, infection seems 

 possible only where a young and actively growing culture is used. 



Infection seems to be favored by an enzym secreted by the advancing 

 mycelial strands. Examination of the roots or tubers infected with 



