conspectus: period of incubation 17 



parasitic organism to multiply inside the plant before it does 

 damage serious enough to be recognized externally as a dis- 

 ease. This is the so-called "period of incubation," during 

 which the parasite is growing and its enzymes and toxins are 

 becoming active. The microscope shows it to be present in the 

 tissues, but the latter have yielded only a httle in the immediate 

 vicinity of the bacterial focus. This time is short or long 

 depending on whether the parasite or the host has the first 

 advantage. If the host is growing rapidly it may either en- 

 tirely outstrip the parasite, or be only so much the more sub- 

 ject to it. All depends on whether the parasite finds the initial 

 conditions entirely suited to its needs, or by means of its secre- 

 tions and excretions can quickly make them so, and conse- 

 quently can from the start make a rapid growth, or must 

 first slowly overcome obstacles of various sorts, such as in- 

 hibiting acids and resistant tissues. The plant may show signs 

 of infection within as short a time as one or two days after in- 

 oculation (various soft rots), or it may be as long a time as one 

 to two months before they appear (Cobb's disease of sugar-cane, 

 Stewart's disease of sweet-corn). In the latter, infection gener- 

 ally occurs in the seedling stage and the maize plant may be three 

 months old and six feet tall before it finally succumbs. Of 

 course, as in case of bacterial animal diseases, the greater the 

 volume of infectious material, the shorter the time. I have 

 seen many instances of that law. In general, the period of 

 latency may be said to vary from one to three weeks (yellow 

 disease of hyacinth, black rot of cabbage, black spot of plum, 

 cucurbit wilt, pear blight, angular leaf-spot of cotton, sorghum 

 leaf -stripe, etc.). The longest period of latency I have observed 

 was two years. This was in crown gall on orange (see Fig. 342). 



RECOVERY FROM DISEASE 



Mention has already been made of the self-hmited spot" 

 diseases and bUghts. As the actively growing season draws 

 to a close such diseases cease their activity. 



Also in some plants well developed signs of vascular dis- 

 ease may be suppressed (squash, maize, sugar-cane, etc.), or 



