518 MICROBIAL DISEASES OF PLANTS. 



turn yellow, then brown, and finally black. If a diseased stem is split 

 lengthwise, black streaks, following the fibro-vascular bundles, can be 

 traced the whole length of the stem and often out into the corresponding 

 leaves. The vessels are packed with bacteria which ooze out on the cut 

 surface as little drops of a dirty white, slightly viscid liquid. The bacillus 

 destroys the parenchyma of the pith and bark and mechanically plugs the 

 water tubes so that the water supply from the soil is shut off and wilting 

 follows. In the tubers of the potato, the rot begins in the blackened 

 vascular ring and spreads in all directions, producing well defined cavities 

 next to the ring. 



METHOD OF INFECTION. Insect enemies are largely responsible for 

 the spread of the wilt, especially above ground, while beneath the surface 

 inoculated soil enters the roots through wounds made either by trans- 

 planting, cultivating, or nematodes. 



CAUSAL ORGANISM. -According to Smith, Bacillus solanacearum is a medium- 

 sized rod, rounded ends; 1.5^ by 0.5/1; motile by several peritrichiate flagella, zooglcea 

 formed in liquid media ; stains readily with aqueous stains. 



Zoogloea produced at the surface in beef broth, copious dirty white sediment, 

 reaction made alkaline. Casein of milk dissolved without precipitation and medium 

 becomes alkaline. On nutrient agar, growth is smooth, wet shining, slightly viscid, at 

 first dirty white becoming yellowish, then brown; agar browned. Gelatin stab growth 

 best at surface, pure white, smooth, wet shining, no liquefaction or very feeble after six 

 weeks. Potato 'wet shining, not wrinkled, copious, dirty white and later brown to 

 black; medium browned. Neither acid not gas produced in any of the culture media or 

 from glucose, etc. Obligate aerobe; ammonia produced in nutrient broth and potato 

 tubes; pigment formation aided by glucose, fructose and saccharose. Grows well at 

 37. Thermal death-point, 52. 



PATHOGENESIS. Pathogenic for tomato, potato, eggplant, tobacco, 

 Jamestown weed, black nightshade, physilis and petunia. 



CONTROL. If the disease is not too general, it is possible to control 

 its spread by removing the dead plants and burning them; the early and 

 complete destruction of all insect pests is important; if available and 

 practical, new land or land which has not been planted to any of the 

 potato family for a period of years, should be used; only those seeds and 

 tubers which have come from plants grown in localities free from the 

 disease should be planted; the use of infected manure or soil should be 

 avoided. 



