630 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 Uquid. 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 aU directions, producing well- 

 J 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 

 transplanting, cultivating, or nematodes. In the case of the nastur- 

 tium, stomatal infections have been demonstrated. 



Causal Organism.— According to Smith, Psevdomonas solanacearum is a 

 medium-sized rod, rounded ends; i.Sm by o-SMi motile by a single polar flagellum, 

 zoogloese formed in liquid media; stains readily with aqueous stains. 



Zoogloefe 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 nor gas produced 

 in any of the culture media or from glucose, etc. Obligate aerobe; ammonia pro- 

 duced in nutrient broth and potato tubes; pigment formation aided by glucose, 

 fructose and saccharose. Grows well at 37°. Thermal death-point, 52°. 



Pathoge'nesis. — Pathogenic for tomato, potato, eggplant, tobacco, 

 Jamestown weed, black nightshade, physilis, petunia and nasturtium. 



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. 



