980 MICROBIAL DISEASES OF PLANTS 



vessels, especially those of the bulb, with a bright yellow bacterial slime. 

 In time, the walls of the vessels are destroyed and large cavities are 

 formed in the nbro-vascular bundles. The disease does not spread 

 rapidly from bundle to bundle in the bulb, but is confined for a long 

 time to the vessels first involved, a year or more being required for the 

 destruction of the host plant. This is due, largely, to the resistance 

 offered by the cells of the parenchyma to bacterial invasion. 



METHOD OF INFECTION. The causal organism enters through 

 wounds in the leaves and through the blossoms, and when the disease is 

 once established, it is probably spread by insects which visit the blos- 

 soms or eat the leaves. Daughter bulbs contract, the infection from 

 mother bulbs. Wakker believed the disease to be transmitted often 

 by knives used around sick plants. 



CAUSAL ORGANISM. Pseudomonas hyacinthi Wakker, according to Erwin F. 

 Smith, is a medium-sized rod with rounded ends, i.o/x to 2.o/* by 0.5^1 to 0.7^, motile 

 by one polar flagellum; non-spore forming. 



It grows well upon the ordinary culture media, on most of which, as well as in 

 the host plant, it produces a bright, chrome-yellow pigment. Gelatin and blood 

 serum are liquefied slowly (six to seven days). Milk is rendered alkaline, and the 

 casein is slowly precipitated. On nutrient agar, growth is copious, yellow, smooth, 

 wet-shining, translucent, spreading. On 20 per cent cane agar, the zooglcea formed 

 gives the growth a papillose, verrucose appearance. Acid but no gas is formed in 

 dextrose and saccharose broth; indol produced slowly. Nitrates not reduced. 

 Feeble growth in Uschinsky's solution. Does not grow at 37; optimum tem- 

 perature 28 to 30; thermal death-point 47.5. 



The hyacinth is the only known host plant. 



CONTROL. Diseased bulbs should be removed from the fields and 

 destroyed; land on which the disease is present should be used for other 

 crops; the use of infected tools without thorough disinfection should be 

 avoided. The selection and breeding of disease resistant varieties, as 

 advised by Wakker, suggests the most practical way of controlling the 

 trouble. 



BLACK LEG OR BASAL STEM ROT OF POTATO 



Bacillus phytophthorm Appel* 



The disease is prevalent in the United States and Europe. It 

 appears to originate in the seed tubers from which it extends upward 



* Appel, Otto, "Untersuchungen u. d. Schwarzbeinigkeit." Arb. Bio. K. G. Amt., Berlin, 

 1903. 



