510 MICROBIAL DISEASES OF PLANTS. 



in the fibro-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 blossoms 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/z to 2.o// by o-5/x 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, trans- 

 lucent, 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 temperature 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. 



BASAL STEM ROT OF POTATO; 

 Bacillus phytophthorus Appel.* 



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

 stems of the potato rot off close to the ground, and the tubers rot in the 

 soil and later in storage. 



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

 1903. 



