204 THE PRINCIPLES OF FLORICULTURE 



perature, will stimulate a soft, succulent growth which is very 

 susceptible to the pathogen. Damp atmospheric conditions, 

 especially at night, sudden changes in temperature, lack of 

 drainage and improper feeding, cause the disease in greenhouses ; 

 while inferior air circulation over the beds, such as is caused by 

 nearby evergreen trees or other shade, and imperfect drainage, 

 favor mildew outdoors. 



183. Preventives. To prevent mildew, grow the plants 

 indoors in large houses, where rapid changes of temperature and 

 drafts can be avoided. Give plenty of ventilation when the 

 plants are first started, to insure a firm, vigorous growth ; keep 

 a little heat in the greenhouses during the nights of late spring 

 and early fall, and keep the ventilators open at the same time, 

 so there will not be an accumulation of moisture in the houses. 

 Remove all dead leaves, especially during the season when rose 

 plants are resting. 



184. Remedies. A remedy for mildew is the frequent dust- 

 ing of the foliage with powdered sulfur. Indoors, a sulfur wash 

 is applied to the heating pipes, and the fumes are destructive 

 to the parasite. 



185. Violet stem-rot. Classification: Ascomycetes, Perispori- 

 acese, Thielavia basicola Zopf. Violet stem-rot is a very com- 

 mon and serious trouble for violet-growers. It first attacks 

 the stems of violet plants, and later may attack the roots. 

 Plants, even when affected with the disease, may make good 

 growth throughout the summer, but after being placed in the 

 house they will wilt for a short time each day, reviving at night. 

 This may occur for a week or more, when they will wilt com- 

 pletely and die. As the fungus acts slowly at first, the disease 

 may not become apparent for a considerable time after the 

 plants are in the houses. 



186. Microscopical nature. The mycelium first attacks the 

 epidermal cells, and bores through the cell walls. Later it 

 attacks the underlying tissue. 



187. Reproduction. Tubeuf * describes the reproduction of 

 violet stem-rot as follows : " Three kinds of reproductive organs 

 are produced on the underground parts of the plants : (1) Cylin- 

 drical, delicate, hyaline chlamydospores produced in pistol-shaped 

 branches of the mycelium. (2) Thick-walled, brown-coated, 

 resting conidia arranged several in a row, like spores of Phrag- 

 midium. (3) Perithecia or little, spherical, permanently closed, 



1 " Diseases of Plants Induced by Cryptogamic Parasites," 

 Tubeuf and Smith, p. 182. 



