DISEASES OF FLOWERING PLANTS 203 



theca pannosa. The rose mildew is exceedingly troublesome 

 on both outdoor and indoor roses, and frequently causes an 

 immense financial loss to commercial growers. To amateurs 

 it is a discouraging element in the culture of outdoor roses. It 

 attacks the younger leaves, and forms small, white patches. 

 Soon it spreads over the entire leaf surface. Young leaves and 

 buds when attacked become deformed, their function is inter- 

 fered with and death may result. Often the foliage of an entire 

 crop is quickly injured by an attack of this fungus, and the 

 plants become defoliated. 



179. Microscopical nature of rose mildew. The vegetative 

 or feeding portion of the fungus spreads over the exterior surface 

 of the leaf. If a spore falls on the moist surface of the leaf, it 

 quickly germinates and a mycelial thread develops. This 

 thread attaches itself to the epidermal cells by means of haus- 

 toria and absorbs the cell contents. The mycelium becomes 

 much branched and in a short time completely covers the sur- 

 face of the leaf. It may also extend over the younger growth 

 of the stem and cover the bud. 



180. Asexual reproduction. When rose plants are growing 

 rapidly and conditions favor rapid mycelial development, the 

 mycelium spreads over the leaf for a short time. Ascending 

 branches are then formed, and from the tips of these, asexual 

 spores of conidia are produced in great numbers. When mature, 

 they separate from the conidiophores or stalks on which they 

 are produced. They are carried by the lightest breeze, and if 

 they fall on young rose tissue, they germinate immediately and 

 rapidly produce new plants. This is the usual and most rapid 

 method of reproduction. 



181. The resting stage. When conditions are unfavorable 

 for the rapid growth of either rose leaves or mycelium, thick- 

 celled and usually dark-colored bodies form on the mycelial 

 strands. They are ascocarps which contain the resting spores 

 of the fungus. Within these thick-walled bodies there are 

 numerous, club-shaped spore-cases placed together in a crowded 

 manner, and within each of these spore-cases several spores are 

 formed. These are the ascospores which, when liberated from 

 the ascocarp, reproduce the fungus. These reproductive bodies 

 or ascospores are formed by nuclear divisions inside the club- 

 shaped cells which are called asci. Because of this manner of 

 reproduction, the mildew exists through a long period if condi- 

 tions are unfavorable for growth. 



182. Ecology of disease. A frequent cause of mildew is a 

 lack of ventilation in rose houses. This, with too high a tern- 



