208 THE PRINCIPLES OF FLORICULTURE 



house, remove at once the affected plants, and a considerable 

 amount of soil in their immediate vicinity. Strong, vigorous 

 plants should be selected to replace diseased ones. If they 

 have a well-developed, central stem, which brings the crown 

 well above the soil, plants are less susceptible to stem-rot. 



207. Fungi Imperfecti. Tubeuf classifies under Fungi Im- 

 perfecti all other species of fungi common on cultivated flower 

 crops. Among the diseases included in this group are the leaf- 

 spots on violets, carnations, and chrysanthemums, violet an- 

 thracnose, mignonette disease and the branch-rot of carnations. 



208. Leaf-spot on violets. Classification: Sphserioidese ; 

 Phyllosticta violce Desm. Phyllosticta on violets is of con- 

 siderable importance and should be carefully kept in check. 

 Phyllosticta infests certain cells, then spreads to adjacent cells, 

 causing a discoloring of the area attacked. The withered tissue 

 falls out, and this is what gives the disease the characteristic 

 name of " shot-hole fungus." Innumerable, colorless conidia 

 form in the affected areas. 



209. Preventives. Keep the plants free from all leaves 

 affected with leaf-spot to prevent the spread of the disease. 



210. Black-spot on roses. Classification: Sphserioidese ; 

 Actinonema rosce Lib. Black-spot is frequently found in green- 

 houses where roses are grown, and in outdoor rose gardens. 

 The disease first appears on the foliage, as a dark-brown spot 

 with an irregular margin. This spot increases in size, and the 

 leaf area surrounding it assumes a pale yellow color and soon 

 defoliation occurs. 



211. Microscopical nature. The mycelium is distributed 

 both inside and outside the leaf tissue. The action of the 

 mycelium on the cell contents renders the protoplasm dark- 

 colored and granular. This gives the leaf the characteristic 

 spot. The yellowing of the surrounding tissue is also caused 

 by the action of the mycelium. 



212. Reproduction of black-spot. On the mycelium in the 

 darkened areas, numerous pycnidia are formed, and from these 

 two-celled conidia are developed. These bring about the rapid 

 spread of the pathogen. When the spores fall on the moist 

 surface of a rose leaf, they quickly germinate and produce a new 

 infecting mycelium. 



213. Ecology of the disease. Imperfect drainage is often 

 responsible for a weakened growth of the rose plants, and this 

 favors the development of the disease. Carelessness in watering, 

 a sudden drop in the temperature of the rose house, and imperfect 

 circulation about the plants, seem to cause a similar result. 



