FUNGOUS DISEASES OF ORNAMENTAL PLANTS. 23 



Fungous Diseases of the Rose. First let us pass into the rose 

 houses. No less than one hundred and sixty-five kinds of 

 fungous diseases of the rose are recorded in the books, and there- 

 fore only a few of the most injurious can be mentioned here. 



The Black Spot {Actinonema rosceFr.) Fig. 1, a, is perhaps the 

 most wide-spread and troublesome of the fungous diseases of the 

 rose. It was first described in 1826 and is so common as scarcely to 

 need mention other than by name, to be brought to the mind of every 

 rosarian. The foliage when attacked by the Actinonema fungus 

 soon develops the characteristic dark circular spots, while 

 elsewhere the leaves become pale and shortly fall to the ground. 

 A house of roses badly affected with the Black Spot will show a 

 lack of leaves, particularly upon the older parts of the plants, and 

 of course the quantity and quality of the blooms are unsatisfactory. 



The compound microscope reveals that the fungus consists of 

 fine threads which feed upon the substance of the rose leaf. 

 Starting at a given point from a spore that has been brought 

 through the air or in water from a mature black spot, the fungus 

 spreads in diverging lines until the circular area is the size of a 

 dime or even a half dollar. After the growth has proceeded for a 

 time small specks may be seen upon the surface of the spot. 

 These are the spore-bearing places, after the skin of ihe leaf has 

 been raptured or thrown off, and multitudes of spores are 

 produced. These spores, small as particles of dust, and as 

 numerous as the sands of the sea-shore, are the " seeds" or the 

 germs of the fungus and are capable, after finding their way to a 

 healthy leaf, of inaugurating a new Black Spot. 



As with many other fungous diseases of plants, some varieties 

 of roses are much more susceptible to the Black Spot than others, 

 and therefore it goes without further saying that, other matters 

 remaining the same, it is an important precaution to grow those 

 sorts that are the least subject to the malady. 



As we glance around in the imaginary rose house, much of the 

 foliage is observed to be more or less covered with a powdery 

 substance (mildew), as if the fine dust from a flouring mill had 

 settled upon the leaves, many of which are somewhat misshapen. 

 This powdery substance consists largely of the spores of a fungus 

 (Sphcerotheca pannosa Wallr.) that bears the familiar name of Rose 

 Powdery Mildew, Fig. 1, c. This fungus is a surface feeder and 

 does not send its filaments deeply into and between the cells of the 

 leaf. In its habit of growth, therefore, this fungus is quite in 



