9 



way until the entire leaf is destroyed. It is seldom, however, that a 

 single spot upon a leaf develops to this extent. More frequentl}' the 

 leaf is attacked at a number of different points (Pis. I, 11, III, IV), 

 and as the disease progresses the spots become larger and one or more 

 of them coalesce, forming large irregular areas or blotches upon the 

 leaf. (Pis. I, II.) A well-developed spot of this disease therefore 

 shows a light-colored central portion, the point of infection, partly or 

 wholly surrounded liy alternate rings of dark and light colored tissue, 

 the lighter colored portions as a rule being ver}' much broader and 

 more conspicuous than the darker. (Pis. I, II.) The majority of these 

 spots are usuall}'^ free from fungous spores exc(?pt under conditions 

 peculiarly favorable to their development. Spores are produced, 

 however, in great abundance upon most of them, especially upon the 

 central or older portions of the spots, after the leaves have been placed 

 in a saturated atmosphere for from twenty-four to forty-eight hours. 

 It is frequently the case that spores are produced in sufficient numbers 

 to be discernible by the unaided eye, but usually the aid of a hand lens 

 or a microscope is necessarj^ to determine their presence. The spores 

 are borne in chains on dark brownish hypha? that rise from the dis- 

 eased surface. PI. V, fig. 2, shows a photomicrograph of some of the 

 mycelium and spores of this fungus taken from a diseased spot in a 

 living leaf. The spores break from their attachment and separate 

 from each other easily, and being very small and light they are car- 

 ried around liy currents of air and finally settle upon other leaves. 



THEORIES AS TO THE CAUSE AND TREATMENT OF THE DISEASE. 



Perhaps no subject relating to floriculture has received more atten- 

 tion in the floricultural and horticultural journals during the past 

 eight or ten years than the disease in question. The most varied 

 opinions have been expressed in regard to it, and the explanations 

 advanced as to its cause and the possible course of treatment are 

 numerous. Some of the more important of these hypotheses are 

 given here. 



WEAKNESS OF THE PLANTS. 



Some writers claim that the plants are of necessity weakened by 

 being forced during the winter into heavy flower production, and 

 that the taking of cuttings from such plants, and the rooting and forc- 

 ing of them in the same way from year to year has resulted in pro- 

 ducing a weak strain peculiarly susceptible to injury of all kinds. 

 They reconnnond fall propagation to secure strong, vigorous, health}^ 

 wood before the plants are weakened by flowering. The cuttings, 

 after being rooted in clean, sharp sand, are transplanted into thumb-pots 

 or into flats and carried through the winter in a house or in frames, 

 where the temperature is kept as low as possible, not allowing the 



