638 INSECTS AFFECTING VEGETATION 



place within a few hours after infection. Tho freshly diseased por- 

 tion of the leaf at first presents a waterlogged appearance, frequently 

 being semi-transparent, and is lighter in color than the adjacent 

 healthy tissue. The diseased portion around the central point of in- 

 fection in a few days fades or bleaches to a yellowish or grayish 

 white, sometimes to a pure white, the time depending somewhat 

 upon the conditions of the weather. The development of the disease 

 may stop at this point and the plants apparently entirely recover 

 from its effects; in which event the diseased portions of the leaves 

 after a few days separate from the healthy tissue and fall out, leav- 

 ing the leaves full of holes. More frequently, however, the disease 

 continues to develop in the parts of the leaf adjoining or surround- 

 ing those already diseased. These freshly diseased areas in turn pass 

 through the same changes as the parts previously attacked. Unless 

 checked by some means the disease continues to spread in this way 

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

 single spot upon a leaf develops to this extent. More frequently the 

 leaf is attacked at a number of different points, 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. A well- 

 developed spot of this disease therefore shows a light-colored central 

 portion, the point of infection, partly or wholly surrounded by alter- 

 nate rings of dark and light colored tissue, the lighter colored por- 

 tions, as a rule, being very much broader and more conspicuous than 

 the darker. The majority of these spots are usually free from 

 fungus spores except 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 necessary to determine their presence. The spores are borne in 

 chains on dark brownish hyphse that rise from the diseased surface. 

 The spores break from their attachment and separate from each 

 other easily, and being very small and light they are carried around 

 by currents of air and finally settle upon other leaves. 



Perhaps no subject relating to floriculture has received more 

 attention 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 ex- 

 planations advanced as to its cause and the possible course of treat- 

 ment are numerous. Some of the more important of these hypo- 

 theses are given here. (Div. Veg. Path, and Path. B. 23.) 



So far as we are aware there is at present no effective remedy 

 for this disease when it has gained a foothold. The principal fungi- 

 cides in common use for the prevention and check of plant diseases 

 have frequently been tried for this trouble, but with varying re- 

 sults. The experiments of the Division in spraying violets with 

 some of the more important of these, among them Bordeaux mix- 



