60 EEPOKT OF THE COMMISSIONER OF AGRICULTURE. 



fall npon the leaves of the cedar, and, vice versa, those produced on the 

 cedar will only vegetate upon the apple tree, or some closely-related 

 i>lant. The conclusion naturally follows that if we would preserve oar 

 orchards from the attacks of the Bcestelia we must exterminate the red 

 cedar that supports the " cedar apples." This is not an isolated exam 

 pie of alternation of forms upon different host plants ; others are known, 

 and it is probable that there are many yet to be discovered. Further 

 information in this direction will no doubt suggest remedies for plant 

 diseases that now seem wholly beyond our control. 



Another way of checking the ravages of fungi is to destroy their so- 

 called "winter spores," spores which are designed to continue tbeir 

 existence through the cold season. These winter spores are known only 

 in a few instances, owing to the diflliculty of tracing their formation. 

 The winter spores of the grape-vine mildew are formed in the tissues of 

 the leaves, probably also in the dead fruit when that is attacked, and 

 in these they remain through the winter or until decomposition sets 

 them free. It is now believed that the fungus which causes the Amer- 

 ican black rot (genus Phoma), maintains its existence through the win- 

 ter in an acigerous form (the mature or perfect form of the Phoma) in 

 the rotted and fallen grapes. From the consideration of these facts, 

 the importance of destroying all the ftillen grape leaves, berries, and 

 trimmings from the vines, which are often allowed to remain where 

 they fall, is sufficiently evident.* 



In addition to these winter or resting spores, which resist the severity 

 of the cold season and continues the existence of the fungus from one 

 year to the next, there are many species whose vegetative portion, or 

 mycelium, is perennial, living and continuing their growth more or less 

 actively through the winter months in the tissues of the host plant. For 

 obvious reasons fungi of this character are more difficult to destroy than 

 the annual species. 



There is another matter relating to the habit of growth of fungi that 

 must be considered in connection with the employment of remedies for 

 plant diseases, and this is whether they grow icithin the tissues of the 

 host i)lant or only npon its surface. Those of the first class are by far 

 the most numerous, and they are also the most difficult to contend with, 

 for, when once established within the tissues of the host, little can be 

 done to check their ravages. 



The white mildew or blights (order Perisporiacccc) includes species 

 growing wholly upon the surface of the plants they attack. Unless oc- 

 curring in great abundance they are not especially injurious, and they 

 may be destroyed, if promptly attended to, with comparative ease. The 

 inycelium consists of long cobweb-like threads which cling closely to 

 the infested part, imbibing nourishment by means of small projections 

 known as haustoria or '' suckers." 



The Pcrisporiacecc produce two forms of spores. The mycelium at 

 tlrsfsends up vertical threads or branches which divide into a number 

 of" squarish or ovoid cells. The topmost cell falls away and those below 

 follow in rapid succession. These constitute the summer or oidiiim 

 spores. They are so small and light that they are easily dispersed by 

 the winds, and when they alight upon a suitable place they quickly ger- 

 minate, and in this way the mildew is rapidly propagated. 

 Later in the season another and more complicated form of spore is 



* The practical value of this vineyard clearing is shown by the experience of Colo- 

 nel Pearson, who believes it to be the surest and most practical method of preventing 

 both the black rot and the mildew. See article on " Grape rot and grape mildew," 

 ia this volume. 



