PLANT DISEASES CAUSED BY FUNGI. 



123 



CHAPTER XIII. 



PLANT DISEASES CAUSED BY FUNGI. 



Rjnd of Food required by Fungi— Structure and 

 Reproduction — Different Modes of Reproduc- 

 tion — Preventing the Spread of a Disease — 

 Preventing a Repetition of a Disease — Fungi- 

 cides. 



The knowledge necessary to enable the horti- 

 culturist, farmer, or forester to utilize to the 

 fullest extent the discoveries of specialists 

 relating to plant diseases, and also to convey 

 to others an intelligent account of the particu- 

 lar form of disease respecting which information 

 is desired, is : First, familiarity with the general 

 appearance of the commoner fungoid parasites, 

 their varied modes of attack, and the special 

 conditions favouring the same; also the rela- 

 tionship between the different forms assumed 

 by certain parasites during different periods 

 of their existence. Second, acquaintance with 

 the preventive measures which, if acted upon, 

 would prevent the wholesale destruction too 

 often experienced from fungoid diseases. Third, 

 a knowledge of what can be done towards 

 effecting a cure when the disease has revealed 

 itself. In the majority of instances where an 

 annual plant is attacked, cure is practically 

 impossible, and in the case of perennials, the 

 prospect of a profitable crop for that year is 

 slight. In such instances, however, the disease 

 can be arrested in its course, and a recurrence 

 prevented. Hence it follows that a knowledge 

 of the broad principles regulating the mode of 

 life of those fungoid parasites which prove so 

 injurious to cultivated plants should be clearly 

 grasped. 



Kind of Food required by Fungi. — 

 Fungi differ from plants possessing green leaves 

 in not being able to obtain their food from the 

 soil, and can only derive nourishment from the 

 bodies of plants or animals, either living or 

 dead. 



Those fungi that live on dead vegetable mat- 

 ter, as rotten tree-trunks, roots, leaves, manure, 

 &c, are called saprophytes. Certain saprophytes 

 that grow on the ground in woods and pastures 

 might naturally be supposed to obtain their 

 food from the soil in the same manner as the 

 flowers and grass among which they are grow- 

 ing. This, however, is not the case; the roots 

 of such fungi will, on careful examination, be 

 found to spring from fragments of buried wood, 

 roots, &e.., or from decaying vegetable humus. 



As saprophytic fungi do not injure living plants 

 they may be dismissed without further comment. 



On the other hand, those fungi that attack 

 and grow on living plants are called parasites. 

 By far the larger number of destructive para- 

 sitic fungi are individually very minute, and 

 do not conform to the popular idea of a 

 fungus as represented by a "toadstool", "puff- 

 ball", or the common mushroom, and are only 

 visible to the naked eye Avhen present in im- 

 mense numbers, as is usually the case, and are 

 then called by such popular names as "rust", 

 "mould", "mildew", "bunt", &c. 



Structure and Reproduction of Fungi. 

 — The wide-spread popular opinion that fungi 

 grow very quickly, springing up in a single 

 night, is a mistake. A moment's consideration 

 as to the period of time intervening between 

 the placing of spawn in a mushroom-bed and 

 the appearance of mushrooms ready for the 

 table should be sufficient to disprove such an 

 idea. The mistake is due to considering the 

 part above-ground as representing the entire 

 fungus, whereas the stalk, cap, and gills col- 

 lectively only represent the seed-bearing por- 

 tion, corresponding in function to the fruit of 

 a flowering plant, the spawn or mycelium as 

 it is properly called, corresponding in use to 

 the root and leaves of a flowering plant, 

 absorbing and assimilating food from the sub- 

 stance in which it is growing. The fruit of the 

 common mushroom first appears as a minute 

 white point on this underground mycelium, 

 and remains underground until its stem, cap, 

 and gills are formed, which requires several 

 weeks to accomplish, when the entire structure 

 is lifted above ground for the purpose of ripen- 

 ing its seed or spores as they will henceforth be 

 called, in the air, where they are readily scat- 

 tered by wind and other agents; these germinate 

 in turn and produce mycelium, which in course 

 of time gives origin to fruit or mushrooms. The 

 same course of development takes place when a 

 living plant is attacked. A spore carried by 

 wind or an insect is deposited on the surface 

 of a living leaf or stem, where it germinates, 

 pierces the skin of the leaf, and enters its 

 tissues ; there a branching mycelium is soon 

 formed, obtaining its food from the cells of the 

 plant on which it is parasitic. The mycelium 

 often spreads quickly in the interior of the 

 plant, and not unfrequently every part of the 

 plant contains mycelium before there is any 

 external evidence of its presence. It is well 

 known that a flowering plant must accumulate 

 a certain amount of reserve material from the 



