Infection 29 



♦of the plant directly attacked by the fungus, the period of iueubatioii is 

 within fourteen days. It has also to be noted that numerous infections nia v 

 occur in the same plant at the same time. Brefeld I'emoved the epidei'mis 

 from young seedlings that had been infected, and found it pierced by distinct 

 holes,' through which the germ-tubes had entered. In favorable preparations 

 he observed the surface riddled as if by drill holes and penneated by numerous 

 in-grown germ-tubes. This would explain such cases as I have met with 

 where ears from the same plant produced spores of both Tilletia intia and 

 T. levis, for if the grain was dusted with both kinds of spores they might both 

 infect the same plant. Even the case mentioned by Sorauer^. where both 

 species were found in the same ear, ould be explained in the same way. It 

 is not unusual to find cases with stinking smut and loose smut on diSerent 

 ears from the same wheat plant, but this is not difficult to account for. The 

 loose smut fungus was already in the grain of the wheat plant, since infection 

 takes place through the flower, and afterwards infection occurred in the seedling 

 stage by means of the spores of stinking smut. 



Modes of Infection. 



There are at least four typical modes of infection at present known, and t)y 

 giving a detailed account of each subsequently it will serve to show the nature 

 of the process and the means most likely to be successful in combating the 

 particular disease. 



1. The most common mode is that the young seedlings are infected, the 

 resulting mycelium grows throughout the entire plant and the smut spore* 

 are produced usually in the flowers or inflorescence, as in oat smut ( Ust^lago 

 avenae). ■ *,' '0 



2. A second tvpe is represented in the case where any young and growing 

 portion of the host-plant is capable of infection, and the mycelium is localized 

 where the infection occurs, as in the American corn smut not known in 

 AustraUa {Ustilago maydis). 



.3. Infection may take place through the stigma of the flower, the spore 

 acting after the manner of a pollen-grain, or by piercing the young ovary wall, 

 the mj^celium developing in the ovary, lying dormant in the ripe grain and 

 growing throughout the entire plant next season, until it reaches the flowering 

 stage, when the smut spores are again formed, as in loose smut of wlieat 

 {Ustilago tritici) and naked smut of barley [Ustilago nuda). 



While in the cereals it is generally considered that the wind is the agent 

 for carrying the spores to the stigma or ovary after fertilization has occurred, 

 it is highly probable that thrips are also concerned in it, for in our northern 

 areas particularly, these insects are in some seasons so common as to affect 

 the development of the ears of wheat. ■ '. * ; ; 



4. Infection may take place as shown by Hecke,^ through the young 

 shoots which arise near the surface of the soil in tlie process known as 

 " tillering." 



It is worthy of note that these different modes of infection are asso- 

 ciated with a different behaviour of the spores on gei'mination. In the 

 first and fourth type sprouting conidia are formed which are capable of 

 living saprophytically in the soil and infecting the plant from that source. 

 In the second aerial conidia are chiefly formed, and infection is mainly brought 

 about by the wind ; and in the third no conidia are formed at all. so that 

 the spores are carried to the stigma and germinate there after the manner of 

 pollen-grains, or they may fall u])on the surface of the very young ovary. 



