27 



favor or check epidemics of such diseases, either directly by in- 

 fluencing the spread and development of the organisms, or indirect- 

 ly by placing the host plant in a condition of greater or less suscepti- 

 bility. But always the causative organism must be present, and 

 control measures must usually be aimed directly at it. 



Occasionally, as in the case of the Powdery ]\Iildew on leaves 

 and young shoots of apple, the fungus may be killed after gaining 

 foothold by applications of a fungicide. In mose cases the aim 

 must be to prevent the first infection ; l^ecause, as a rule, when the 

 organism has become established, there is no hope of eradicating 

 it from invaded parts. 



For infection to occur, three conditions must hold : There must 

 be a source of contagion, there must be a susceptible host plant, and 

 the general environmental conditions must fai^or the infection. To 

 prevent infection, we must take these things into account, and the 

 special measures will vary for each disease according to its peculiar- 

 ities with reference to these three conditions. For illustration think 

 of well known treatment for apple Scab, which calls for three appli- 

 cations of a proper fungicide, just before the buds open, just after 

 the petals fall, and a third two weeks later. The coating of fungi- 

 cide on fruit and leaves makes an unfavorable environment for the 

 development of the scab fungus there. This is made when the 

 parts in question are young and in their most susceptible condition. 

 And it so happens that the source of early contagion for Scab is 

 the so-called winter-spores that form slowly during winter on fallen 

 apple leaves infected the previous year with Scab, and reach ma- 

 turity, are scattered, and retain their vitality for three or four 

 weeks only, about the apple blossoming time. I know of cases 

 where elimination of the fallen leaves, by plovving them under be- 

 fore the time indicated, or by burning them, has given successful 

 control ; but such measures do not commend themselves on the score 

 of general practicability. A'arietal susceptibility influences very 

 much Scab infection, and should influence our treatment of it. 

 Baldwin, York Imperial, Ben Davis and Jonathan are aft'ected little 

 or not at all, while Stayman AA'inesap, ]\IcIntosh, Spitzenburg, and 

 Xorthern Spy are quite susceptible. Discrimination in the treat- 

 ment of varieties varying in susceptibility will promote efflciency 

 and economy not only for scab, but for other troubles. A wet, cool 

 spring favors Scab. Our climatic conditions, fortunatelv. are less 

 conducive to Scab development than those farther north ; and we 

 can for this reason safely omit, in the aAcrage season and on the 

 average variety, the early application, just before the blossoms 

 open. 



In the case of Cedar or Orange Rust, unprotected voung leaves 

 and fruit of certain A'arieties are the endangered parts ; the infec- 

 tive material in this case comes from red cedars that mav harbor 

 the fungus in the familiar "cedar-apples ;" and infection is favored 

 by periods of continuous wet Aveather for tAvo or three davs. AMien 

 these conditions occur together. Avhich i^ only occasional! v. Ave have 

 an outbreak of Cedar Rust. For this disease the removal of en- 

 dangering red cedars from the vicinity of orchards has proved more 

 constantly eifective than spraying. 



