368 ANNUAL REPORT OF THE Off. Doc. 



APPLE DISEASE 



By PROF. H. R. FULTON, State College, Pa. 



I have been asked to speak on the subject of apple diseases. Fully 

 twenty of these, affecting all parts of the apple tree, have come 

 to my notice in Pennsylvania. Fortunately the majority are only 

 slightly injurious; several that are very serious in other sections 

 of the country occur very infrequently with us. We can consider 

 only the most important apple diseases to-day. 



These diseases, for our purpose, may be classified as fungus and 

 bacterial diseases, and physiological [diseases, remembering that 

 bacteria are, after all, merely a special kind of fungi. Those of 

 the first class are caused by living plant organisms of very small 

 size, that may spread from plant to plant; and these diseases are in- 

 fectious or contagious in character. However, climatic and local 

 weather conditions, as well as other conditions of environment, may 

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

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

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

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

 control measures must usuallv be aimed directly at it. 



Occasionally, as in the case of the Powdery Mildew on leaves 

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

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

 must be to prevent the first infection ; because, 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 favor 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 a 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 Aveeks 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 it 

 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 plowing 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 



