PEACH YELLOWS. 299 



• HYPOTHESES PROBABLY RULED OUT. 



Among supposed causes deserving furtlier inquiry I should place root- 

 aphides and root-fungi. I am iucliued to believe that neither one is at the 

 bottom of the trouble ; yet another summer in the field would enable me to 

 speak more positively. 



REMAINIKG PROBABLE HYPOTHESES. 



"What then remains? The larger fungi are out of the question, and I can 

 think of nothing else but micro-organisms. The spread of yellows from dis- 

 eased buds to healthy stocks, which I have carefully verified, points strongly 

 to some contagmm vivum as the cause of the disease. If a micro-organism 

 be really the cause, it probably occurs quite constantly in some part of each 

 diseased tree, and this must be established beyond question; it must also be 

 clearly distinguished from similar organisms not related to the disease; and, 

 finally, it must be isolated by cultivation in suitable nutritive media and be 

 able to produce the disease when inserted into healthy trees. If, from a pure 

 culture of some micro-organism peach yellows can be induced in healthy 

 trees, then the case is closed and there can be but one verdict. I write this 

 paragraph with ease, but the work itself is full of difficulties. Nature does 

 not yield her secrets upon the mere asking. Only those engaged in similar 

 inquiries can have any adequate conception of the labor involved or of the 

 perplexities which beset one at every step. Moreover, in such an inquiry 

 nothing can be promised in advance. The investigator and the public alike 

 must take their chances on the results However, as I have elsewhere stated, 

 there seems to be every encouragement for the renewed and persistent prose- 

 cution of this inquiry. By such effort sources of error will be discovered, 

 difficulties overcome, and the truth finally established. 



The remainder of Prof. Smith's work embraces Appendix A, chemical 

 analyses of both healthy and diseased leaves, trunks, branches and fruit, of 

 the peach, showing considerable variation of constituents between the state 

 of disease and that of health. Appendix B, legal enactments of several 

 States, Michigan having been the first to pass a law for the restriction of yel- 

 lows; several maps of orchards, showing hight of land, position as to water, 

 etc., and distribution of infected trees; a map of the United States and one 

 of the Philadelphia region, showing infected portions, degree of infection, 

 and territory wholly free of yellows; and thirty-seven plates, some in colors, 

 showing healthy and diseased orchards, trees, limbs, leaves and fruit, both 

 as to yellows and borers. It is matter for regret that the means of this society 

 do not permit reproduction of some of them in this volume. 



While but little appears in the way of revelation of the cause of yellows, 

 or as to its prevention or cure, it must be borne in mind that the report is a 

 preliminary one and the investigation only just begun. In this light the 

 report must be regarded as of very great value. It has certainly been made 

 with thorough and intelligent research and with a carefulness as to detail 

 that causes strong hope of excellent results of Mr. Smith's future work. 



