SCIENCE. 



[N. S. Vol. XII. No. 310. 



has sometimes been found to have little or 

 no effect in facilitating transmission. The 

 facts which have been taken to indicate a 

 gradual spread of the diseases, and thus 

 have been used as an argument for an 

 epidemic type of infection, are as well or 

 even better provided for in the above sug- 

 gestion, if proper allowance be made for 

 the fact that such diseases are brought to 

 the attention of pathologists only when 

 they threaten serious damage to com- 

 mercial interests in the hands of intelli- 

 gent horticulturists. This would be by no 

 means the first instance where the exten- 

 sion of our knowledge of a disease has been 

 interpreted as an extension of the disease 

 itself. As with most other plant diseases, 

 neither the yellows nor the rosette was rec- 

 ognized as a definite malady until an or- 

 ganized industry was attacked. Both were 

 at first supposed to be local, and though 

 a wide distribution has been ascertained, 

 there is nothing to indicate that either was 

 introduced from Europe or has spread to 

 California, facts which also militate strongly 

 against theories of the constitutional origin 

 of the diseases, though possibly compar- 

 able with the failure of the above-mentioned 

 orange-rust mites to establish themselves in 

 the dry atmosphere of California. 



That the yellows has not gone south and 

 that the rosette has not come north are also 

 indications that the diseases are not spread 

 entirelj' from the peach, if we admit that 

 both disorders had probably existed for 

 many decades before their scientific recogni- 

 tion. In fact, the various considerations of 

 distribution are sufficient to warrant a be- 

 lief in local origin and external infection, 

 and it is not even necessary to insist that 

 they are transferred in nature from one 

 peach tree to another, since the existence of 

 the injurious species in the neighborhood 

 of an orchard would mean that infection 

 might be indefinitely repeated. It is also 

 to be remembered that under the theory of 



infection of an entire tree by a single punc- 

 ture it is to be expected that there will be 

 many trees with yellows where none of the 

 injurious mites can be found, since only 

 when fertilized females or both sexes are 

 transferred will the species be multiplied. 

 Thus might the yellows be ' contagious ' 

 from some trees, where colonies are formed^ 

 while not contagious from others. 



But though the mite should be found to 

 breed upon the peach, insecticides would 

 remain useless, since the infected trees are 

 injured beyond repair and may as well be re- 

 moved and burned, as Dr. Smith has urged. 

 That this policy has appeared to keep the 

 yellows in check in some parts of Michigan 

 may mean that either our supposititious 

 mite or the agent of its transfer to the 

 peach is locally rare. We have, in fact, all 

 gradations from an apparent epidemic in 

 some parts of the East to a merely sporadic 

 condition in Illinois, where competent in- 

 vestigators are not yet convinced that the 

 disease is contagious. 



It is a fact well known to entomologists 

 that the numbers of many species of in- 

 sects and other short-lived arthropoda vary 

 enormously in different years or periods of 

 years. An unfavorable season, an active 

 enemy, or a disease may reduce a species to 

 practical non-existence for a time, until the 

 return of favorable conditions permits a 

 gradual increase to the former numbers, or 

 even to unrecorded abundance. These 

 fiuctuations are also closely comparable with 

 those which appear iu connection with par- 

 asitic fungi and bacteria, species previously 

 so rare as to have remained quite unknown 

 to the botanist often appearing suddenly as 

 the agents of extensive injury. Seasonal 

 and periodic as well as local differences in 

 the apparent ' virulence ' of peach-yellows 

 would thus be readily explainable on the 

 present theory, to which plant pathologists 

 need not object because of reasons drawn 

 from the history of the malady. 



