Recent Studies on Peach Yellows and Little Peach 55 



Several such contradictory observations have already been men- 

 tioned in this bulletin, as, for example, the fact that Carman ap- 

 peared to be less susceptible to disease than most other varieties in 

 orchard no. 1, but was quite susceptible in no. 2. 



Does Disease Spread from Local Centers? 



It has been the general observation that where diseased trees are 

 left growing in an orchard they become centers of infection and that 

 surrounding trees gradually become affected. Whether this is act- 

 ually due to direct infection in some manner or to environmental 

 conditions favoring the disease in that area appears to be an open 

 question. 



A peach orchard completely infested with yellows during the epi- 

 demic of 1905-1907 adjoined the Vineland orchards at the northwest 

 corner from 1907-1909, after which it was removed. The spread of 

 disease in that corner of the experiment orchard has been serious. 

 Figure 26 illustrates replants growing in one portion of this area. 



An equally serious center of spread occurred at the southwest 

 corner of the same orchard, which was bordered by a narrow hedge- 

 row of native trees. One may also note by the colored diagrams 

 healthy trees surrounded by areas from which diseased trees have been 

 removed ; and so the contradictory comparisons may be multiplied. 



There is no definite evidence in these orchards that these diseases 

 spread from tree to tree ; yet, on the other hand, there is no strong evi- 

 dence to the contrary. 



Two negative facts in regard to their spread appear to be estab- 

 lished : (1) they are not distributed by the pollen of diseased trees, 

 and (2) the evidence is strong against their spread by means of the 

 soil. 



Some have suggested pruning tools as a possible agency of in- 

 fection, but this also appears to be a factor to be placed in the very 

 doubtful class. 



