PECAlSr EOSETTE. 13 



The spongy parenchyma of these parts is also more compact, and 

 the intercellular spaces are smaller than in the green areas. The 

 chloroplastids are decidedly smaller and often are pressed so closely 

 against the cell wall as to be almost invisible. In the fruit the 

 structural derangements are similar in general to those occurring in 

 the diseased leaves. 



Dilutions of the virus up to 1 to 1,000 were found to be just as 

 potent as the undiluted juice expressed from diseased plants, but at 

 dilutions greater than 1 to 10,000 no infections took place. Where 

 infections took place with the higher dilutions, the incubation period 

 was no longer than when undiluted juice was used, thus showing a 

 rapid reproduction of the virus within the plants. This virus was 

 found to be entirely removed by passage of the expressed juices 

 through porcelain filters of the finer grades (29). 



Taubenhaus (76) reported experiments in which mosaic of sweet 

 peas was transmitted by insects and by needle inoculations with plant 

 juices. 



Reddick and Stewart (63, 64, 75) found mosaic of beans trans- 

 missible by rubbing the young leaves of normal seedlings with 

 crushed leaves from mosaic plants and obtained a high percentage 

 of mosaic by sowing seeds from diseased plants. In cases of inocu- 

 lation external signs usually appeared after about four weeks. 



Many other chlorotic diseases such as aster yellows, cassava leaf- 

 curl, mulberry dwarf, cotton leaf-curl, little-leaf of the vine, citrus 

 mottle-leaf, raspberry leaf-curl, apple rosette, and mosaics of peony 

 and sweet potato present characters suggesting a possible relation to 

 the group of infectious diseases. However, they have scarcely re- 

 ceived sufficient study for any final statements regarding infectivity 

 or causes. With this brief general review the particular disease 

 under investigation may now be considered. 



STUDIES OF PECAN ROSETTE. 

 RESULTS OF PREVIOUS WORK. 



Pecan rosette was recognized by orchardists as far back as 1900, 

 but no early published references to the disease have been found. 

 Field investigations by W. A. Orton were undertaken in 1902 and 

 continued about four years. During the years 1910 to 1913 field 

 studies were carried out independently by the writer. The results 

 of these two sets of field studies were brought together and published 

 as a joint paper (56). With the exception of a few brief references 

 to the 'disease, this was the first published account of pecan rosette. 



The disease is fairly well distributed over the pecan-growing 

 regions of the Southern States, but has not been reported from the 

 northern limits of pecan culture. 



