32 BULLETIN 1038, U. S. DEPARTMEITT OF AGRICULTUEE. 



differentiation and lack of tissue differentiation, hypotrophy and 

 hypertrophy of cells may all occur together within an area only a 

 few millimeters in diameter. There seem to be focal centers out from 

 which these alterations spread, and the resulting abnormal develop- 

 ment appears to be controlled from these centers rather than by the 

 normal morphogenic forces. The factors controlling morphogenesis 

 as related to cell differentiation, to development of tissues in dis- 

 eased areas, and to general pattern of leaf are profoundly altered. 

 No less far-reaching are the derangements in metabolism evidenced 

 by the altered assimilation and translocation of starch. 



Pecan rosette frequently appears first on the tip leaves of a single 

 branch and seems as likely to affect a part of the tree first as to occur 

 at once over the whole top. 



Careful field observations have given no evidence of varietal dif- 

 ferences in susceptibility or resistance to pecan rosette. This same 

 statement would apply equally well to most infectious chloroses. 



Fertilizer- and transplanting experiments and field observations 

 indicate that rosette is affected, at least indirectly, by soil and cli- 

 matic conditions, but similar relations are exhibited by chloroses of 

 known infectious nature as well as by many diseases of bacterial or 

 fungous origin. Furthermore, great reliance is not to be placed upon 

 the results of plat soil experiments unless such results are unusually 

 definite, or unless in a large number of similar tests the data all 

 point in the same direction. 



In view of the refractory attitude exhibited toward grafting and 

 cross-inoculating experiments by certain of the infectious chloroses it 

 is considered that this type of experimentation has not yet given a 

 conclusive answer to the question of possible infectivity in pecan 

 rosette. In the early experiments a small portion of the diseased 

 buds developed the disease, though in no larger percentage than in 

 contiguous nursery trees worked with supposedly healthy buds in 

 the ordinary commercial propagation. These experiments need repe- 

 tition on a large scale under controlled conditions and with extreme 

 care in selection both of diseased and healthy buds and stocks. More- 

 over, the possibility of insect transmission has not been touched upon 

 from the standpoint of experiment. 



The disease has not been definitely and experimentally caused by 

 a set of known conditions. Though it is more prevalent and severe 

 under certain environmental conditions, it occurs to some extent in 

 practically every soil type where the tree has been observed, with 

 the possible exception of swamp land. Under these conditions the 

 tree makes very little growth and presents a starved and stunted 

 appearance followed by a general form of chlorosis bearing no 

 resemblance to rosette. Ultimate proof of the cause must account 

 for all cases of the disease. 



