174 Journal of Agricultural Research [voi.iii. No. a 



of isolation cultures and inoculation tests, the varying presence and non- 

 presence of mycorrhiza on both healthy and rosetted trees, the budding 

 and grafting tests, and the transplanting experiments. 



It appears from the results of experiments in pruning and cutting 

 back, transplanting tests, fertilizer experiments, results of subsoil 

 dynamiting, and orchard records that the disease is directly or indirectly 

 caused by some soil relation. On account of their variable character 

 the ash analyses have shed but little light on the problem. 



Leaf-hopper injury has been observed on pecans, but is distinct from 

 rosette and has occurred both in the presence and the absence of the latter 

 disease. Sun scald, or "winterkill," manifests itself in the death of 

 the cambium at the base of the trunk and is not likely to be confused 

 with rosette. Frost injury may simulate rosette in the killing back of 

 the terminals, but the other rosette symptoms are lacking. Rosette and 

 yellows of peach in a general way suggest pecan rosette, but though some 

 symptoms may be common to all three diseases the complete clinical 

 picture is distinct in each case. A striking resemblance is to be observed 

 between pecan rosette and ordinary chlorosis of various trees, where all 

 gradations occur from mere yellowing of the leaves to cases where the 

 symptoms closely simulate rosette of pecans. The spike disease of pine- 

 apples also bears some general resemblance to the rosette of pecans, 

 both as to effect and apparent cause. 



Observations and experimental evidence point to the conclusion that 

 pecan rosette belongs among the chlorotic diseases of plants grouped 

 by Sorauer into two main classes: (i) Noninheritable and noninfectious 

 diseases, due mostly to improper nutritive supply or to injurious physical 

 conditions, and (2) inheritable and infectious diseases, due probably to 

 enzymatic disturbances. It seems legitimate to conclude from the data 

 outlined in this paper that pecan rosette belongs in the first group. The 

 evidence strongly points in the direction that the disease is caused by 

 improper nutritive supply, and it seems probable that it is directly re- 

 lated to a lack of balance between two or more soil ingredients. The 

 possibility of some relation to soil organisms is not entirely precluded, 

 but it is thought that the direct cause will ultimately be found in some 

 lack of balance in the nutritive supply, or possibly in some toxic organic 

 substances in the soil. 



There appears to be little doubt as to a difference in resisting power 

 toward rosette, but orchard records and observations tend to show that 

 this difference is usually manifested through the stock rather than 

 through the variety worked upon it. Good care and fertilization are 

 to be recommended, but the effects of lime should be tested upon a few 

 trees before using it on a commercial scale. Pruning is of no avail as a 

 remedial measure. Trees showing only traces of rosette may be left in 

 the orchard, but all advanced cases should be cut out and replanted. On 

 account of resistance versus susceptibility of stock, the discarding of all 

 rosetted nursery trees is to be strongly advised. 



