1204 RESISTANT PLANTS 



which has now become necessary : i) owing to the increasing spread of 

 this disease and the extensive damage it occasions : 2) owing to the inipos- 

 sibiHty of detecting young plants with incipient disease before planting : 

 3) owing to the persistance of the germs in the soil, which germs later on 

 attack the healthy plants. 



The plan adopted in order to determine the degree of resistance to Bac- 

 ternim iumefaciens is to inoculate pure cultures (in standard agar with the 

 addition of 0.5 % of dextrose), at one week's interval, from May till Sep- 

 tember for the purpose of testing the plant in all phases of growth. 



Three lots of bacteria were used : " No. 694 " isolated from infected 

 young peach plants ; " No. 753 " from the galls of an almond tree (14th April 

 1913); and " No. 790 " isolated from an adult peach tree. 



The species and varieties of Primus examined were the following : 

 P. aUeghaniensis ; P. americana ; P. Amygdalus ; P. Andersoni ; P. Arnie- 

 niaca ;P. Armeniaca var. Mikado ; P. avium ; P. Eesseyi ; P. caroliniana ; 

 P. cerasifera var. planteriensis ; different varieties and various types of 

 P. domestica ; P. eriogyna ; P. glandulosa ; P. hortidana ; P. ilicifolia ; 

 P. integrijolia ; P. Mume ; P. Mimsoniana ; P. nigra ; P. orthosepala ; 

 P. -pennsylvanica ; P. Persica : P. platicarpa ; P. pitmila ; P. serotina . 

 P. Simonii and P. Uatsoni. 



The various degrees of susceptibility are measured b}^ means of the 

 frequency of infestation and the size of the galls, which vary from 5 cm. 

 in diameter to such small dimensions that it is diihcult to distinguish them, 

 from the surrounding tissue. 



In the appended table are found the data arranged in comparative 

 form. For P. ilicifolia, P. caroliniana and P. Amygdalus, a complete series 

 of observations are wanting. Nevertheless these experiments show that 

 the first two species are highly resistant, almost immune. On the other hand, 

 P. amygdalus is highely susceptible. 



Of the resistant species therefore the best would be P. ptimila and 

 the two varieties " Italian prune " and " German prune " of P. domestica. 

 Nevertheless P. pumila is not adapted for re-stocking owing to its tendency 

 to dwarf the varieties grafted on it. On the other hand, such good results 

 were obtained with the two above varieties of P. domestica that it would be 

 advisable to continue experiments in this direction, so as to isolate in the 

 domestica group practicalh' inmiune varieties or types, the possibility of 

 existence of which must not be precluded. P. hortulana is considered by 

 many fruit-growers as destined to ^aeld good results specially in the indig- 

 enous varieties. The resistance to Bact. tumefaciens, is however not consid- 

 erable, except in the variety " Golden Beauty ". In the " Damson " group 

 P. institutia, known under the name of P. pendula, is rather susceptible ; 

 the other kinds, on the contrary, present a high degree of resistance and are 

 certainly capable of supplying good material for further work. 



For the peach tree (P. Persica) , conclusive data are not available. Never- 

 theless the varieties " Elberta ", " Saucer " or " Peento ", " Salway ", 

 " Lovell " and " Muir " seedlings hitherto examined, did not show resistance, 



Fourteen kinds of almond trees (3'oung plants growing at Davis, Cali- 



