30 A " Wif/ier Tip " of Plum Trees 



must have grown out and become infected during the previous year. 

 On the surface of these older dead twigs were numerous Monilia pustules 

 and it appeared probable that these had served as sources of infection 

 for the young growth. 



When a twig is killed early in the season the buds towards its base 

 which normally would have remained dormant until the following year 

 may be stimulated to precocious devclo])ment and several weak shoots 

 with pale green leaves grow out. Thus the vigorous leading shoot, 

 which, under ordinary circumstances, was destined to produce a normal 

 supply of young "'wood" for the production of fruit buds in the succeed- 

 ing second or third year, becomes replaced by a number of feeble, 

 miniature "unripened" shoots which would probably be killed during a 

 hard winter (PI. VIII, Fig. 3). 



It was noticed that the plum trees that year were suffering from a 

 severe attack by aphides (Aphis pruni Reaumur) and these insects were 

 also present in considerable numbers on trees affected by the '" Wither 

 Tip" in other plantations in Kent. 



Although inoculation experiments carried out at Wye with pure 

 cultures of the fungus have hitherto failed to induce infection of the 

 leaves^, observations in the open appear to indicate that the young shoots 

 must in many cases (i.e. in the typical "Wither Tip" disease) become 

 attacked through infected leaves. Duggar(4) in reference to the "Brown 

 Rot of Stone Fruits" states^ that "The twigs are also susceptible, but it 

 has been definitely shown that infection of the twigs results only when 

 either flowers or fruit produced on the twigs have already fallen prey to 

 the disease." I have pointed out however (lO) that there are reasons for 

 suspecting that the American Brown Rot fungus is a form of Monilia 

 quite distinct in some respects from either M. fructigena or M. cinerea 

 as occurring in this country. 



Frank and Kruger(7) have described and figured an instance of 

 infection occurring in the lamina of a cherry leaf, but in this case the 

 leaf was in actual contact with an infected flower. In the "wither-tip" 

 disease of the plum, infection of leaves far removed from any other source 

 of infection is of frequent occurrence. 



Aderhold(i) records and figures the wilting of young shoots but only 

 when infected through a node bearing diseased flowers. Frank and 

 Kriiger also illustrate a similar instance. 



1 Vide i>. :J8 - Loc. cit. p. 18i). 



