50 A ''Wither Tip'' of Pbnn Trees 



which had expanded their petals during the preceding 24 hours) were 

 selected, for in the experiments of the previous years it had been found 

 that such flowers had been more readily attacked than older flowers. 



Three of the flowers (numbered 1, 2 and 3) were inoculated on May 14; 

 two others of the same age were left untreated and labelled as controls 

 (q and C2). By the following day other flowers had opened and four of 

 these were inoculated (numbered 4, 5, 6 and 7); three similar ones were 

 kept as controls (Cg, c^ and ^5). No difficulty was experienced in obtaining 

 conidia for the inoculations as this strain of Monilia produces numerous 

 conidia on sterilized potato. 



The result is shown in the table (p. 51). 



The early inception of a brown discoloration of the inoculated flowers 

 afforded evidence that infection had occurred. Later, all the inoculated 

 flowers fell away and in no case was there any evidence that the disease 

 had passed from the infected flowers into the tissues of the spur, and the 

 leaves borne on these spurs remained green and healthy. It would 

 appear therefore that the strain of Monilia cinerea used in this experi- 

 ment is able to infect and cause the death of apple flowers but is unable 

 to penetrate further than the base of the pedicels. 



This result was particularly interesting as it suggested that there 

 were strains within the same morphological species, Monilia cinerea, 

 with different powers of infection. For confirmation of this it was 

 decided to repeat the experiment side by side with one in which a strain 

 from an apple tree was used. To obtain results strictly comparable one 

 tree was selected, one of the Warner's King variety growing in the 

 College plantation; umbels on one side of the tree were inoculated with 

 the plum strain, those on the other with the apple strain. Each of the 

 two strains had been isolated during the previous winter so that there 

 was no question that either had lost its virulency from long continued 

 growth on artificially prepared culture media and it had been already 

 ascertained that the plum strain readily attacked plum flowers and spurs. 



Experiment 2. 



Method. Particles bearing conidial tufts were cut from a culture of 

 the "Wither Tip" strain growing on sterilized potato. These particles 

 were placed in a sterile petri dish and taken to the plantation, where, by 

 means of needles (previously flamed), they were inserted in the flowers 

 to be inoculated in such a way that the conidiophores were brought in 

 contact with the stigmas. That many of the conidia adhered was evident. 



