38 A " Wither T'qy " of Plum Trees 



two days several of the resulting sporelings were seen to be uncon- 

 taminated and three were transferred to another plate. All three grew 

 out in a similar manner and were indistinguishable in their mode of 

 growth from other strains of Monilia cinerea; sub-cultures from one of 

 these were used in the inoculation experiments. On the agar plates the 

 fungus failed to produce conidia and as these were necessary when 

 infection of flowers via the stigma was attempted, cultures growing on 

 sterilized potato were obtained since on this medium conidia readily 

 developed within a week. 



(a) Inoculation of Plum Leaves. 



Since observations in the open suggested that infection of the twigs 

 arose in the leaves, attempts were made to produce the disease by 

 inoculating leaves with the fungus. Experiments were made on the 

 leaves of young growing shoots both in the greenhouse and in the open, 

 conidia being placed (1) directly on the uninjured surface of the leaves, 

 or (2) on the glands at the base of the lamina, or (3) on punctures made 

 through leaves by means of a sterilized needle. In no case however did 

 inoculation result in infection of the leaves and no difference could be 

 detected between inoculated leaves and controls, repeated attempts 

 giving invariably negative results. 



It would appear that the leaves must receive some special form of 

 injury before the fungus is able to invade the tissues and the results lend 

 support to the hypothesis that aphides provide the kind of injury 

 necessary for the fungus to establish itself in the leaf. 



(b) Inoculation of Plum Flowers. 



Experiment 1. Inoculation of flowers on a Victoria plum tree in the 

 College Plantation. 



A culture of the fungus growing on sterilized potato in a test tube was 

 prepared and when numerous conidia had developed the potato block 

 was transferred from the tube to a sterile petri dish; small portions 

 bearing tufts of conidiophores were cut off with flamed scalpels and 

 transferred to another dish which was then taken to the tree where the 

 experiment was to be carried out. The actual inoculation of a flower was 

 made by removing from the dish one of the particles of potato on the 

 point of a sterilized mounted steel needle, and the conidia-bearing surface 

 was applied to the stigma of the flower. 



The tree selected was free from diseased twias or "mummied" fruit. 



