32 A " Wither Tij) " of Plum Trees 



bloom. Many of these withered twigs retained their leaves throughout 

 the winter; the ])rcsence of such twigs still bearing leaves at that season 

 of the year is an almost certain indication that Monilia pustules are to 

 be found. 



When the trees were examined in spring a few cankers bearing pus- 

 tules of the fungus were found on the branches; in each case the canker 

 had arisen round a node, the branch having been invaded through a 

 short shoot or flowering spur. 



Since considerable numbers of the dead twigs bearing the fungus were 

 present on the trees under examination it was expected that there would 

 be an outbreak of the disease during the summer of 1917. Very few 

 examples of the typical "Wither Tip" condition were to be found how- 

 ever and it was noticed also that very few aphides were present on the 

 leaves. This suggests that the aphides either aid in the distribution of 

 the conidia or that their presence on the leaves reduces the vitality of 

 the latter to such a degree that they are rendered particularly susceptible 

 to attack by the fungus. The wounds caused by the aphis probably 

 enable the germ tubes to penetrate the epidermal layer. 



That outbreaks of the "Wither Tip" disease are dependent on aphis 

 attacks has not yet been established, but the observations made during 

 the past two seasons are suggestive since, although in 1917 (when 

 aphides were few in number) the sources of infection, in the form of 

 withered twigs bearing Monilia pustules, were more numerous than in 

 the previous season, instances of twigs killed that season were very few. 



(6) Cultural Studies. 



In order to compare the fungus obtained from the dead twigs with 

 strains of Monilia from other sources it was necessary to grow the 

 organism in pure culture, so that its behaviour when growing on varied 

 artificially prepared media could be studied ; cultures of the fungus known 

 to be uncontaminated with other organisms were also required for the 

 inoculation experiments described later in this paper. For this purpose 

 infected twigs were obtained from various sources in Kent and in all 

 eleven strains have been grown. 



Cultures were obtained initially in two ways, viz. : 



(1) If the fungus was producing conidia, as when occurring on the 

 dead twigs in tlie winter and spring following infection, or as sometimes 

 found on the newly killed leaves in the summer, the conidia were isolated 

 on agar plates and the resulting sporehngs which weie found to be un- 



